#picodexed — Public Fediverse posts

I finally have a good enough Eurorack version of my picoDexed.

https://diyelectromusic.com/2025/05/03/picodexed-minidexed-eurorack/

This is using a neat pico-to-zero adaptor from bablokb, released on Github, which allows me to use my existing MiniDexed PCBs and module with no other hardware changes.

#picoDexed #RaspberryPiPico #EuroRack #SynthDIY

I finally have a good enough Eurorack version of my picoDexed.

https://diyelectromusic.com/2025/05/03/picodexed-minidexed-eurorack/

This is using a neat pico-to-zero adaptor from bablokb, released on Github, which allows me to use my existing MiniDexed PCBs and module with no other hardware changes.

#picoDexed #RaspberryPiPico #EuroRack #SynthDIY

I finally have a good enough Eurorack version of my picoDexed.

https://diyelectromusic.com/2025/05/03/picodexed-minidexed-eurorack/

This is using a neat pico-to-zero adaptor from bablokb, released on Github, which allows me to use my existing MiniDexed PCBs and module with no other hardware changes.

#picoDexed #RaspberryPiPico #EuroRack #SynthDIY

I finally have a good enough Eurorack version of my picoDexed.

https://diyelectromusic.com/2025/05/03/picodexed-minidexed-eurorack/

This is using a neat pico-to-zero adaptor from bablokb, released on Github, which allows me to use my existing MiniDexed PCBs and module with no other hardware changes.

#picoDexed #RaspberryPiPico #EuroRack #SynthDIY

I finally have a good enough Eurorack version of my picoDexed.

https://diyelectromusic.com/2025/05/03/picodexed-minidexed-eurorack/

This is using a neat pico-to-zero adaptor from bablokb, released on Github, which allows me to use my existing MiniDexed PCBs and module with no other hardware changes.

#picoDexed #RaspberryPiPico #EuroRack #SynthDIY

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

@rpimag I went back to my picoDexed (Synth_Dexed on a Pico) and updated it for the latest Pico SDK and added a simple display and encoder.

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

I've not quite got to the stage shown in the photo - the sound isn't working on this one yet - but it's getting close (it worked fine on my breadboard version)!

And yes, this is one of my MiniDexed front panels...

#WorkInProgress #MakerMonday #picoDexed

@rpimag I went back to my picoDexed (Synth_Dexed on a Pico) and updated it for the latest Pico SDK and added a simple display and encoder.

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

I've not quite got to the stage shown in the photo - the sound isn't working on this one yet - but it's getting close (it worked fine on my breadboard version)!

And yes, this is one of my MiniDexed front panels...

#WorkInProgress #MakerMonday #picoDexed

@rpimag I went back to my picoDexed (Synth_Dexed on a Pico) and updated it for the latest Pico SDK and added a simple display and encoder.

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

I've not quite got to the stage shown in the photo - the sound isn't working on this one yet - but it's getting close (it worked fine on my breadboard version)!

And yes, this is one of my MiniDexed front panels...

#WorkInProgress #MakerMonday #picoDexed

@rpimag I went back to my picoDexed (Synth_Dexed on a Pico) and updated it for the latest Pico SDK and added a simple display and encoder.

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

I've not quite got to the stage shown in the photo - the sound isn't working on this one yet - but it's getting close (it worked fine on my breadboard version)!

And yes, this is one of my MiniDexed front panels...

#WorkInProgress #MakerMonday #picoDexed

@rpimag I went back to my picoDexed (Synth_Dexed on a Pico) and updated it for the latest Pico SDK and added a simple display and encoder.

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

I've not quite got to the stage shown in the photo - the sound isn't working on this one yet - but it's getting close (it worked fine on my breadboard version)!

And yes, this is one of my MiniDexed front panels...

#WorkInProgress #MakerMonday #picoDexed

I thought this would be an easy fix - using a Pi Zero-format RP2040 board in my MiniDexed EuroRack module...

... but it ended up an afternoon of frustration and unfortunately I still can't get it making sound or booting reliably.

It does look cool though :)

Anyone else have any experience of these Stacky-Pi "RP2040 on a Pi Zero shaped PCB" boards?

https://diyelectromusic.com/2025/04/12/picodexed-stackypi-minidexed-eurorack/

#PicoDexed #MIDI #StackyPi #MakerFail

I thought this would be an easy fix - using a Pi Zero-format RP2040 board in my MiniDexed EuroRack module...

... but it ended up an afternoon of frustration and unfortunately I still can't get it making sound or booting reliably.

It does look cool though :)

Anyone else have any experience of these Stacky-Pi "RP2040 on a Pi Zero shaped PCB" boards?

https://diyelectromusic.com/2025/04/12/picodexed-stackypi-minidexed-eurorack/

#PicoDexed #MIDI #StackyPi #MakerFail

I thought this would be an easy fix - using a Pi Zero-format RP2040 board in my MiniDexed EuroRack module...

... but it ended up an afternoon of frustration and unfortunately I still can't get it making sound or booting reliably.

It does look cool though :)

Anyone else have any experience of these Stacky-Pi "RP2040 on a Pi Zero shaped PCB" boards?

https://diyelectromusic.com/2025/04/12/picodexed-stackypi-minidexed-eurorack/

#PicoDexed #MIDI #StackyPi #MakerFail

I thought this would be an easy fix - using a Pi Zero-format RP2040 board in my MiniDexed EuroRack module...

... but it ended up an afternoon of frustration and unfortunately I still can't get it making sound or booting reliably.

It does look cool though :)

Anyone else have any experience of these Stacky-Pi "RP2040 on a Pi Zero shaped PCB" boards?

https://diyelectromusic.com/2025/04/12/picodexed-stackypi-minidexed-eurorack/

#PicoDexed #MIDI #StackyPi #MakerFail

I thought this would be an easy fix - using a Pi Zero-format RP2040 board in my MiniDexed EuroRack module...

... but it ended up an afternoon of frustration and unfortunately I still can't get it making sound or booting reliably.

It does look cool though :)

Anyone else have any experience of these Stacky-Pi "RP2040 on a Pi Zero shaped PCB" boards?

https://diyelectromusic.com/2025/04/12/picodexed-stackypi-minidexed-eurorack/

#PicoDexed #MIDI #StackyPi #MakerFail

picoDexed + StackyPi + MiniDexed EuroRack

Now that I have a picoDexed with a display an encoder it is very tempting to create a version of my MiniDexed EuroRack PCB for it.

But as a short diversion, there is another possibility – can I use a RP2040 that is already in a Pi Zero form factor with my existing MiniDexed EuroRack PCB Design?

Spoilers: The answer, it turns out, is no. Something is not quite letting this work for me – it’s close, but I’m just not there yet. Here is another attempt where it works: picoDexed + MiniDexed EuroRack.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

I’m aware of a couple of products that were available to put a RP2040 into a Raspberry Pi Zero form factor:

• Red Robots Pico to Zero adaptor.
• Stacky-Pi by SB Components.
• Waveshare RP2040 PiZero.
• Pico to Pi HAT.
• Update: I’ve just found the following open source design: https://github.com/bablokb/pcb-pico-pi-base – details of how I’ve used this can be found here: picoDexed + MiniDexed EuroRack

The first two were available on Tindie and other places, and as far as I can see, both appear to use the same mapping of RP2040 GPIO to RPi 40-pin header. The Red Robot board doesn’t seem to be available anymore, and whilst there are a few Stacky-Pis on Tindie, in most places it seems to be discontinued.

I don’t know anything about the Waveshare or Pico to Pi, but might grab myself one of each.

I’ll give the open source design a try (and possibly save me designing my own…).

As can be seen above, the Red Robots board is designed to take an actual Pico, whereas the Stacky-Pi is its own board with an on-board RP2040.

Mapping over to MiniDexed/RPi Zero

The following table lists the RPi GPIO connections I used in my MiniDexed board and how they map onto the RP2040 using the above boards.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP3 and 4.

The I2S interface is going to be tricky, as with a BCLK on GP28, it will be expecting LCLK on GP29 rather than the GP13 it is currently routed to on the board. Quite apart from the fact that GP29 isn’t even broken out on a Pico.

The obvious thing to explore is if the BCLK connection can be routed through to GP13 on the RPi header, or GP12 for the RP2040. That would be handy if so.

Connecting into GP13 should be possible as this pin is currently unconnected on the MiniDexed PCB. It will involve cutting a track however for GP18. The existing track and its new destination is highlighted below in blue.

Changing I2C Bus and UARTs

There is one other complication however. The picoDexed configuration has the ssd1306 connected to GP2 and GP3 which are multiplexed onto the I2C bus 1.

The adaptor configuration maps SDA/SCL onto GP20/21 which are multiplexed onto I2C bus 0. I updated the code to support changing I2C bus in addition to changing IO pins. Arguably, I should probably have supported this in the first place anyway…

A similar issue exists for the UART, but unfortunately that isn’t quite so easy to change.

I can take a similar approach to the above for the Serial MIDI link – allowing it to use either UART0 or UART1.

But that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    15
#define I2S_BCLK_PIN    12
#define I2S_LRCLK_PIN   13  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  0
#define DISPLAY_I2C_SDA  20
#define DISPLAY_I2C_SCL  21
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  3
#define ENCODER_B_PIN  4 // Not used
#define ENCODER_SW_PIN 2 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

Bringing it all together…

I decided to hack on my (already pretty hacked) prototype MiniDexed EuroRack board and cut and patch the trace to the DAC as described above.

The complication being the GPIO header pin I need to get to is under the OLED display, but I could just about do it – see below.

It doesn’t work. Unfortunately.

First of, I have to say, I’ve not found the Stacky-Pi particularly reliable. It took me ages to get it to successfully boot up into accepting a uf2 file, and once installed, I was struggling to get it to reliably boot and run.

Weirdly it seems to work best when I have a finger on the flash chip which seems to imply some board layout/grounding/stability issues to me…

From what I can see, the display, MIDI, serial debug and encoder are working fine though once it does get up and running.

But I just can’t get any reliable sound out of the thing at all. I managed sound once, but that was it. It is all quiet unreliable for me – far too unreliable to be useful.

Shame, as it actually looks really cool!

Closing Thoughts

It has been a frustrating afternoon. I’m having to leave this one here for now as there are just too many unknowns at the moment to really get to the bottom of what is going on.

I thought the Stacky-Pi would be a quick and easy fix, but haven’t ever used them before, and the fact that they are discontinued and there is very little information that I can find online about them makes me think perhaps they aren’t worth persevering with.

So I have a number of options now:

• Try a Waveshare RP2040 PiZero. As there are a lot more peripherals, I’m not sure how much what I’m doing will translate across, to be honest, it cost wise, it’s essentially the same as a Zero itself, which I know “just works”.
• Do I design my own Pico to RPI GPIO converter board to let me use that with my existing MiniDexed EuroRack design? Tempting and probably not that hard.
  • Update: I’ve since found (and ordered) https://github.com/bablokb/pcb-pico-pi-base
    • Update 2: And it works! picoDexed + MiniDexed EuroRack
• Do I attempt to do something with the Pico version of my EuroRack 6HP MCU Experimenter Module? Sounds initially easy but I suspect forcing a MiniDexed into this module eventually will hit other at the moment unforeseen issues.
• Or do I just go for it and put together a special picoDexed EuroRack module itself.

I might have one more go with the Stacky-Pi. I haven’t quite given up. I’ll have to do some research – maybe sprinkling a few capacitors around the board or some updated GND connections might help. Answers on a postcard (or in the comments) if you have any ideas.

I’ll get a Waveshare RP2040 PiZero on order, as I’ve had quite a bit of success with their own “Zero” miniature boards so far, and now I’d like it know if it would work 🙂

To be continued…

Kevin

#define #midi #minidexed #picodexed #raspberryPiPico #StackyPi

picoDexed + StackyPi + MiniDexed EuroRack

Now that I have a picoDexed with a display an encoder it is very tempting to create a version of my MiniDexed EuroRack PCB for it.

But as a short diversion, there is another possibility – can I use a RP2040 that is already in a Pi Zero form factor with my existing MiniDexed EuroRack PCB Design?

Spoilers: The answer, it turns out, is no. Something is not quite letting this work for me – it’s close, but I’m just not there yet. Here is another attempt where it works: picoDexed + MiniDexed EuroRack.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

I’m aware of a couple of products that were available to put a RP2040 into a Raspberry Pi Zero form factor:

• Red Robots Pico to Zero adaptor.
• Stacky-Pi by SB Components.
• Waveshare RP2040 PiZero.
• Pico to Pi HAT.
• Update: I’ve just found the following open source design: https://github.com/bablokb/pcb-pico-pi-base – details of how I’ve used this can be found here: picoDexed + MiniDexed EuroRack

The first two were available on Tindie and other places, and as far as I can see, both appear to use the same mapping of RP2040 GPIO to RPi 40-pin header. The Red Robot board doesn’t seem to be available anymore, and whilst there are a few Stacky-Pis on Tindie, in most places it seems to be discontinued.

I don’t know anything about the Waveshare or Pico to Pi, but might grab myself one of each.

I’ll give the open source design a try (and possibly save me designing my own…).

As can be seen above, the Red Robots board is designed to take an actual Pico, whereas the Stacky-Pi is its own board with an on-board RP2040.

Mapping over to MiniDexed/RPi Zero

The following table lists the RPi GPIO connections I used in my MiniDexed board and how they map onto the RP2040 using the above boards.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP3 and 4.

The I2S interface is going to be tricky, as with a BCLK on GP28, it will be expecting LCLK on GP29 rather than the GP13 it is currently routed to on the board. Quite apart from the fact that GP29 isn’t even broken out on a Pico.

The obvious thing to explore is if the BCLK connection can be routed through to GP13 on the RPi header, or GP12 for the RP2040. That would be handy if so.

Connecting into GP13 should be possible as this pin is currently unconnected on the MiniDexed PCB. It will involve cutting a track however for GP18. The existing track and its new destination is highlighted below in blue.

Changing I2C Bus and UARTs

There is one other complication however. The picoDexed configuration has the ssd1306 connected to GP2 and GP3 which are multiplexed onto the I2C bus 1.

The adaptor configuration maps SDA/SCL onto GP20/21 which are multiplexed onto I2C bus 0. I updated the code to support changing I2C bus in addition to changing IO pins. Arguably, I should probably have supported this in the first place anyway…

A similar issue exists for the UART, but unfortunately that isn’t quite so easy to change.

I can take a similar approach to the above for the Serial MIDI link – allowing it to use either UART0 or UART1.

But that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    15
#define I2S_BCLK_PIN    12
#define I2S_LRCLK_PIN   13  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  0
#define DISPLAY_I2C_SDA  20
#define DISPLAY_I2C_SCL  21
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  3
#define ENCODER_B_PIN  4 // Not used
#define ENCODER_SW_PIN 2 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

Bringing it all together…

I decided to hack on my (already pretty hacked) prototype MiniDexed EuroRack board and cut and patch the trace to the DAC as described above.

The complication being the GPIO header pin I need to get to is under the OLED display, but I could just about do it – see below.

It doesn’t work. Unfortunately.

First of, I have to say, I’ve not found the Stacky-Pi particularly reliable. It took me ages to get it to successfully boot up into accepting a uf2 file, and once installed, I was struggling to get it to reliably boot and run.

Weirdly it seems to work best when I have a finger on the flash chip which seems to imply some board layout/grounding/stability issues to me…

From what I can see, the display, MIDI, serial debug and encoder are working fine though once it does get up and running.

But I just can’t get any reliable sound out of the thing at all. I managed sound once, but that was it. It is all quiet unreliable for me – far too unreliable to be useful.

Shame, as it actually looks really cool!

Closing Thoughts

It has been a frustrating afternoon. I’m having to leave this one here for now as there are just too many unknowns at the moment to really get to the bottom of what is going on.

I thought the Stacky-Pi would be a quick and easy fix, but haven’t ever used them before, and the fact that they are discontinued and there is very little information that I can find online about them makes me think perhaps they aren’t worth persevering with.

So I have a number of options now:

• Try a Waveshare RP2040 PiZero. As there are a lot more peripherals, I’m not sure how much what I’m doing will translate across, to be honest, it cost wise, it’s essentially the same as a Zero itself, which I know “just works”.
• Do I design my own Pico to RPI GPIO converter board to let me use that with my existing MiniDexed EuroRack design? Tempting and probably not that hard.
  • Update: I’ve since found (and ordered) https://github.com/bablokb/pcb-pico-pi-base
    • Update 2: And it works! picoDexed + MiniDexed EuroRack
• Do I attempt to do something with the Pico version of my EuroRack 6HP MCU Experimenter Module? Sounds initially easy but I suspect forcing a MiniDexed into this module eventually will hit other at the moment unforeseen issues.
• Or do I just go for it and put together a special picoDexed EuroRack module itself.

I might have one more go with the Stacky-Pi. I haven’t quite given up. I’ll have to do some research – maybe sprinkling a few capacitors around the board or some updated GND connections might help. Answers on a postcard (or in the comments) if you have any ideas.

I’ll get a Waveshare RP2040 PiZero on order, as I’ve had quite a bit of success with their own “Zero” miniature boards so far, and now I’d like it know if it would work 🙂

To be continued…

Kevin

#define #midi #minidexed #picodexed #raspberryPiPico #StackyPi

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

Raspberry Pi Pico Synth_Dexed – Revisited

I thought it was time I took another look at my Raspberry Pi Pico Synth_Dexed. I’ve a couple of main aims with coming back to this project:

• Update the build for the latest Pico SDK and Synth_Dexed.
• See what is involved in getting it running on an RP2350.
• See if a simple I2C display can be added and possibly an encoder or other controls.
• Actually document the architecture!

The current hardware is described in detail in Raspberry Pi Pico Synth_Dexed? – Part 5 and supports serial and USB MIDI, I2S or PWM audio output, voice selection (over MIDI) and up to 16 note polyphony if using an overclocked (to 250MHz) RP2040 based Pico and lower sample rate.

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Core Updates

Since I first put the PicoDexed together, V2 of the Raspberry Pi Pico SDK has been released. There aren’t many changes, but as noted by okyeron, an additional include file is required in main.c.

But as I was updating things, I also found a number of changes to Synth_Dexed too that broke the build. The most irritating of which was including a definition of DEXED_SAMPLE_RATE which now clashes with my own definition. I’ve gone through and changed a number of DEXED_ definitions to PICODEXED_ to avoid that in the future too.

A few other changes have also been introduced, which have necessitated an update to my diff file for Synth_Dexed/src/Dexed.cpp.

But on the whole, there was nothing major in the end. This has all been uploaded to GitHub as a v0.02.

PicoDexed Architecture

I had to go back and attempt to reacquaint myself with what I’d done last time, so as part of that I’ve drawn out a rough architecture diagram as shown below.

This expands on the design notes I’ve already written up in Part 3.

The key principles are as follows:

• Core 0 is used for all IO and is largely driven by MIDI events received over serial or USB.
• Core 1 is used for all audio processing and is run continually using the Pico’s audio driver (either I2S or PWM) to use a callback function to fill a buffer with samples to be played. The samples come from Synth_Dexed.
• All synthesis parameters (including keyup/down events for playing notes) is handled within picoDexed in response to the MIDIMessageHandler receiving MIDI messages from the MIDIParser.

Looking at this, I should be able to include some additional IO handling in the main Process loop of picoDexed that runs on core 0.

I2C Displays

Hunting around for libraries to support an SSD1306 display with the Pico SDK, so far I’ve found four options:

• pico-examples – has an example app that can put simple text or an image on an SSD1306. Note this isn’t created as a library that could be used however – it is all in a single example.
• sharkis/pico-ssd1306 – what appears to be a couple of files that can be included in your own code. It is pretty low-level with minimal options for any text or graphics and there is no documentation or licensing information as far as I can see.
• daschr/pico-ssd1306 – a higher level interface that provides basic graphics primitives and simple font support. MIT license. Again a couple of files to include in your project.
• Harbys/pico-ssd1306 – the most complete support I’ve found so far. What I’d consider to be a “proper” library with what seems to be good documentation and a wide range of usage options.

Whilst tempted to go with the “proper” library, it might be a little over the top for what I really need right now, and I don’t want to include an additional third-party GitHub link in my code at this time.

So I’m going to try daschr’s pico-ssd1306 as I can just grab the files, maintain the license information, and include it directly into my code.

To include this code in the build I’ve done the following:

• Created a libs area and copied in the files: ssd1306.c, ssd1306.h, font.h
• Added the c file to the CMakeLists.txt file.
• Added libs to the include path.
• Added hardware_i2c as an included library.

My new CMakeLists.txt file now looks as follows.

cmake_minimum_required(VERSION 3.13)
include(pico_sdk_import.cmake)
include(pico_extras_import.cmake)
project(picodexed)
pico_sdk_init()
add_executable(picodexed)

target_sources(picodexed PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}/src/main.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/pico_perf.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/mididevice.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/picodexed.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/serialmidi.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/sounddevice.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/usbmidi.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/usbtask.c
    ${CMAKE_CURRENT_LIST_DIR}/src/usb_descriptors.c
    ${CMAKE_CURRENT_LIST_DIR}/libs/ssd1306.c
)

add_subdirectory(synth_dexed)

target_include_directories(picodexed PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}/src
    ${CMAKE_CURRENT_LIST_DIR}/libs
    ${CMAKE_CURRENT_LIST_DIR}/synth_dexed/Synth_Dexed/src
    ${CMAKE_CURRENT_LIST_DIR}/synth_dexed
)

target_link_libraries(picodexed PUBLIC synth_dexed pico_stdlib pico_multicore tinyusb_device tinyusb_board pico_audio_i2s pico_audio_pwm hardware_i2c)

target_compile_definitions(picodexed PRIVATE
    PICO_AUDIO_I2S_MONO_OUTPUT=1
    PICO_AUDIO_I2S_MONO_INPUT=1
USE_AUDIO_I2S=1
    USE_AUDIO_PWM=1
)

pico_add_extra_outputs(picodexed)

The most basic usage is as follows:

static ssd1306_t disp;

i2c_init(i2c1, 400000);
gpio_set_function(2, GPIO_FUNC_I2C);
gpio_set_function(3, GPIO_FUNC_I2C);
gpio_pull_up(2);
gpio_pull_up(3);
    
disp.external_vcc=false;
ssd1306_init(&disp, 128, 32, 0x3c, i2c1);
ssd1306_clear(&disp);

ssd1306_draw_string(&disp, 8, 8, 2, "picoDexed");
ssd1306_show(&disp);

This initialises I2C bus 1 on GPIO 2 and 3 for device at address 0x3C.

Rotary Encoder

Taking the same approach with a rotary encoder, I’ve so far found the following as candidates for use:

• matteomeneghetti/pico-quadrature-encoder – provides a C++ API and uses PIO on two consecutive GPIO pins.
• jamon/pi-pico-pio-quadrature-encoder – simpler, and slightly lower level, PIO implementation only.
• GitJer/Some_RPI-Pico_stuff – example code of how to drive an encoder, again using PIO.
• pimoroni/pimoroni-pico/tree/encoder-pio/drivers/encoder-pio – Pimoroni driver for encoders using PIO.

The first is a more complete library, the second more “bare bones”. The last seems more of an example rather than a reusable object.

I was initially inclined towards the second which seemed likely to be easier to integrate into my own code, but as it was pretty low level and required a fair bit of glue around it.

Eventually I actually opted for something based on “GitJer”‘s example. There is a full explanation of how the code works here: https://github.com/GitJer/Some_RPI-Pico_stuff/tree/main/Rotary_encoder

I’ve had to change how voices are selected slightly to ensure the display, MIDI BANKSEL/PC and encoder can stay reasonably intuitive.

I’ve used the following logic:

• IF BANKSEL/PC both received then treat as Bank=0-7; Voice=0-31.
• IF PC on its own, then treat as Voice=0-127 across Banks 0 to 3.
• IF UI changes voices, then treat as Bank=0-7; Voice=0-31.

In each case the display shows the current bank and voice number in 0-7/0-31 format.

So just to be clear, MIDI Program Select on its own now (so 0..128) will always only select from the first four banks of 32 voices each. This is a change from the previous version which would allow PC to select based on the currently selected banks and the three following banks.

The encoder will automatically switch from one bank to the next, and wrap around at either end, so is able to select all voices across all installed banks.

Raspberry Pi Pico PIO Use

One issue to keep an eye on is PIO state-machine use.

I’m still getting my head around PIO (I’ve just not really devoted any significant time to figuring it out yet) but as I understand things, each PIO instance has a 32 instruction memory which is shared across four state machines.

So if there are several programmes to run and they all combined fit in the 32 instruction memory, then they could all use the same PIO instance whilst being attached to different state machines.

But if there are two vastly different programs to be running then it may be that they have to be split across the two PIO instances. But there can still be up to four instance of each program, one for each state machine in a PIO instance.

The I2S audio driver uses PIO to implement I2S. As far as I can see which PIO to use is determined by the following definitions:

PICO_AUDIO_PIO
PICO_AUDIO_I2S_PIO
PICO_AUDIO_PWM_PIO

If PICO_AUDIO_PIO is not defined then I2S_PIO and PWM_PIO are set to 0, which is then turned into “pio0” via the following in i2s_audio.c:

#define audio_pio __CONCAT(pio, PICO_AUDIO_I2S_PIO)

Which later on then uses the following to claim a free statemachine as part of audio_i2s_setup():

pio_sm_claim(audio_pio, sm);

I don’t know for certain, but it appears to me that the I2S PIO program is quite complete and so highly likely to fill the 32 word instruction memory.

On that basis, then the rotary encoder PIO program will have to use PIO instance 1 for its own code.

Summary of PIO use:

Update GPIO Usage

Expanding now on the table from Part 5, updated with the above, now gives the following GPIO usage map.

PicoDexed on a Breadboard

Closing Thoughts

I feel like this is really starting to get interesting now. I have a few choices to make now – do I attempt to go for a more complete menu system similar to MiniDexed, or do I stay with MIDI control and basic voice selection as it is now?

Future enhancements might include:

• It will be interesting to see what supporting the RP2350 could bring.
• It would be useful to be able to set the MIDI channel – either in menu, or perhaps more ideally in hardware somehow (e.g. switches, resistors on an ADC, etc).
• Act as a USB host for USB MIDI devices.

In the video I’ve hooked it up to a serial MIDI controller and am just cycling through some of the voices showing how the UI works.

Kevin

#dac #dx7 #i2c #midi #picodexed #raspberryPiPico #rotaryEncoder #synthDexed

Raspberry Pi Pico Synth_Dexed – Revisited

I thought it was time I took another look at my Raspberry Pi Pico Synth_Dexed. I’ve a couple of main aims with coming back to this project:

• Update the build for the latest Pico SDK and Synth_Dexed.
• See what is involved in getting it running on an RP2350.
• See if a simple I2C display can be added and possibly an encoder or other controls.
• Actually document the architecture!

The current hardware is described in detail in Raspberry Pi Pico Synth_Dexed? – Part 5 and supports serial and USB MIDI, I2S or PWM audio output, voice selection (over MIDI) and up to 16 note polyphony if using an overclocked (to 250MHz) RP2040 based Pico and lower sample rate.

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Core Updates

Since I first put the PicoDexed together, V2 of the Raspberry Pi Pico SDK has been released. There aren’t many changes, but as noted by okyeron, an additional include file is required in main.c.

But as I was updating things, I also found a number of changes to Synth_Dexed too that broke the build. The most irritating of which was including a definition of DEXED_SAMPLE_RATE which now clashes with my own definition. I’ve gone through and changed a number of DEXED_ definitions to PICODEXED_ to avoid that in the future too.

A few other changes have also been introduced, which have necessitated an update to my diff file for Synth_Dexed/src/Dexed.cpp.

But on the whole, there was nothing major in the end. This has all been uploaded to GitHub as a v0.02.

PicoDexed Architecture

I had to go back and attempt to reacquaint myself with what I’d done last time, so as part of that I’ve drawn out a rough architecture diagram as shown below.

This expands on the design notes I’ve already written up in Part 3.

The key principles are as follows:

• Core 0 is used for all IO and is largely driven by MIDI events received over serial or USB.
• Core 1 is used for all audio processing and is run continually using the Pico’s audio driver (either I2S or PWM) to use a callback function to fill a buffer with samples to be played. The samples come from Synth_Dexed.
• All synthesis parameters (including keyup/down events for playing notes) is handled within picoDexed in response to the MIDIMessageHandler receiving MIDI messages from the MIDIParser.

Looking at this, I should be able to include some additional IO handling in the main Process loop of picoDexed that runs on core 0.

I2C Displays

Hunting around for libraries to support an SSD1306 display with the Pico SDK, so far I’ve found four options:

• pico-examples – has an example app that can put simple text or an image on an SSD1306. Note this isn’t created as a library that could be used however – it is all in a single example.
• sharkis/pico-ssd1306 – what appears to be a couple of files that can be included in your own code. It is pretty low-level with minimal options for any text or graphics and there is no documentation or licensing information as far as I can see.
• daschr/pico-ssd1306 – a higher level interface that provides basic graphics primitives and simple font support. MIT license. Again a couple of files to include in your project.
• Harbys/pico-ssd1306 – the most complete support I’ve found so far. What I’d consider to be a “proper” library with what seems to be good documentation and a wide range of usage options.

Whilst tempted to go with the “proper” library, it might be a little over the top for what I really need right now, and I don’t want to include an additional third-party GitHub link in my code at this time.

So I’m going to try daschr’s pico-ssd1306 as I can just grab the files, maintain the license information, and include it directly into my code.

To include this code in the build I’ve done the following:

• Created a libs area and copied in the files: ssd1306.c, ssd1306.h, font.h
• Added the c file to the CMakeLists.txt file.
• Added libs to the include path.
• Added hardware_i2c as an included library.

My new CMakeLists.txt file now looks as follows.

cmake_minimum_required(VERSION 3.13)
include(pico_sdk_import.cmake)
include(pico_extras_import.cmake)
project(picodexed)
pico_sdk_init()
add_executable(picodexed)

target_sources(picodexed PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}/src/main.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/pico_perf.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/mididevice.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/picodexed.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/serialmidi.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/sounddevice.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/usbmidi.cpp
    ${CMAKE_CURRENT_LIST_DIR}/src/usbtask.c
    ${CMAKE_CURRENT_LIST_DIR}/src/usb_descriptors.c
    ${CMAKE_CURRENT_LIST_DIR}/libs/ssd1306.c
)

add_subdirectory(synth_dexed)

target_include_directories(picodexed PUBLIC
    ${CMAKE_CURRENT_LIST_DIR}/src
    ${CMAKE_CURRENT_LIST_DIR}/libs
    ${CMAKE_CURRENT_LIST_DIR}/synth_dexed/Synth_Dexed/src
    ${CMAKE_CURRENT_LIST_DIR}/synth_dexed
)

target_link_libraries(picodexed PUBLIC synth_dexed pico_stdlib pico_multicore tinyusb_device tinyusb_board pico_audio_i2s pico_audio_pwm hardware_i2c)

target_compile_definitions(picodexed PRIVATE
    PICO_AUDIO_I2S_MONO_OUTPUT=1
    PICO_AUDIO_I2S_MONO_INPUT=1
USE_AUDIO_I2S=1
    USE_AUDIO_PWM=1
)

pico_add_extra_outputs(picodexed)

The most basic usage is as follows:

static ssd1306_t disp;

i2c_init(i2c1, 400000);
gpio_set_function(2, GPIO_FUNC_I2C);
gpio_set_function(3, GPIO_FUNC_I2C);
gpio_pull_up(2);
gpio_pull_up(3);
    
disp.external_vcc=false;
ssd1306_init(&disp, 128, 32, 0x3c, i2c1);
ssd1306_clear(&disp);

ssd1306_draw_string(&disp, 8, 8, 2, "picoDexed");
ssd1306_show(&disp);

This initialises I2C bus 1 on GPIO 2 and 3 for device at address 0x3C.

Rotary Encoder

Taking the same approach with a rotary encoder, I’ve so far found the following as candidates for use:

• matteomeneghetti/pico-quadrature-encoder – provides a C++ API and uses PIO on two consecutive GPIO pins.
• jamon/pi-pico-pio-quadrature-encoder – simpler, and slightly lower level, PIO implementation only.
• GitJer/Some_RPI-Pico_stuff – example code of how to drive an encoder, again using PIO.
• pimoroni/pimoroni-pico/tree/encoder-pio/drivers/encoder-pio – Pimoroni driver for encoders using PIO.

The first is a more complete library, the second more “bare bones”. The last seems more of an example rather than a reusable object.

I was initially inclined towards the second which seemed likely to be easier to integrate into my own code, but as it was pretty low level and required a fair bit of glue around it.

Eventually I actually opted for something based on “GitJer”‘s example. There is a full explanation of how the code works here: https://github.com/GitJer/Some_RPI-Pico_stuff/tree/main/Rotary_encoder

I’ve had to change how voices are selected slightly to ensure the display, MIDI BANKSEL/PC and encoder can stay reasonably intuitive.

I’ve used the following logic:

• IF BANKSEL/PC both received then treat as Bank=0-7; Voice=0-31.
• IF PC on its own, then treat as Voice=0-127 across Banks 0 to 3.
• IF UI changes voices, then treat as Bank=0-7; Voice=0-31.

In each case the display shows the current bank and voice number in 0-7/0-31 format.

So just to be clear, MIDI Program Select on its own now (so 0..128) will always only select from the first four banks of 32 voices each. This is a change from the previous version which would allow PC to select based on the currently selected banks and the three following banks.

The encoder will automatically switch from one bank to the next, and wrap around at either end, so is able to select all voices across all installed banks.

Raspberry Pi Pico PIO Use

One issue to keep an eye on is PIO state-machine use.

I’m still getting my head around PIO (I’ve just not really devoted any significant time to figuring it out yet) but as I understand things, each PIO instance has a 32 instruction memory which is shared across four state machines.

So if there are several programmes to run and they all combined fit in the 32 instruction memory, then they could all use the same PIO instance whilst being attached to different state machines.

But if there are two vastly different programs to be running then it may be that they have to be split across the two PIO instances. But there can still be up to four instance of each program, one for each state machine in a PIO instance.

The I2S audio driver uses PIO to implement I2S. As far as I can see which PIO to use is determined by the following definitions:

PICO_AUDIO_PIO
PICO_AUDIO_I2S_PIO
PICO_AUDIO_PWM_PIO

If PICO_AUDIO_PIO is not defined then I2S_PIO and PWM_PIO are set to 0, which is then turned into “pio0” via the following in i2s_audio.c:

#define audio_pio __CONCAT(pio, PICO_AUDIO_I2S_PIO)

Which later on then uses the following to claim a free statemachine as part of audio_i2s_setup():

pio_sm_claim(audio_pio, sm);

I don’t know for certain, but it appears to me that the I2S PIO program is quite complete and so highly likely to fill the 32 word instruction memory.

On that basis, then the rotary encoder PIO program will have to use PIO instance 1 for its own code.

Summary of PIO use:

Update GPIO Usage

Expanding now on the table from Part 5, updated with the above, now gives the following GPIO usage map.

PicoDexed on a Breadboard

Closing Thoughts

I feel like this is really starting to get interesting now. I have a few choices to make now – do I attempt to go for a more complete menu system similar to MiniDexed, or do I stay with MIDI control and basic voice selection as it is now?

Future enhancements might include:

• It will be interesting to see what supporting the RP2350 could bring.
• It would be useful to be able to set the MIDI channel – either in menu, or perhaps more ideally in hardware somehow (e.g. switches, resistors on an ADC, etc).
• Act as a USB host for USB MIDI devices.

In the video I’ve hooked it up to a serial MIDI controller and am just cycling through some of the voices showing how the UI works.

Kevin

#dac #dx7 #i2c #midi #picodexed #raspberryPiPico #rotaryEncoder #synthDexed

PicoDexed UI and Voice Demonstration

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

PicoDexed UI and Voice Demonstration

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

PicoDexed UI and Voice Demonstration

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

PicoDexed UI and Voice Demonstration

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

PicoDexed UI and Voice Demonstration

https://makertube.net/w/tY1u9qFz85NprRYPmtdvEj

My picoDexed now has very rudimentary ssd1306 OLED display support.

All it shows is voice name at the moment.

#picodexed

My picoDexed now has very rudimentary ssd1306 OLED display support.

All it shows is voice name at the moment.

#picodexed

My picoDexed now has very rudimentary ssd1306 OLED display support.

All it shows is voice name at the moment.

#picodexed

My picoDexed now has very rudimentary ssd1306 OLED display support.

All it shows is voice name at the moment.

#picodexed

My picoDexed now has very rudimentary ssd1306 OLED display support.

All it shows is voice name at the moment.

#picodexed

@[email protected] @[email protected] Oh, I forgot it was #MakersHour

Oh well. I'll perhaps sit and wander through the responses in the morning :)

(I was buried in the code for my #picoDexed after not really touching it for a year)

@[email protected] @[email protected] Oh, I forgot it was #MakersHour

Oh well. I'll perhaps sit and wander through the responses in the morning :)

(I was buried in the code for my #picoDexed after not really touching it for a year)

@[email protected] @[email protected] Oh, I forgot it was #MakersHour

Oh well. I'll perhaps sit and wander through the responses in the morning :)

(I was buried in the code for my #picoDexed after not really touching it for a year)

@[email protected] @[email protected] Oh, I forgot it was #MakersHour

Oh well. I'll perhaps sit and wander through the responses in the morning :)

(I was buried in the code for my #picoDexed after not really touching it for a year)

@[email protected] @[email protected] Oh, I forgot it was #MakersHour

Oh well. I'll perhaps sit and wander through the responses in the morning :)

(I was buried in the code for my #picoDexed after not really touching it for a year)

One last thing I definitely wanted for my PicoDexed was the option for PWM output. This post is by way of a short coda detailing how to do PWM on a Raspberry Pi Pico.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 5 details different options for building hardware to run PicoDexed.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

PWM Output Circuit

The official “Hardware Design with the RP2040” guide includes a PWM circuit as follows (see section 3.4.1).

Several others have used a slightly simplified version of this circuit, essentially omitting the buffer stage (U3) and perhaps only implementing a single channel. See for example – Raspberry Pi Pico PWM Audio Output Circuit (Jeremy S. Cook) and “pico-pwm-audio” (Robin Grosset).

An alternative is this circuit used by Tod Kurt for his Circuitpython-tricks – “The output circuitry to get line-out levels is a simple 10:1 voltage-divider and a 1uF capacitor to recenter the signal around 0 volts”:

I’m going with Jeremy S. Cook and Robin Grosset and using the following (diagram from here):

This is my solderless breadboard version of the above:

Raspberry Pi Pico Audio PWM

There appears to be two ways of getting audio style PWM signals out of a Pico:

• Use the PWM peripherals.
• Use the PIO.

It would appear that Raspberry Pi’s pico_audio library in the pico-extras repository uses PIO. I haven’t found a clear explanation as to why the built-in PWM peripherals haven’t been used.

Here are some other resources that show alternative descriptions of PWM for audion on a Pico:

• Greg Chadwick’s “Playing with the Pico” series: Part 3 – “PWM Audio” – walks through the basics of using PWM and DMA.
• Ben Everard’s PIO tutorial in Hackspace magazine – a simplified PIO PWM driver.
• chipfire’s rppico-pwm-sound synthesizer – uses the Pico SDK and DMA to the PWM peripheral.
• Pico Playground sine wave example – uses the Pico Extra’s pico_audio library and PIO PWM.
• Robin Grosset’s Raspberry Pi Pico PWM Audio Project – uses the Pico SDK and the PWM peripheral.

As I’m using the pico_audio library for I2S audio, I’m going to use the default PWM pico_audio library too.

To initialise the pico_audio PWM library requires the following:

const audio_pwm_channel_config_t config =
{
  .core = {
    .base_pin = base_pin,
    .dma_channel = dma_ch,
    .pio_sm = pio_sm
  },
  .pattern = 3,
};

const struct audio_format *output_format;
output_format = audio_pwm_setup(pAudioFormat, -1, &config);    
bool status = audio_pwm_default_connect(pBufferPool, false);
audio_pwm_set_enabled(true);

Everything else is the same as for I2S.

At present, overclocking the Pico causes the PWM frequencies to be messed up, so for now the recommended configuration is 8-note polyphony, 24000 sample rate and no overclocking.

The build uses GPIO 20 for PWM.

Closing Thoughts

I’ve uploaded a prototype PWM UF2 file to GitHub too now in case anyone wants to give that a go too: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/19/raspberry-pi-pico-synth_dexed-part-6/

#dx7 #midi #picodexed #pwm #raspberryPiPico #usbMidi

One last thing I definitely wanted for my PicoDexed was the option for PWM output. This post is by way of a short coda detailing how to do PWM on a Raspberry Pi Pico.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 5 details different options for building hardware to run PicoDexed.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

PWM Output Circuit

The official “Hardware Design with the RP2040” guide includes a PWM circuit as follows (see section 3.4.1).

Several others have used a slightly simplified version of this circuit, essentially omitting the buffer stage (U3) and perhaps only implementing a single channel. See for example – Raspberry Pi Pico PWM Audio Output Circuit (Jeremy S. Cook) and “pico-pwm-audio” (Robin Grosset).

An alternative is this circuit used by Tod Kurt for his Circuitpython-tricks – “The output circuitry to get line-out levels is a simple 10:1 voltage-divider and a 1uF capacitor to recenter the signal around 0 volts”:

I’m going with Jeremy S. Cook and Robin Grosset and using the following (diagram from here):

This is my solderless breadboard version of the above:

Raspberry Pi Pico Audio PWM

There appears to be two ways of getting audio style PWM signals out of a Pico:

• Use the PWM peripherals.
• Use the PIO.

It would appear that Raspberry Pi’s pico_audio library in the pico-extras repository uses PIO. I haven’t found a clear explanation as to why the built-in PWM peripherals haven’t been used.

Here are some other resources that show alternative descriptions of PWM for audion on a Pico:

• Greg Chadwick’s “Playing with the Pico” series: Part 3 – “PWM Audio” – walks through the basics of using PWM and DMA.
• Ben Everard’s PIO tutorial in Hackspace magazine – a simplified PIO PWM driver.
• chipfire’s rppico-pwm-sound synthesizer – uses the Pico SDK and DMA to the PWM peripheral.
• Pico Playground sine wave example – uses the Pico Extra’s pico_audio library and PIO PWM.
• Robin Grosset’s Raspberry Pi Pico PWM Audio Project – uses the Pico SDK and the PWM peripheral.

As I’m using the pico_audio library for I2S audio, I’m going to use the default PWM pico_audio library too.

To initialise the pico_audio PWM library requires the following:

const audio_pwm_channel_config_t config =
{
  .core = {
    .base_pin = base_pin,
    .dma_channel = dma_ch,
    .pio_sm = pio_sm
  },
  .pattern = 3,
};

const struct audio_format *output_format;
output_format = audio_pwm_setup(pAudioFormat, -1, &config);    
bool status = audio_pwm_default_connect(pBufferPool, false);
audio_pwm_set_enabled(true);

Everything else is the same as for I2S.

At present, overclocking the Pico causes the PWM frequencies to be messed up, so for now the recommended configuration is 8-note polyphony, 24000 sample rate and no overclocking.

The build uses GPIO 20 for PWM.

Closing Thoughts

I’ve uploaded a prototype PWM UF2 file to GitHub too now in case anyone wants to give that a go too: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/19/raspberry-pi-pico-synth_dexed-part-6/

#dx7 #midi #picodexed #pwm #raspberryPiPico #usbMidi

One last thing I definitely wanted for my PicoDexed was the option for PWM output. This post is by way of a short coda detailing how to do PWM on a Raspberry Pi Pico.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 5 details different options for building hardware to run PicoDexed.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

PWM Output Circuit

The official “Hardware Design with the RP2040” guide includes a PWM circuit as follows (see section 3.4.1).

Several others have used a slightly simplified version of this circuit, essentially omitting the buffer stage (U3) and perhaps only implementing a single channel. See for example – Raspberry Pi Pico PWM Audio Output Circuit (Jeremy S. Cook) and “pico-pwm-audio” (Robin Grosset).

An alternative is this circuit used by Tod Kurt for his Circuitpython-tricks – “The output circuitry to get line-out levels is a simple 10:1 voltage-divider and a 1uF capacitor to recenter the signal around 0 volts”:

I’m going with Jeremy S. Cook and Robin Grosset and using the following (diagram from here):

This is my solderless breadboard version of the above:

Raspberry Pi Pico Audio PWM

There appears to be two ways of getting audio style PWM signals out of a Pico:

• Use the PWM peripherals.
• Use the PIO.

It would appear that Raspberry Pi’s pico_audio library in the pico-extras repository uses PIO. I haven’t found a clear explanation as to why the built-in PWM peripherals haven’t been used.

Here are some other resources that show alternative descriptions of PWM for audion on a Pico:

• Greg Chadwick’s “Playing with the Pico” series: Part 3 – “PWM Audio” – walks through the basics of using PWM and DMA.
• Ben Everard’s PIO tutorial in Hackspace magazine – a simplified PIO PWM driver.
• chipfire’s rppico-pwm-sound synthesizer – uses the Pico SDK and DMA to the PWM peripheral.
• Pico Playground sine wave example – uses the Pico Extra’s pico_audio library and PIO PWM.
• Robin Grosset’s Raspberry Pi Pico PWM Audio Project – uses the Pico SDK and the PWM peripheral.

As I’m using the pico_audio library for I2S audio, I’m going to use the default PWM pico_audio library too.

To initialise the pico_audio PWM library requires the following:

const audio_pwm_channel_config_t config =
{
  .core = {
    .base_pin = base_pin,
    .dma_channel = dma_ch,
    .pio_sm = pio_sm
  },
  .pattern = 3,
};

const struct audio_format *output_format;
output_format = audio_pwm_setup(pAudioFormat, -1, &config);    
bool status = audio_pwm_default_connect(pBufferPool, false);
audio_pwm_set_enabled(true);

Everything else is the same as for I2S.

At present, overclocking the Pico causes the PWM frequencies to be messed up, so for now the recommended configuration is 8-note polyphony, 24000 sample rate and no overclocking.

The build uses GPIO 20 for PWM.

Closing Thoughts

I’ve uploaded a prototype PWM UF2 file to GitHub too now in case anyone wants to give that a go too: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/19/raspberry-pi-pico-synth_dexed-part-6/

#dx7 #midi #picodexed #pwm #raspberryPiPico #usbMidi

Ok, ignore my last comment about Part 4 being my last post on this topic. I thought I ought to pull together all the hardware notes on how to build one as I haven’t really written that down anywhere.

So this shows the connections required between the Pico, I2S DAC and MIDI IN.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 6 includes details of how to use PWM output.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

The Circuit

The easiest way to get a PicoDexed up and running for me, is to use one of my MIDI Proto PCBs and the Pimoroni I2S Audio Pack. This does not allow for any debug output.

Note the jumpers are set to use UART 1 on GP4 and GP5.

If only USB-MIDI is required, then the Audio Pack can be plugged directly onto the back of the Pico, in which case simply plugging into the Pico via USB to a computer and connected up the audio out will work.

It is also possible to use any 3V3 compatible MIDI module. The following shows the use of a Pimoroni “dual expander” with a MIDI module connected to GND, 3V3_OUT and GP5 (RX1).

This has the advantage that all other GPIO pins are available, so it is relatively straight forward to include a debug link via GP0/GP1 for UART 0.

A Solderless Breadboard Circuit

Of course it is also possible to build everything on a solderless breadboard.

The diagram below shows how a cheap GY-PCM5102 module can be used as the I2S DAC and a serial MIDI interface (optional) can be built using my standard 3V3 MIDI IN circuit.

This also allows for some debug output using UART 0 (GP0/GP1).

Things to note:

• The PCM5102 board will have to have solder jumpers on the rear set as follows: 1=L, 2=L, 3=H, 4=L. Sometimes these come preconfigured with solder bridges, sometimes with zero-ohm SMT resistors, and sometimes with no connection made at all. More details here.
• I always get pins 4 and 5 mixed up on MIDI DIN sockets. Here is my MIDI Connections Cheat Sheet which may help.

Pico GPIO Usage

The following GPIO pins are in use or allocated:

PWM Audio Output

It is possible to use PWM audio output on the Pico. Full details can be found here: Raspberry Pi Pico Synth_Dexed? – Part 6.

But this is a lot more limited than I2S and the quality is a lot poorer too. As a DAC can be obtained quite cheaply, the use of an I2S DAC is strongly recommended.

Closing Thoughts

I’ve uploaded a prototype UF2 file to GitHub in case anyone wants to give it a go: everything can be found here: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/18/raspberry-pi-pico-synth_dexed-part-5/

#dx7 #midi #pcm5102 #picodexed #raspberryPiPico #usbMidi

Ok, ignore my last comment about Part 4 being my last post on this topic. I thought I ought to pull together all the hardware notes on how to build one as I haven’t really written that down anywhere.

So this shows the connections required between the Pico, I2S DAC and MIDI IN.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 6 includes details of how to use PWM output.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

The Circuit

The easiest way to get a PicoDexed up and running for me, is to use one of my MIDI Proto PCBs and the Pimoroni I2S Audio Pack. This does not allow for any debug output.

Note the jumpers are set to use UART 1 on GP4 and GP5.

If only USB-MIDI is required, then the Audio Pack can be plugged directly onto the back of the Pico, in which case simply plugging into the Pico via USB to a computer and connected up the audio out will work.

It is also possible to use any 3V3 compatible MIDI module. The following shows the use of a Pimoroni “dual expander” with a MIDI module connected to GND, 3V3_OUT and GP5 (RX1).

This has the advantage that all other GPIO pins are available, so it is relatively straight forward to include a debug link via GP0/GP1 for UART 0.

A Solderless Breadboard Circuit

Of course it is also possible to build everything on a solderless breadboard.

The diagram below shows how a cheap GY-PCM5102 module can be used as the I2S DAC and a serial MIDI interface (optional) can be built using my standard 3V3 MIDI IN circuit.

This also allows for some debug output using UART 0 (GP0/GP1).

Things to note:

• The PCM5102 board will have to have solder jumpers on the rear set as follows: 1=L, 2=L, 3=H, 4=L. Sometimes these come preconfigured with solder bridges, sometimes with zero-ohm SMT resistors, and sometimes with no connection made at all. More details here.
• I always get pins 4 and 5 mixed up on MIDI DIN sockets. Here is my MIDI Connections Cheat Sheet which may help.

Pico GPIO Usage

The following GPIO pins are in use or allocated:

PWM Audio Output

It is possible to use PWM audio output on the Pico. Full details can be found here: Raspberry Pi Pico Synth_Dexed? – Part 6.

But this is a lot more limited than I2S and the quality is a lot poorer too. As a DAC can be obtained quite cheaply, the use of an I2S DAC is strongly recommended.

Closing Thoughts

I’ve uploaded a prototype UF2 file to GitHub in case anyone wants to give it a go: everything can be found here: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/18/raspberry-pi-pico-synth_dexed-part-5/

#dx7 #midi #pcm5102 #picodexed #raspberryPiPico #usbMidi

Ok, ignore my last comment about Part 4 being my last post on this topic. I thought I ought to pull together all the hardware notes on how to build one as I haven’t really written that down anywhere.

So this shows the connections required between the Pico, I2S DAC and MIDI IN.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 6 includes details of how to use PWM output.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

The Circuit

The easiest way to get a PicoDexed up and running for me, is to use one of my MIDI Proto PCBs and the Pimoroni I2S Audio Pack. This does not allow for any debug output.

Note the jumpers are set to use UART 1 on GP4 and GP5.

If only USB-MIDI is required, then the Audio Pack can be plugged directly onto the back of the Pico, in which case simply plugging into the Pico via USB to a computer and connected up the audio out will work.

It is also possible to use any 3V3 compatible MIDI module. The following shows the use of a Pimoroni “dual expander” with a MIDI module connected to GND, 3V3_OUT and GP5 (RX1).

This has the advantage that all other GPIO pins are available, so it is relatively straight forward to include a debug link via GP0/GP1 for UART 0.

A Solderless Breadboard Circuit

Of course it is also possible to build everything on a solderless breadboard.

The diagram below shows how a cheap GY-PCM5102 module can be used as the I2S DAC and a serial MIDI interface (optional) can be built using my standard 3V3 MIDI IN circuit.

This also allows for some debug output using UART 0 (GP0/GP1).

Things to note:

• The PCM5102 board will have to have solder jumpers on the rear set as follows: 1=L, 2=L, 3=H, 4=L. Sometimes these come preconfigured with solder bridges, sometimes with zero-ohm SMT resistors, and sometimes with no connection made at all. More details here.
• I always get pins 4 and 5 mixed up on MIDI DIN sockets. Here is my MIDI Connections Cheat Sheet which may help.

Pico GPIO Usage

The following GPIO pins are in use or allocated:

PWM Audio Output

It is possible to use PWM audio output on the Pico. Full details can be found here: Raspberry Pi Pico Synth_Dexed? – Part 6.

But this is a lot more limited than I2S and the quality is a lot poorer too. As a DAC can be obtained quite cheaply, the use of an I2S DAC is strongly recommended.

Closing Thoughts

I’ve uploaded a prototype UF2 file to GitHub in case anyone wants to give it a go: everything can be found here: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/18/raspberry-pi-pico-synth_dexed-part-5/

#dx7 #midi #pcm5102 #picodexed #raspberryPiPico #usbMidi

I was going to leave things at Part 3 blog-wise, and just get on with filling in the gaps in code now, but I’ve come back to add a few more notes. But this is likely to be the final part now.

Recall so far, I have:

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.

This is building on the last part and includes notes on how I’ve implemented the following:

• Fuller MIDI support, including control change, program change and pitch bend messages.
• Voice and voice banks, selectable over MIDI.
• MIDI SysEx messages for voice parameters.
• USB MIDI device support.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

MIDI Support

I’m not going to walk through all the details of how I’ve added MIDI but suffice to say that once again the implementation owes a lot to MiniDexed and the Arduino MIDI Library.

At the time of writing the following are all supported as they were already supported in Synth_Dexed, so I just needed to glue the bits together.

Channel Voice Messages (only channel 1 at present)

Channel Control Change Messages

* There is a bug with the master tuning. It ought to accept -99 to 99 I believe, but only 0..99 will actually register and there is no way to send -99 via MIDI at the moment. I need to read up on what is going on here and what it ought to do!

** The Mono Mode parameter has the option for specifying how many of the playable voices can be dedicated to mono mode (at least I think that is what it is saying). I only support a value of 0 which I believe is meant to mean “all available voices”.

System Messages

System Exclusive Messages

Any valid Yamaha (DX) system exclusive messages are passed straight into Synth_Dexed. A Yamaha (DX) message has the following format (see the “DX7IIFD/D Supplemental Booklet: Advanced MIDI Data and Charts”):

F0 - start SysEx message
43 - Yamaha manufacturer ID
sd - s=substatus (command class:0,1,2); d=device ID (0..F)
.. data ..
F7 - end SysEx message

The device ID can be set using the UI on a real DX7 to a value between 1 and 16, which becomes a value between 0 and 15 (0..F) as part of the SysEx message (see “DX7IIFD/D Supplemental Booklet: Advanced MIDI Applications, Section 8”). It is a Systems Exclusive value analogous to the MIDI channel for regular channel messages.

There are a range of Sys Ex parameter settings that have been passed onto Synth_Dexed as follows:

At this stage, all of the MIDI support is on a “it’s probably something like this” basis, so it will evolve as I find out what it is meant to be doing!

Voice and Bank Loading

Banks of voices are programmed directly into the code. There is a python script from Synth_Dexed that will take a .syx format voice bank and generate a block of C code. I’ve included a script to download the main 8 banks of standard DX voices and run the script:

#!/bin/sh

# Get voices from
# https://yamahablackboxes.com/collection/yamaha-dx7-synthesizer/patches/

mkdir -p voices

DIR="https://yamahablackboxes.com/patches/dx7/factory"

wget -c "${DIR}"/rom1a.syx -O voices/rom1a.syx
wget -c "${DIR}"/rom1b.syx -O voices/rom1b.syx
wget -c "${DIR}"/rom2a.syx -O voices/rom2a.syx
wget -c "${DIR}"/rom2b.syx -O voices/rom2b.syx
wget -c "${DIR}"/rom3a.syx -O voices/rom3a.syx
wget -c "${DIR}"/rom3b.syx -O voices/rom3b.syx
wget -c "${DIR}"/rom4a.syx -O voices/rom4a.syx
wget -c "${DIR}"/rom4b.syx -O voices/rom4b.syx

./synth_dexed/Synth_Dexed/tools/sysex2c.py voices/* > src/voices.h

This only needs to be run once to create the src/voices.h file which is then included in the build.

Voices have the following format:

uint8_t progmem_bank[8][32][128] PROGMEM =
{
    {      // Bank 1
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
    {      // Bank 2
        ...
    }
    ...
    {      // Bank 8
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
}

The system assumes 8 banks of 32 voices each, in the “packed” SYX header format, meaning each voice consists of 128 bytes.

MIDI Bank and Voice Selection

As there are only 8 banks, only BANKSEL (LSB) values 0..7 are valid. Program Change will work in two ways however:

• 0..31 will select voices 1 to 32 in the current bank.
• 31..127 will select voices from the following three adjacent banks.

To select any voice in all 8 banks thus requires the following sequence:

BANKSEL MSB = 0
BANKSEL LSB = 0..7
PROG CHANGE = 0..31

But if bank selection is skipped, then Program Change messages can still be used to select one of the first 128 voices across four consecutive banks.

USB MIDI

The Raspberry Pi Pico SDK uses the TinyUSB protocol stack to implement USB device or host modes and there is an additional option to implement a second USB host port using the Pico’s PIO.

However, USB MIDI appears to only be supported for USB devices at the time of writing, so I’m just using the built-in USB port as a USB device, based on the code provided as part of the TinyUSB examples (more details of how to get basic USB MIDI running here).

TinyUSB MIDI supports two interfaces for reading data, and this wasn’t immediately obvious from the example as that is only sending data and ignores anything coming in.

• USB MIDI Stream mode: this will fill a provided buffer with MIDI data received over USB.
• USB MIDI Packet mode: this will return each 4-byte USB packet individually.

From what I can see of the USB MIDI Spec, all MIDI messages are turned into 4-byte packets for transferring over USB. All normal MIDI messages will consist of 1, 2 or 3 byte messages, and so will fit in a packet each – any unused bytes are padded with 0.

However SysEx messages are a little more complicated and have to be split across multiple packets.

This is the format for a USB MIDI Event Packet (see the “Universal Serial Bus Device Class Definition for MIDI Devices”, Release 1.0):

The code index number is an indication of the contents of each packet. For channel messages, this is basically a repeat of the MIDI command, so a MIDI Note On message might look something like the following:

09 92 3C 64
   Cable 0
   Code Index Number 9
   MIDI Cmd 0x90 (Note On)
   MIDI Channel 3 (0x0=1; 0x1=2; 0x2=3; ... 0xF=16)
   Note 0x3C (60 = C4)
   Velocity 0x64 (100)

But things get a little more complex with System Common or System Exclusive messages which have their own set of codes, depending on the chunking of the packets required.

The critical ones for SysEx are CIN=4,5,6,7 which correspond to SysEx start and then various versions of continuation or end packets. So a larger SysEx message might look something like the following

04 F0 43 10 -- SysEx Start or Continuation
04 34 44 4D -- SysEx Start or Continuation
06 3E F7 00 -- SysEx End after two bytes

Complete message: F0 43 10 34 44 4D 3E F7

So, if I opt to use the packet interface to TinyUSB MIDI then all this has to be sorted out in user code myself. However, the streaming interface will take care of all this for me and just return a buffer full of “traditional” MIDI messages.

Note that there is no concept of Running Status in USB MIDI. Even the oldest USB standard protocol speeds are an order of magnitude, or more, higher than serial MIDI so it isn’t necessary. Every MIDI message will either be a complete 1,2,3 byte message in a single USB packet, or a SysEx multi-packet message as described above.

The basic structure of the USB MIDI handler is as follows:

Init:
   Initialise TinyUSB MIDI stack

Process:
   Run the TinyUSB MIDI task
   IF TinyUSB says MIDI data available:
      Call the stream API to fill our RX buffer
      WHILE data in the RX buffer:
         Call the MIDIParser which reads from the RX buffer
         IF MIDI messages found:
            Call the MIDI Message Handler

Read:
   Grab the next byte from the RX buffer

I’ve actually split this over two files: usbmidi.cpp is the companion to serialmidi.cpp and provides the class that inherits from MIDIDevice (which provides the parser and message handler); usbtask.c provides the interface into the TinyUSB C driver code.

I haven’t done anything special with a USB manufacturer/vendor and device ID yet – so at some point I should see what TinyUSB is using by default and find something unique to PicoDexed (assuming I take it forward in any useful way).

Closing Thoughts

I have a fairly complete implementation now, which is quite nice. I do need to find some way to properly exercise the voice loading over SysEx and it would be good to get some idea of the performance when I throw a MIDI file at it over USB!

I’ve tested some of the parameter changes using the PC version of Dexed. When configured correctly, this can be used to send voice parameter changes to PicoDexed, but I haven’t found a way to download the entire voice as yet.

It’s a shame I can’t just plug in a USB MIDI controller and play it now, but I’ll work on some kind of interface board that should allow me to do it. It will need to be independently powered to act as a USB host anyway.

This is probably going to be my last blog post on PicoDexed for now, but I plan to keep tinkering away at the GitHub repository to see how things go. There are still a couple of limitations, the main one being that everything has to be hard-coded in at present. It would be nice to be able to have some kind of system configuration facility for the MIDI channel if nothing else.

At some point it would also be nice to have a build on the GitHub so others can try it too. And I still need to decide how best to manage the changes I needed to make to Synth_Dexed.

Kevin

https://diyelectromusic.wordpress.com/2024/02/16/raspberry-pi-pico-synth_dexed-part-4/

#dx7 #midi #picodexed #raspberryPiPico #usbMidi

I was going to leave things at Part 3 blog-wise, and just get on with filling in the gaps in code now, but I’ve come back to add a few more notes. But this is likely to be the final part now.

Recall so far, I have:

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.

This is building on the last part and includes notes on how I’ve implemented the following:

• Fuller MIDI support, including control change, program change and pitch bend messages.
• Voice and voice banks, selectable over MIDI.
• MIDI SysEx messages for voice parameters.
• USB MIDI device support.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

MIDI Support

I’m not going to walk through all the details of how I’ve added MIDI but suffice to say that once again the implementation owes a lot to MiniDexed and the Arduino MIDI Library.

At the time of writing the following are all supported as they were already supported in Synth_Dexed, so I just needed to glue the bits together.

Channel Voice Messages (only channel 1 at present)

Channel Control Change Messages

* There is a bug with the master tuning. It ought to accept -99 to 99 I believe, but only 0..99 will actually register and there is no way to send -99 via MIDI at the moment. I need to read up on what is going on here and what it ought to do!

** The Mono Mode parameter has the option for specifying how many of the playable voices can be dedicated to mono mode (at least I think that is what it is saying). I only support a value of 0 which I believe is meant to mean “all available voices”.

System Messages

System Exclusive Messages

Any valid Yamaha (DX) system exclusive messages are passed straight into Synth_Dexed. A Yamaha (DX) message has the following format (see the “DX7IIFD/D Supplemental Booklet: Advanced MIDI Data and Charts”):

F0 - start SysEx message
43 - Yamaha manufacturer ID
sd - s=substatus (command class:0,1,2); d=device ID (0..F)
.. data ..
F7 - end SysEx message

The device ID can be set using the UI on a real DX7 to a value between 1 and 16, which becomes a value between 0 and 15 (0..F) as part of the SysEx message (see “DX7IIFD/D Supplemental Booklet: Advanced MIDI Applications, Section 8”). It is a Systems Exclusive value analogous to the MIDI channel for regular channel messages.

There are a range of Sys Ex parameter settings that have been passed onto Synth_Dexed as follows:

At this stage, all of the MIDI support is on a “it’s probably something like this” basis, so it will evolve as I find out what it is meant to be doing!

Voice and Bank Loading

Banks of voices are programmed directly into the code. There is a python script from Synth_Dexed that will take a .syx format voice bank and generate a block of C code. I’ve included a script to download the main 8 banks of standard DX voices and run the script:

#!/bin/sh

# Get voices from
# https://yamahablackboxes.com/collection/yamaha-dx7-synthesizer/patches/

mkdir -p voices

DIR="https://yamahablackboxes.com/patches/dx7/factory"

wget -c "${DIR}"/rom1a.syx -O voices/rom1a.syx
wget -c "${DIR}"/rom1b.syx -O voices/rom1b.syx
wget -c "${DIR}"/rom2a.syx -O voices/rom2a.syx
wget -c "${DIR}"/rom2b.syx -O voices/rom2b.syx
wget -c "${DIR}"/rom3a.syx -O voices/rom3a.syx
wget -c "${DIR}"/rom3b.syx -O voices/rom3b.syx
wget -c "${DIR}"/rom4a.syx -O voices/rom4a.syx
wget -c "${DIR}"/rom4b.syx -O voices/rom4b.syx

./synth_dexed/Synth_Dexed/tools/sysex2c.py voices/* > src/voices.h

This only needs to be run once to create the src/voices.h file which is then included in the build.

Voices have the following format:

uint8_t progmem_bank[8][32][128] PROGMEM =
{
    {      // Bank 1
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
    {      // Bank 2
        ...
    }
    ...
    {      // Bank 8
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
}

The system assumes 8 banks of 32 voices each, in the “packed” SYX header format, meaning each voice consists of 128 bytes.

MIDI Bank and Voice Selection

As there are only 8 banks, only BANKSEL (LSB) values 0..7 are valid. Program Change will work in two ways however:

• 0..31 will select voices 1 to 32 in the current bank.
• 31..127 will select voices from the following three adjacent banks.

To select any voice in all 8 banks thus requires the following sequence:

BANKSEL MSB = 0
BANKSEL LSB = 0..7
PROG CHANGE = 0..31

But if bank selection is skipped, then Program Change messages can still be used to select one of the first 128 voices across four consecutive banks.

USB MIDI

The Raspberry Pi Pico SDK uses the TinyUSB protocol stack to implement USB device or host modes and there is an additional option to implement a second USB host port using the Pico’s PIO.

However, USB MIDI appears to only be supported for USB devices at the time of writing, so I’m just using the built-in USB port as a USB device, based on the code provided as part of the TinyUSB examples (more details of how to get basic USB MIDI running here).

TinyUSB MIDI supports two interfaces for reading data, and this wasn’t immediately obvious from the example as that is only sending data and ignores anything coming in.

• USB MIDI Stream mode: this will fill a provided buffer with MIDI data received over USB.
• USB MIDI Packet mode: this will return each 4-byte USB packet individually.

From what I can see of the USB MIDI Spec, all MIDI messages are turned into 4-byte packets for transferring over USB. All normal MIDI messages will consist of 1, 2 or 3 byte messages, and so will fit in a packet each – any unused bytes are padded with 0.

However SysEx messages are a little more complicated and have to be split across multiple packets.

This is the format for a USB MIDI Event Packet (see the “Universal Serial Bus Device Class Definition for MIDI Devices”, Release 1.0):

The code index number is an indication of the contents of each packet. For channel messages, this is basically a repeat of the MIDI command, so a MIDI Note On message might look something like the following:

09 92 3C 64
   Cable 0
   Code Index Number 9
   MIDI Cmd 0x90 (Note On)
   MIDI Channel 3 (0x0=1; 0x1=2; 0x2=3; ... 0xF=16)
   Note 0x3C (60 = C4)
   Velocity 0x64 (100)

But things get a little more complex with System Common or System Exclusive messages which have their own set of codes, depending on the chunking of the packets required.

The critical ones for SysEx are CIN=4,5,6,7 which correspond to SysEx start and then various versions of continuation or end packets. So a larger SysEx message might look something like the following

04 F0 43 10 -- SysEx Start or Continuation
04 34 44 4D -- SysEx Start or Continuation
06 3E F7 00 -- SysEx End after two bytes

Complete message: F0 43 10 34 44 4D 3E F7

So, if I opt to use the packet interface to TinyUSB MIDI then all this has to be sorted out in user code myself. However, the streaming interface will take care of all this for me and just return a buffer full of “traditional” MIDI messages.

Note that there is no concept of Running Status in USB MIDI. Even the oldest USB standard protocol speeds are an order of magnitude, or more, higher than serial MIDI so it isn’t necessary. Every MIDI message will either be a complete 1,2,3 byte message in a single USB packet, or a SysEx multi-packet message as described above.

The basic structure of the USB MIDI handler is as follows:

Init:
   Initialise TinyUSB MIDI stack

Process:
   Run the TinyUSB MIDI task
   IF TinyUSB says MIDI data available:
      Call the stream API to fill our RX buffer
      WHILE data in the RX buffer:
         Call the MIDIParser which reads from the RX buffer
         IF MIDI messages found:
            Call the MIDI Message Handler

Read:
   Grab the next byte from the RX buffer

I’ve actually split this over two files: usbmidi.cpp is the companion to serialmidi.cpp and provides the class that inherits from MIDIDevice (which provides the parser and message handler); usbtask.c provides the interface into the TinyUSB C driver code.

I haven’t done anything special with a USB manufacturer/vendor and device ID yet – so at some point I should see what TinyUSB is using by default and find something unique to PicoDexed (assuming I take it forward in any useful way).

Closing Thoughts

I have a fairly complete implementation now, which is quite nice. I do need to find some way to properly exercise the voice loading over SysEx and it would be good to get some idea of the performance when I throw a MIDI file at it over USB!

I’ve tested some of the parameter changes using the PC version of Dexed. When configured correctly, this can be used to send voice parameter changes to PicoDexed, but I haven’t found a way to download the entire voice as yet.

It’s a shame I can’t just plug in a USB MIDI controller and play it now, but I’ll work on some kind of interface board that should allow me to do it. It will need to be independently powered to act as a USB host anyway.

This is probably going to be my last blog post on PicoDexed for now, but I plan to keep tinkering away at the GitHub repository to see how things go. There are still a couple of limitations, the main one being that everything has to be hard-coded in at present. It would be nice to be able to have some kind of system configuration facility for the MIDI channel if nothing else.

At some point it would also be nice to have a build on the GitHub so others can try it too. And I still need to decide how best to manage the changes I needed to make to Synth_Dexed.

Kevin

https://diyelectromusic.wordpress.com/2024/02/16/raspberry-pi-pico-synth_dexed-part-4/

#dx7 #midi #picodexed #raspberryPiPico #usbMidi

I was going to leave things at Part 3 blog-wise, and just get on with filling in the gaps in code now, but I’ve come back to add a few more notes. But this is likely to be the final part now.

Recall so far, I have:

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.

This is building on the last part and includes notes on how I’ve implemented the following:

• Fuller MIDI support, including control change, program change and pitch bend messages.
• Voice and voice banks, selectable over MIDI.
• MIDI SysEx messages for voice parameters.
• USB MIDI device support.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

MIDI Support

I’m not going to walk through all the details of how I’ve added MIDI but suffice to say that once again the implementation owes a lot to MiniDexed and the Arduino MIDI Library.

At the time of writing the following are all supported as they were already supported in Synth_Dexed, so I just needed to glue the bits together.

Channel Voice Messages (only channel 1 at present)

Channel Control Change Messages

* There is a bug with the master tuning. It ought to accept -99 to 99 I believe, but only 0..99 will actually register and there is no way to send -99 via MIDI at the moment. I need to read up on what is going on here and what it ought to do!

** The Mono Mode parameter has the option for specifying how many of the playable voices can be dedicated to mono mode (at least I think that is what it is saying). I only support a value of 0 which I believe is meant to mean “all available voices”.

System Messages

System Exclusive Messages

Any valid Yamaha (DX) system exclusive messages are passed straight into Synth_Dexed. A Yamaha (DX) message has the following format (see the “DX7IIFD/D Supplemental Booklet: Advanced MIDI Data and Charts”):

F0 - start SysEx message
43 - Yamaha manufacturer ID
sd - s=substatus (command class:0,1,2); d=device ID (0..F)
.. data ..
F7 - end SysEx message

The device ID can be set using the UI on a real DX7 to a value between 1 and 16, which becomes a value between 0 and 15 (0..F) as part of the SysEx message (see “DX7IIFD/D Supplemental Booklet: Advanced MIDI Applications, Section 8”). It is a Systems Exclusive value analogous to the MIDI channel for regular channel messages.

There are a range of Sys Ex parameter settings that have been passed onto Synth_Dexed as follows:

At this stage, all of the MIDI support is on a “it’s probably something like this” basis, so it will evolve as I find out what it is meant to be doing!

Voice and Bank Loading

Banks of voices are programmed directly into the code. There is a python script from Synth_Dexed that will take a .syx format voice bank and generate a block of C code. I’ve included a script to download the main 8 banks of standard DX voices and run the script:

#!/bin/sh

# Get voices from
# https://yamahablackboxes.com/collection/yamaha-dx7-synthesizer/patches/

mkdir -p voices

DIR="https://yamahablackboxes.com/patches/dx7/factory"

wget -c "${DIR}"/rom1a.syx -O voices/rom1a.syx
wget -c "${DIR}"/rom1b.syx -O voices/rom1b.syx
wget -c "${DIR}"/rom2a.syx -O voices/rom2a.syx
wget -c "${DIR}"/rom2b.syx -O voices/rom2b.syx
wget -c "${DIR}"/rom3a.syx -O voices/rom3a.syx
wget -c "${DIR}"/rom3b.syx -O voices/rom3b.syx
wget -c "${DIR}"/rom4a.syx -O voices/rom4a.syx
wget -c "${DIR}"/rom4b.syx -O voices/rom4b.syx

./synth_dexed/Synth_Dexed/tools/sysex2c.py voices/* > src/voices.h

This only needs to be run once to create the src/voices.h file which is then included in the build.

Voices have the following format:

uint8_t progmem_bank[8][32][128] PROGMEM =
{
    {      // Bank 1
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
    {      // Bank 2
        ...
    }
    ...
    {      // Bank 8
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
}

The system assumes 8 banks of 32 voices each, in the “packed” SYX header format, meaning each voice consists of 128 bytes.

MIDI Bank and Voice Selection

As there are only 8 banks, only BANKSEL (LSB) values 0..7 are valid. Program Change will work in two ways however:

• 0..31 will select voices 1 to 32 in the current bank.
• 31..127 will select voices from the following three adjacent banks.

To select any voice in all 8 banks thus requires the following sequence:

BANKSEL MSB = 0
BANKSEL LSB = 0..7
PROG CHANGE = 0..31

But if bank selection is skipped, then Program Change messages can still be used to select one of the first 128 voices across four consecutive banks.

USB MIDI

The Raspberry Pi Pico SDK uses the TinyUSB protocol stack to implement USB device or host modes and there is an additional option to implement a second USB host port using the Pico’s PIO.

However, USB MIDI appears to only be supported for USB devices at the time of writing, so I’m just using the built-in USB port as a USB device, based on the code provided as part of the TinyUSB examples (more details of how to get basic USB MIDI running here).

TinyUSB MIDI supports two interfaces for reading data, and this wasn’t immediately obvious from the example as that is only sending data and ignores anything coming in.

• USB MIDI Stream mode: this will fill a provided buffer with MIDI data received over USB.
• USB MIDI Packet mode: this will return each 4-byte USB packet individually.

From what I can see of the USB MIDI Spec, all MIDI messages are turned into 4-byte packets for transferring over USB. All normal MIDI messages will consist of 1, 2 or 3 byte messages, and so will fit in a packet each – any unused bytes are padded with 0.

However SysEx messages are a little more complicated and have to be split across multiple packets.

This is the format for a USB MIDI Event Packet (see the “Universal Serial Bus Device Class Definition for MIDI Devices”, Release 1.0):

The code index number is an indication of the contents of each packet. For channel messages, this is basically a repeat of the MIDI command, so a MIDI Note On message might look something like the following:

09 92 3C 64
   Cable 0
   Code Index Number 9
   MIDI Cmd 0x90 (Note On)
   MIDI Channel 3 (0x0=1; 0x1=2; 0x2=3; ... 0xF=16)
   Note 0x3C (60 = C4)
   Velocity 0x64 (100)

But things get a little more complex with System Common or System Exclusive messages which have their own set of codes, depending on the chunking of the packets required.

The critical ones for SysEx are CIN=4,5,6,7 which correspond to SysEx start and then various versions of continuation or end packets. So a larger SysEx message might look something like the following

04 F0 43 10 -- SysEx Start or Continuation
04 34 44 4D -- SysEx Start or Continuation
06 3E F7 00 -- SysEx End after two bytes

Complete message: F0 43 10 34 44 4D 3E F7

So, if I opt to use the packet interface to TinyUSB MIDI then all this has to be sorted out in user code myself. However, the streaming interface will take care of all this for me and just return a buffer full of “traditional” MIDI messages.

Note that there is no concept of Running Status in USB MIDI. Even the oldest USB standard protocol speeds are an order of magnitude, or more, higher than serial MIDI so it isn’t necessary. Every MIDI message will either be a complete 1,2,3 byte message in a single USB packet, or a SysEx multi-packet message as described above.

The basic structure of the USB MIDI handler is as follows:

Init:
   Initialise TinyUSB MIDI stack

Process:
   Run the TinyUSB MIDI task
   IF TinyUSB says MIDI data available:
      Call the stream API to fill our RX buffer
      WHILE data in the RX buffer:
         Call the MIDIParser which reads from the RX buffer
         IF MIDI messages found:
            Call the MIDI Message Handler

Read:
   Grab the next byte from the RX buffer

I’ve actually split this over two files: usbmidi.cpp is the companion to serialmidi.cpp and provides the class that inherits from MIDIDevice (which provides the parser and message handler); usbtask.c provides the interface into the TinyUSB C driver code.

I haven’t done anything special with a USB manufacturer/vendor and device ID yet – so at some point I should see what TinyUSB is using by default and find something unique to PicoDexed (assuming I take it forward in any useful way).

Closing Thoughts

I have a fairly complete implementation now, which is quite nice. I do need to find some way to properly exercise the voice loading over SysEx and it would be good to get some idea of the performance when I throw a MIDI file at it over USB!

I’ve tested some of the parameter changes using the PC version of Dexed. When configured correctly, this can be used to send voice parameter changes to PicoDexed, but I haven’t found a way to download the entire voice as yet.

It’s a shame I can’t just plug in a USB MIDI controller and play it now, but I’ll work on some kind of interface board that should allow me to do it. It will need to be independently powered to act as a USB host anyway.

This is probably going to be my last blog post on PicoDexed for now, but I plan to keep tinkering away at the GitHub repository to see how things go. There are still a couple of limitations, the main one being that everything has to be hard-coded in at present. It would be nice to be able to have some kind of system configuration facility for the MIDI channel if nothing else.

At some point it would also be nice to have a build on the GitHub so others can try it too. And I still need to decide how best to manage the changes I needed to make to Synth_Dexed.

Kevin

https://diyelectromusic.wordpress.com/2024/02/16/raspberry-pi-pico-synth_dexed-part-4/

#dx7 #midi #picodexed #raspberryPiPico #usbMidi

The story so far…

• In Part 1 I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• In Part 2 I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.

This post describes how I’ve set things up for some further development and the decisions I’ve made to get to the point where it can receive MIDI and actually be somewhat playable within the limitations of 10 note polyphony, a 24000 sample rate, and a single voice only on MIDI channel 1!

Update: By overclocking the Pico to 250MHz I can do 16 note polyphony at 24000 or 8 note polyphony at 48000!

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Optimised Dexed->getSamples

I left things in part 2 noting that Dexed itself is essentially a fully integer-implemented synth engine, so why did I need the floating point calculations. I concluded it is all due to the filter that has been added, which is based on the LP filter code from https://obxd.wordpress.com/ which was added in Dexed, but wasn’t in the original “music synthesizer for Android“.

So I’ve decided not to bother with it. If I feel like it is really missing out, then I have stumbled across the following which looks promising: https://beammyselfintothefuture.wordpress.com/2015/02/16/simple-c-code-for-resonant-lpf-hpf-filters-and-high-low-shelving-eqs/

So, here is my integer-only version of Dexed->getSamples.

void Dexed::getSamples(int16_t* buffer, uint16_t n_samples)
{
  if (refreshVoice)
  {
    for (uint8_t i = 0; i < max_notes; i++)
    {
      if ( voices[i].live )
        voices[i].dx7_note->update(data, voices[i].midi_note, voices[i].velocity, voices[i].porta, &controllers);
    }
    lfo.reset(data + 137);
    refreshVoice = false;
  }

  for (uint16_t i = 0; i < n_samples; ++i)
  {
    buffer[i] = 0;
  }

  for (uint16_t i = 0; i < n_samples; i += _N_)
  {
    AlignedBuf<int32_t, _N_> audiobuf;

    for (uint8_t j = 0; j < _N_; ++j)
    {
      audiobuf.get()[j] = 0;
    }

    int32_t lfovalue = lfo.getsample();
    int32_t lfodelay = lfo.getdelay();

    for (uint8_t note = 0; note < max_notes; note++)
    {
      if (voices[note].live)
      {
        voices[note].dx7_note->compute(audiobuf.get(), lfovalue, lfodelay, &controllers);

        for (uint8_t j = 0; j < _N_; ++j)
        {
          int16_t tmp = audiobuf.get()[j] >> 16;
          buffer[i + j] += tmp;
          audiobuf.get()[j] = 0;
        }
      }
    }
  }
}

With this in place, I appear to be able to comfortably cope with 8-note polyphony. At least with my test chords.

Debug Output

Now before I go too far, I want a simple way to get some output out of the device. The Pico Getting Started documentation gives an example of how to get some standard output (stdio) working. There are two options for this output (see chapter 4 “Saying “Hello World” in C”):

• Using the built-in USB serial port.
• Outputting to the UART serial port.

To use USB requires building in TinyUSB, but I’m planning on using that later. It also adds quite a lot of overhead apparently, so the default is to output to the serial port via GP0 (TX) and GP1 (RX). All that is required is to find a way to connect this up to a computer or terminal device.

There are several options: some kind of 3V3 supporting USB<->Serial converter – there are several, based on the CH240 of FTDI devices for example, although not many of the cheap ones are 3V3 compatible (don’t use a 5V board it could damage the Pico!); or using a native Raspberry Pi development environment, then simply using GPIO directly to connect the Pico to the Pi’s UART.

It is also possible to use the picoprobe firmware running on another Pico I believe, but I haven’t tried that. It wasn’t totally clear to me if that supports the USB to serial link, although it is strongly implied. The official Raspberry Pi Debug Probe definitely does however, but I haven’t got one of those at the moment.

I initially opted to use another Pico as a serial to USB gateway by running Circuitpython and the following script on boot by saving it as code.py:

import board
import busio
import digitalio

uart = busio.UART(tx=board.GP0, rx=board.GP1, baudrate=115200, timeout=0.1)

while True:
    readbytes = uart.read()
    if readbytes != None:
        print (''.join([chr(b) for b in readbytes]))

Now this just needs connected to the Pico running PicoDexed as follows:

PicoDexed        Debug Pico
   GP0    <---->    GP1
   GND    <---->    GND

As this is running Circuitpython it means I also get the CIRCUITPY virtual drive appear and mounted too which isn’t ideal but not really a big issue.

Then I had a rummage in my Pico drawer looking for a neater solution and found a Waveshare RP2040-One that I’d forgotten I had! This is perfect as it has a USB plug at one end (via a shaped PCB) and GPIO at the other, including pins connected to UART 0.

I dropped Micropython onto the board this time, with the following script.

import time
from machine import UART, Pin

# Use one of the GPIO as a GND pin for the serial
gndpin = Pin(11, Pin.OUT)
gndpin.value(0)

print ("Initialising UART 0 on pins gnd=11, tx=12, rx=13...")
uart = UART(0, baudrate=115200, tx=Pin(12), rx=Pin(13))
print ("Ready")

while True:
    # Read raw data version
    rxdata = bytes()
    while uart.any() > 0:
        rxdata += uart.read(1)
        time.sleep_ms(10)

    if rxdata:
        print(rxdata.decode('utf-8'))

To keep the connections simple, I used GPIO 11 as an additional GND pin as there is only one on the board and it isn’t so convenient.

PicoDexed        RP2040-One
   GP0    <---->    GP13
   GND    <---->    GP11

To ensure the code can output text just needs something like the following:

#include <stdio.h>

void main () {
  stdio_init_all();
  printf("PicoDexed...");
}

Then with both devices connected to my virtual Ubuntu Linux machine, I can run minicom (once installed – it isn’t installed by default):

$ sudo minicom -b 115200 -D /dev/ttyACM0

Here is the output.

Welcome to minicom 2.8

OPTIONS: I18n 
Port /dev/ttyACM0, 13:27:28
Press CTRL-A Z for help on special keys

PicoDexed...

Connecting PIO I2S audio
Copying mono to mono at 24000 Hz

Note, to exit minicom use CTRL-A then X.

Alternatively I could use PuTTY on Windows on the COM port associated with the “debugging” Pico.

At some point I’ll probably need to set up proper SWD debugging, but this should do for the time being.

I might also need to switch UARTs if I want to use UART 0 for MIDI, but apparently there are some defines that can be changed in the CMakeLists.txt file:

target_compile_definitions(picodexed PRIVATE
  PICO_DEFAULT_UART=0
  PICO_DEFAULT_UART_TX_PIN=0
  PICO_DEFAULT_UART_RX_PIN=1
)

PicoDexed design

It is time to start thinking seriously if I can turn this into something interesting or not, so borrowing from some of the design principles encapsulated in MiniDexed, I’ve now got a project that looks as follows:

• main.cpp -> Basic IO, initialisation and main update loop.
• picodexed.cpp -> The core synthesizer wrapper with the following key interface:
  • CPicoDexed::Init -> perform all the required synthesizer initialisation.
  • CPicoDexed::Process -> perform a single “tick” of the synthesizer functions, including updating the sample buffers from Dexed.
• mididevice.cpp, serialmidi.cpp, usbmidi.cpp -> placeholder classes that will eventually support MIDI message passing and a serial and USB MIDI interface. This borrows heavily from the way it is done in MiniDexed. These classes will support the following interface:
  • CSerialMIDI::Init -> Initialise the hardware (same for USB).
  • CSerialMIDI::Process -> poll the hardware (same for USB).
  • CMIDIDevice::MIDIMessageHandler -> will be called by the lower-level devices when a MIDI message is ready to be processed. Once parsed, it will trigger calls into the PicoDexed main synthesizer to update its state.
• soundevice.cpp -> Encapsulating the interface to the pico_audio library to use I2S audio, with the following key interface:
  • CSoundDevice::Init -> Set the sample rate and I2S interface pins.
  • CSoundDevice::Update -> Fill the sample buffer using the provided callback, which will be a call to the Dexed->getSamples code above.
• config.h -> contains some system-wide configuration items, such as sample rate and polyphony.

PicoDexed will include the functions required to control the synthesizer. Examples include keydown and keyup functions for when MIDI NoteOn and NoteOff messages, and so on. It also includes a means to set the MIDI channel and to load a voice.

I don’t know yet if the MIDI handling will be interrupt driven or polled. I need to read up on how the Pico SDK handles USB and serial data, but I suspect a polled interface should be fine for my purposes as long as it doesn’t hold up the sample calculations, buffer filling, and sample playing.

With my first pass of this code, there is no external interface – it is still playing a test chord only. But at least most of the structure is now in place to hook it up to MIDI.

The “to do” list so far:

• Ideally find a way to better manage the Synth_Dexed changes. I should submit a PR to Holger, the creator of Synth_Dexed and discuss some conditional compilation steps.
• Hook up USB MIDI so that the Pico can act as a MIDI device and play the synth that way.
• Hook up serial MIDI too.
• Implement volume. Without the filter there is currently no volume changing.
• Implement some basic voice and bank loading.
• Connect up some more core MIDI functionality for program change, BANKSEL, channel volume, master volume, and so on.
• Think about how best to utilise the second core – in theory it should be possible to expand it to 16-note polyphony by using both cores. Or an alternative might be two instances of Synth_Dexed running, so making a second tone generator.

MIDI/Serial Handling

Rather than jump into USB, I’ve opted to get serial MIDI working first. The serial port handling I’ve implemented in serialmidi.cpp borrows heavily from the “advanced” UART example: https://github.com/raspberrypi/pico-examples/tree/master/uart/uart_advanced

It is interrupt driven and shares a simple circular buffer with the main Read function based on the implementation described here: https://embedjournal.com/implementing-circular-buffer-embedded-c/.

The basic design of the serial MIDI interface is as follows:

Interrupt Handler:
  Empty the serial hardware of data writing it to the circular buffer

Init function:
  Initialise the UART as per the uart_advanced example
  Install the interrupt handler and enable interrupts

Process function:
  Call the MIDI device data parser to piece together any MIDI messages
  Call the MIDI device msg handler to handle any complete MIDI messages

Read function:
  Read the next byte out of the circular buffer

There is a common MIDI device that the serial MIDI device inherits from (and that I plan to also use with USB MIDI support when I get that far). This has the following essential functionality:

MIDIParser:
  Read a byte from the transport interface (e.g. the serial MIDI Read)
  IF starting a new message THEN
    Initialise MIDI msg structures
    IF a single byte message THEN
      Fill in MIDI msg structures for single-byte message
      return TRUE
    IF there is a valid Running Status byte stored THEN
      IF this now completes a valid two-byte msg THEN
        Fill in MIDI msg structures for a two-byte message
        return TRUE
  Otherwise process as a two or three byte message
  IF message now complete THEN
    Fill in MIDI msg structures
    return TRUE
  return FALSE

MIDI Message Parser:
  IF MIDI msg already processed THEN return
  IF on correct channel or OMNI THEN
    Based on received MIDI command:
      Extract parameters
      Call appropriate picoDexed function
  Mark MIDI msg as processed.

I had a fun bug where in the serial handling, I was writing to a byte one-out in the circular buffer which meant that the MIDI handling largely worked, but only when using a controller with ActiveSensing – basically the reception of the extra byte “pushed through” the previous message. But it was a bit unresponsive, and occasionally a note of a chord would sound after the others.

I spent the better part of a day instrumenting the code, attempting to work out where the delays might be coming from. Eventually I got so fed up with the active sensing reception clouding my analysis (and triggering my scope when I didn’t want it to) that I filtered it out in the serial interrupt routine – so as early as I could.

This the made the delay a whole pile worse! That was the point I realised it was continually essentially one message behind. As a consequence I had another look at the buffer handling and that was when I realised the mistake.

Multicore support

My initial thought on the above problem was that it was a performance issue – that the MIDI handling wasn’t responsive enough. So I pushed ahead and moved all the synthesis code over to the second core. This is something I wanted to do anyway as I always had the plan of splitting the functionality across the two cores.

To enable multicore support requires including pico_multicore in the list of libaries in the CMakeLists.txt file and then it should largely be a case of doing the following:

#include "pico/multicore.h"

void core1_entry (void)
{
  // stuff to do to initialise core 1

  while (1)
  {
    // Stuff to do repeatedly on core 1
  }
}

// Rest of "normal" (core 0) initialisation code
multicore_launch_core1 (core1_entry);

The question is where to enable this. Eventually I settled on implementing this in picoDexed itself to split out the ProcessSound function over to the second core. This required the following:

• PicoDexed::Init – initialise multi-core support and start the second core running.
• PicoDexed::Process – no longer calls ProcessSound.
• PicoDexed::core1_entry – now calls ProcessSound in an infinite loop.

In order to ensure that I don’t get Dexed into an inconsistent state, I’ve protected the calls into Dexed from the Dexed_Adaptor with spinlocks (mirroring what was happing in MiniDexed) as shown in the following extract:

class CDexedAdapter : public Dexed
{
public:
    CDexedAdapter (uint8_t maxnotes, int rate)
    : Dexed (maxnotes, rate)
    {
        spinlock_num = spin_lock_claim_unused(true);
        spinlock = spin_lock_init(spinlock_num);
    }

    void getSamples (int16_t* buffer, uint16_t n_samples)
    {
        spin_lock_unsafe_blocking(spinlock);
        Dexed::getSamples (buffer, n_samples);
        spin_unlock_unsafe(spinlock);
    }

private:
    int spinlock_num;
    spin_lock_t *spinlock;
}

Spinlocks are described in chapter 4.1.19 of the RPi C/C++ SDK and are part of the hardware_sync library.

In order to ensure that the spin_locks are not held too long, and to allow things like keyup/down events to be registered in a timely manner and not hold up core 0 whilst core 1 is calculating samples, I’ve now reduced the sample buffer to 64 bytes.

As core 1 is essentially free-running calculating samples now, I figured it wouldn’t make much difference how many samples are calculated in each “chunk” but going to 64 from 256 gives four times the number of break points in the cycle where other events can be processed between the spin_locks.

Once consequence of running multi-core seems to be that I can now push the polyphony up to 10 simultaneous notes without any artefacts.

If I can find a way to keep some of the sound generation on core 0 too, I might be able to increase that even further, although getting 6 additional sound engines running to get up to the magic 16 note polyphony might be stretching things still. The trick will be finding a way to trigger and mix samples from the sound generators in the two cores, as all of that current happens within Dexed itself.

Overclocking the Pico

There have been a number of experiments already in seeing how far a Pico can be pushed. There is a standard API call to set the system clock: set_sys_clock_khz(), although not all values can be accurately configured. But general wisdom seems to be that running the Pico at 250MHz isn’t a big deal…

Of course, at this point it is running outside of the “normal” spec, so the long term effects may well reduce the life of the Pico…

But by doing this, the Pico is now running twice as fast and so can now easily cope with 16 note polyphony at a sample rate of 24000, or up the sample rate to 48000 and stick with 8 note polyphony.

It might even raise the possibility of running two tone generators, one on each core! It really does open up a wide range of possibilities!

Closing Thoughts

I’m really pleased with the progress so far. I was starting to think there wouldn’t be a usable combination possible, but 10-note polyphony at a sample rate of 24000 isn’t too bad for a 133MHz CPU with no FPU.

I think my basic design goal would be for something usable with a MIDI controller. I’m not looking to implement a UI like there is with MiniDexed as part of this build. But I do need a bit more MIDI functionality first and I would like to find a way to squeeze some sample calculations out of core 0 when it isn’t handling MIDI.

I also want to get USB MIDI up and running too. I’m not sure if I want to push for both device and host USB support though. I’ll see how complicated it all is!

In the video, I’ve used my Raspberry Pi Pico MIDI Proto Expander. It just needs the addition of the Pimoroni audio board and it is ready to go!

Of course the key question is: would I recommend this to anyone? Answer: no! No way – get yourself a Raspberry Pi Zero V2 and run a full-blown set of 8 DX7s using MiniDexed 🙂

Still for me, this is a bit of fun a really good excuse to do something that’s been on my “to do” list for ages – start getting to grips with the Raspberry Pi Pico C/C++ SDK and the RP2040.

Kevin

https://diyelectromusic.wordpress.com/2024/02/04/raspberry-pi-pico-synth_dexed-part-3/

#dx7 #midi #picodexed #raspberryPiPico

The story so far…

• In Part 1 I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• In Part 2 I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.

This post describes how I’ve set things up for some further development and the decisions I’ve made to get to the point where it can receive MIDI and actually be somewhat playable within the limitations of 10 note polyphony, a 24000 sample rate, and a single voice only on MIDI channel 1!

Update: By overclocking the Pico to 250MHz I can do 16 note polyphony at 24000 or 8 note polyphony at 48000!

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Optimised Dexed->getSamples

I left things in part 2 noting that Dexed itself is essentially a fully integer-implemented synth engine, so why did I need the floating point calculations. I concluded it is all due to the filter that has been added, which is based on the LP filter code from https://obxd.wordpress.com/ which was added in Dexed, but wasn’t in the original “music synthesizer for Android“.

So I’ve decided not to bother with it. If I feel like it is really missing out, then I have stumbled across the following which looks promising: https://beammyselfintothefuture.wordpress.com/2015/02/16/simple-c-code-for-resonant-lpf-hpf-filters-and-high-low-shelving-eqs/

So, here is my integer-only version of Dexed->getSamples.

void Dexed::getSamples(int16_t* buffer, uint16_t n_samples)
{
  if (refreshVoice)
  {
    for (uint8_t i = 0; i < max_notes; i++)
    {
      if ( voices[i].live )
        voices[i].dx7_note->update(data, voices[i].midi_note, voices[i].velocity, voices[i].porta, &controllers);
    }
    lfo.reset(data + 137);
    refreshVoice = false;
  }

  for (uint16_t i = 0; i < n_samples; ++i)
  {
    buffer[i] = 0;
  }

  for (uint16_t i = 0; i < n_samples; i += _N_)
  {
    AlignedBuf<int32_t, _N_> audiobuf;

    for (uint8_t j = 0; j < _N_; ++j)
    {
      audiobuf.get()[j] = 0;
    }

    int32_t lfovalue = lfo.getsample();
    int32_t lfodelay = lfo.getdelay();

    for (uint8_t note = 0; note < max_notes; note++)
    {
      if (voices[note].live)
      {
        voices[note].dx7_note->compute(audiobuf.get(), lfovalue, lfodelay, &controllers);

        for (uint8_t j = 0; j < _N_; ++j)
        {
          int16_t tmp = audiobuf.get()[j] >> 16;
          buffer[i + j] += tmp;
          audiobuf.get()[j] = 0;
        }
      }
    }
  }
}

With this in place, I appear to be able to comfortably cope with 8-note polyphony. At least with my test chords.

Debug Output

Now before I go too far, I want a simple way to get some output out of the device. The Pico Getting Started documentation gives an example of how to get some standard output (stdio) working. There are two options for this output (see chapter 4 “Saying “Hello World” in C”):

• Using the built-in USB serial port.
• Outputting to the UART serial port.

To use USB requires building in TinyUSB, but I’m planning on using that later. It also adds quite a lot of overhead apparently, so the default is to output to the serial port via GP0 (TX) and GP1 (RX). All that is required is to find a way to connect this up to a computer or terminal device.

There are several options: some kind of 3V3 supporting USB<->Serial converter – there are several, based on the CH240 of FTDI devices for example, although not many of the cheap ones are 3V3 compatible (don’t use a 5V board it could damage the Pico!); or using a native Raspberry Pi development environment, then simply using GPIO directly to connect the Pico to the Pi’s UART.

It is also possible to use the picoprobe firmware running on another Pico I believe, but I haven’t tried that. It wasn’t totally clear to me if that supports the USB to serial link, although it is strongly implied. The official Raspberry Pi Debug Probe definitely does however, but I haven’t got one of those at the moment.

I initially opted to use another Pico as a serial to USB gateway by running Circuitpython and the following script on boot by saving it as code.py:

import board
import busio
import digitalio

uart = busio.UART(tx=board.GP0, rx=board.GP1, baudrate=115200, timeout=0.1)

while True:
    readbytes = uart.read()
    if readbytes != None:
        print (''.join([chr(b) for b in readbytes]))

Now this just needs connected to the Pico running PicoDexed as follows:

PicoDexed        Debug Pico
   GP0    <---->    GP1
   GND    <---->    GND

As this is running Circuitpython it means I also get the CIRCUITPY virtual drive appear and mounted too which isn’t ideal but not really a big issue.

Then I had a rummage in my Pico drawer looking for a neater solution and found a Waveshare RP2040-One that I’d forgotten I had! This is perfect as it has a USB plug at one end (via a shaped PCB) and GPIO at the other, including pins connected to UART 0.

I dropped Micropython onto the board this time, with the following script.

import time
from machine import UART, Pin

# Use one of the GPIO as a GND pin for the serial
gndpin = Pin(11, Pin.OUT)
gndpin.value(0)

print ("Initialising UART 0 on pins gnd=11, tx=12, rx=13...")
uart = UART(0, baudrate=115200, tx=Pin(12), rx=Pin(13))
print ("Ready")

while True:
    # Read raw data version
    rxdata = bytes()
    while uart.any() > 0:
        rxdata += uart.read(1)
        time.sleep_ms(10)

    if rxdata:
        print(rxdata.decode('utf-8'))

To keep the connections simple, I used GPIO 11 as an additional GND pin as there is only one on the board and it isn’t so convenient.

PicoDexed        RP2040-One
   GP0    <---->    GP13
   GND    <---->    GP11

To ensure the code can output text just needs something like the following:

#include <stdio.h>

void main () {
  stdio_init_all();
  printf("PicoDexed...");
}

Then with both devices connected to my virtual Ubuntu Linux machine, I can run minicom (once installed – it isn’t installed by default):

$ sudo minicom -b 115200 -D /dev/ttyACM0

Here is the output.

Welcome to minicom 2.8

OPTIONS: I18n 
Port /dev/ttyACM0, 13:27:28
Press CTRL-A Z for help on special keys

PicoDexed...

Connecting PIO I2S audio
Copying mono to mono at 24000 Hz

Note, to exit minicom use CTRL-A then X.

Alternatively I could use PuTTY on Windows on the COM port associated with the “debugging” Pico.

At some point I’ll probably need to set up proper SWD debugging, but this should do for the time being.

I might also need to switch UARTs if I want to use UART 0 for MIDI, but apparently there are some defines that can be changed in the CMakeLists.txt file:

target_compile_definitions(picodexed PRIVATE
  PICO_DEFAULT_UART=0
  PICO_DEFAULT_UART_TX_PIN=0
  PICO_DEFAULT_UART_RX_PIN=1
)

PicoDexed design

It is time to start thinking seriously if I can turn this into something interesting or not, so borrowing from some of the design principles encapsulated in MiniDexed, I’ve now got a project that looks as follows:

• main.cpp -> Basic IO, initialisation and main update loop.
• picodexed.cpp -> The core synthesizer wrapper with the following key interface:
  • CPicoDexed::Init -> perform all the required synthesizer initialisation.
  • CPicoDexed::Process -> perform a single “tick” of the synthesizer functions, including updating the sample buffers from Dexed.
• mididevice.cpp, serialmidi.cpp, usbmidi.cpp -> placeholder classes that will eventually support MIDI message passing and a serial and USB MIDI interface. This borrows heavily from the way it is done in MiniDexed. These classes will support the following interface:
  • CSerialMIDI::Init -> Initialise the hardware (same for USB).
  • CSerialMIDI::Process -> poll the hardware (same for USB).
  • CMIDIDevice::MIDIMessageHandler -> will be called by the lower-level devices when a MIDI message is ready to be processed. Once parsed, it will trigger calls into the PicoDexed main synthesizer to update its state.
• soundevice.cpp -> Encapsulating the interface to the pico_audio library to use I2S audio, with the following key interface:
  • CSoundDevice::Init -> Set the sample rate and I2S interface pins.
  • CSoundDevice::Update -> Fill the sample buffer using the provided callback, which will be a call to the Dexed->getSamples code above.
• config.h -> contains some system-wide configuration items, such as sample rate and polyphony.

PicoDexed will include the functions required to control the synthesizer. Examples include keydown and keyup functions for when MIDI NoteOn and NoteOff messages, and so on. It also includes a means to set the MIDI channel and to load a voice.

I don’t know yet if the MIDI handling will be interrupt driven or polled. I need to read up on how the Pico SDK handles USB and serial data, but I suspect a polled interface should be fine for my purposes as long as it doesn’t hold up the sample calculations, buffer filling, and sample playing.

With my first pass of this code, there is no external interface – it is still playing a test chord only. But at least most of the structure is now in place to hook it up to MIDI.

The “to do” list so far:

• Ideally find a way to better manage the Synth_Dexed changes. I should submit a PR to Holger, the creator of Synth_Dexed and discuss some conditional compilation steps.
• Hook up USB MIDI so that the Pico can act as a MIDI device and play the synth that way.
• Hook up serial MIDI too.
• Implement volume. Without the filter there is currently no volume changing.
• Implement some basic voice and bank loading.
• Connect up some more core MIDI functionality for program change, BANKSEL, channel volume, master volume, and so on.
• Think about how best to utilise the second core – in theory it should be possible to expand it to 16-note polyphony by using both cores. Or an alternative might be two instances of Synth_Dexed running, so making a second tone generator.

MIDI/Serial Handling

Rather than jump into USB, I’ve opted to get serial MIDI working first. The serial port handling I’ve implemented in serialmidi.cpp borrows heavily from the “advanced” UART example: https://github.com/raspberrypi/pico-examples/tree/master/uart/uart_advanced

It is interrupt driven and shares a simple circular buffer with the main Read function based on the implementation described here: https://embedjournal.com/implementing-circular-buffer-embedded-c/.

The basic design of the serial MIDI interface is as follows:

Interrupt Handler:
  Empty the serial hardware of data writing it to the circular buffer

Init function:
  Initialise the UART as per the uart_advanced example
  Install the interrupt handler and enable interrupts

Process function:
  Call the MIDI device data parser to piece together any MIDI messages
  Call the MIDI device msg handler to handle any complete MIDI messages

Read function:
  Read the next byte out of the circular buffer

There is a common MIDI device that the serial MIDI device inherits from (and that I plan to also use with USB MIDI support when I get that far). This has the following essential functionality:

MIDIParser:
  Read a byte from the transport interface (e.g. the serial MIDI Read)
  IF starting a new message THEN
    Initialise MIDI msg structures
    IF a single byte message THEN
      Fill in MIDI msg structures for single-byte message
      return TRUE
    IF there is a valid Running Status byte stored THEN
      IF this now completes a valid two-byte msg THEN
        Fill in MIDI msg structures for a two-byte message
        return TRUE
  Otherwise process as a two or three byte message
  IF message now complete THEN
    Fill in MIDI msg structures
    return TRUE
  return FALSE

MIDI Message Parser:
  IF MIDI msg already processed THEN return
  IF on correct channel or OMNI THEN
    Based on received MIDI command:
      Extract parameters
      Call appropriate picoDexed function
  Mark MIDI msg as processed.

I had a fun bug where in the serial handling, I was writing to a byte one-out in the circular buffer which meant that the MIDI handling largely worked, but only when using a controller with ActiveSensing – basically the reception of the extra byte “pushed through” the previous message. But it was a bit unresponsive, and occasionally a note of a chord would sound after the others.

I spent the better part of a day instrumenting the code, attempting to work out where the delays might be coming from. Eventually I got so fed up with the active sensing reception clouding my analysis (and triggering my scope when I didn’t want it to) that I filtered it out in the serial interrupt routine – so as early as I could.

This the made the delay a whole pile worse! That was the point I realised it was continually essentially one message behind. As a consequence I had another look at the buffer handling and that was when I realised the mistake.

Multicore support

My initial thought on the above problem was that it was a performance issue – that the MIDI handling wasn’t responsive enough. So I pushed ahead and moved all the synthesis code over to the second core. This is something I wanted to do anyway as I always had the plan of splitting the functionality across the two cores.

To enable multicore support requires including pico_multicore in the list of libaries in the CMakeLists.txt file and then it should largely be a case of doing the following:

#include "pico/multicore.h"

void core1_entry (void)
{
  // stuff to do to initialise core 1

  while (1)
  {
    // Stuff to do repeatedly on core 1
  }
}

// Rest of "normal" (core 0) initialisation code
multicore_launch_core1 (core1_entry);

The question is where to enable this. Eventually I settled on implementing this in picoDexed itself to split out the ProcessSound function over to the second core. This required the following:

• PicoDexed::Init – initialise multi-core support and start the second core running.
• PicoDexed::Process – no longer calls ProcessSound.
• PicoDexed::core1_entry – now calls ProcessSound in an infinite loop.

In order to ensure that I don’t get Dexed into an inconsistent state, I’ve protected the calls into Dexed from the Dexed_Adaptor with spinlocks (mirroring what was happing in MiniDexed) as shown in the following extract:

class CDexedAdapter : public Dexed
{
public:
    CDexedAdapter (uint8_t maxnotes, int rate)
    : Dexed (maxnotes, rate)
    {
        spinlock_num = spin_lock_claim_unused(true);
        spinlock = spin_lock_init(spinlock_num);
    }

    void getSamples (int16_t* buffer, uint16_t n_samples)
    {
        spin_lock_unsafe_blocking(spinlock);
        Dexed::getSamples (buffer, n_samples);
        spin_unlock_unsafe(spinlock);
    }

private:
    int spinlock_num;
    spin_lock_t *spinlock;
}

Spinlocks are described in chapter 4.1.19 of the RPi C/C++ SDK and are part of the hardware_sync library.

In order to ensure that the spin_locks are not held too long, and to allow things like keyup/down events to be registered in a timely manner and not hold up core 0 whilst core 1 is calculating samples, I’ve now reduced the sample buffer to 64 bytes.

As core 1 is essentially free-running calculating samples now, I figured it wouldn’t make much difference how many samples are calculated in each “chunk” but going to 64 from 256 gives four times the number of break points in the cycle where other events can be processed between the spin_locks.

Once consequence of running multi-core seems to be that I can now push the polyphony up to 10 simultaneous notes without any artefacts.

If I can find a way to keep some of the sound generation on core 0 too, I might be able to increase that even further, although getting 6 additional sound engines running to get up to the magic 16 note polyphony might be stretching things still. The trick will be finding a way to trigger and mix samples from the sound generators in the two cores, as all of that current happens within Dexed itself.

Overclocking the Pico

There have been a number of experiments already in seeing how far a Pico can be pushed. There is a standard API call to set the system clock: set_sys_clock_khz(), although not all values can be accurately configured. But general wisdom seems to be that running the Pico at 250MHz isn’t a big deal…

Of course, at this point it is running outside of the “normal” spec, so the long term effects may well reduce the life of the Pico…

But by doing this, the Pico is now running twice as fast and so can now easily cope with 16 note polyphony at a sample rate of 24000, or up the sample rate to 48000 and stick with 8 note polyphony.

It might even raise the possibility of running two tone generators, one on each core! It really does open up a wide range of possibilities!

Closing Thoughts

I’m really pleased with the progress so far. I was starting to think there wouldn’t be a usable combination possible, but 10-note polyphony at a sample rate of 24000 isn’t too bad for a 133MHz CPU with no FPU.

I think my basic design goal would be for something usable with a MIDI controller. I’m not looking to implement a UI like there is with MiniDexed as part of this build. But I do need a bit more MIDI functionality first and I would like to find a way to squeeze some sample calculations out of core 0 when it isn’t handling MIDI.

I also want to get USB MIDI up and running too. I’m not sure if I want to push for both device and host USB support though. I’ll see how complicated it all is!

In the video, I’ve used my Raspberry Pi Pico MIDI Proto Expander. It just needs the addition of the Pimoroni audio board and it is ready to go!

Of course the key question is: would I recommend this to anyone? Answer: no! No way – get yourself a Raspberry Pi Zero V2 and run a full-blown set of 8 DX7s using MiniDexed 🙂

Still for me, this is a bit of fun a really good excuse to do something that’s been on my “to do” list for ages – start getting to grips with the Raspberry Pi Pico C/C++ SDK and the RP2040.

Kevin

https://diyelectromusic.wordpress.com/2024/02/04/raspberry-pi-pico-synth_dexed-part-3/

#dx7 #midi #picodexed #raspberryPiPico

The story so far…

• In Part 1 I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• In Part 2 I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.

This post describes how I’ve set things up for some further development and the decisions I’ve made to get to the point where it can receive MIDI and actually be somewhat playable within the limitations of 10 note polyphony, a 24000 sample rate, and a single voice only on MIDI channel 1!

Update: By overclocking the Pico to 250MHz I can do 16 note polyphony at 24000 or 8 note polyphony at 48000!

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Optimised Dexed->getSamples

I left things in part 2 noting that Dexed itself is essentially a fully integer-implemented synth engine, so why did I need the floating point calculations. I concluded it is all due to the filter that has been added, which is based on the LP filter code from https://obxd.wordpress.com/ which was added in Dexed, but wasn’t in the original “music synthesizer for Android“.

So I’ve decided not to bother with it. If I feel like it is really missing out, then I have stumbled across the following which looks promising: https://beammyselfintothefuture.wordpress.com/2015/02/16/simple-c-code-for-resonant-lpf-hpf-filters-and-high-low-shelving-eqs/

So, here is my integer-only version of Dexed->getSamples.

void Dexed::getSamples(int16_t* buffer, uint16_t n_samples)
{
  if (refreshVoice)
  {
    for (uint8_t i = 0; i < max_notes; i++)
    {
      if ( voices[i].live )
        voices[i].dx7_note->update(data, voices[i].midi_note, voices[i].velocity, voices[i].porta, &controllers);
    }
    lfo.reset(data + 137);
    refreshVoice = false;
  }

  for (uint16_t i = 0; i < n_samples; ++i)
  {
    buffer[i] = 0;
  }

  for (uint16_t i = 0; i < n_samples; i += _N_)
  {
    AlignedBuf<int32_t, _N_> audiobuf;

    for (uint8_t j = 0; j < _N_; ++j)
    {
      audiobuf.get()[j] = 0;
    }

    int32_t lfovalue = lfo.getsample();
    int32_t lfodelay = lfo.getdelay();

    for (uint8_t note = 0; note < max_notes; note++)
    {
      if (voices[note].live)
      {
        voices[note].dx7_note->compute(audiobuf.get(), lfovalue, lfodelay, &controllers);

        for (uint8_t j = 0; j < _N_; ++j)
        {
          int16_t tmp = audiobuf.get()[j] >> 16;
          buffer[i + j] += tmp;
          audiobuf.get()[j] = 0;
        }
      }
    }
  }
}

With this in place, I appear to be able to comfortably cope with 8-note polyphony. At least with my test chords.

Debug Output

Now before I go too far, I want a simple way to get some output out of the device. The Pico Getting Started documentation gives an example of how to get some standard output (stdio) working. There are two options for this output (see chapter 4 “Saying “Hello World” in C”):

• Using the built-in USB serial port.
• Outputting to the UART serial port.

To use USB requires building in TinyUSB, but I’m planning on using that later. It also adds quite a lot of overhead apparently, so the default is to output to the serial port via GP0 (TX) and GP1 (RX). All that is required is to find a way to connect this up to a computer or terminal device.

There are several options: some kind of 3V3 supporting USB<->Serial converter – there are several, based on the CH240 of FTDI devices for example, although not many of the cheap ones are 3V3 compatible (don’t use a 5V board it could damage the Pico!); or using a native Raspberry Pi development environment, then simply using GPIO directly to connect the Pico to the Pi’s UART.

It is also possible to use the picoprobe firmware running on another Pico I believe, but I haven’t tried that. It wasn’t totally clear to me if that supports the USB to serial link, although it is strongly implied. The official Raspberry Pi Debug Probe definitely does however, but I haven’t got one of those at the moment.

I initially opted to use another Pico as a serial to USB gateway by running Circuitpython and the following script on boot by saving it as code.py:

import board
import busio
import digitalio

uart = busio.UART(tx=board.GP0, rx=board.GP1, baudrate=115200, timeout=0.1)

while True:
    readbytes = uart.read()
    if readbytes != None:
        print (''.join([chr(b) for b in readbytes]))

Now this just needs connected to the Pico running PicoDexed as follows:

PicoDexed        Debug Pico
   GP0    <---->    GP1
   GND    <---->    GND

As this is running Circuitpython it means I also get the CIRCUITPY virtual drive appear and mounted too which isn’t ideal but not really a big issue.

Then I had a rummage in my Pico drawer looking for a neater solution and found a Waveshare RP2040-One that I’d forgotten I had! This is perfect as it has a USB plug at one end (via a shaped PCB) and GPIO at the other, including pins connected to UART 0.

I dropped Micropython onto the board this time, with the following script.

import time
from machine import UART, Pin

# Use one of the GPIO as a GND pin for the serial
gndpin = Pin(11, Pin.OUT)
gndpin.value(0)

print ("Initialising UART 0 on pins gnd=11, tx=12, rx=13...")
uart = UART(0, baudrate=115200, tx=Pin(12), rx=Pin(13))
print ("Ready")

while True:
    # Read raw data version
    rxdata = bytes()
    while uart.any() > 0:
        rxdata += uart.read(1)
        time.sleep_ms(10)

    if rxdata:
        print(rxdata.decode('utf-8'))

To keep the connections simple, I used GPIO 11 as an additional GND pin as there is only one on the board and it isn’t so convenient.

PicoDexed        RP2040-One
   GP0    <---->    GP13
   GND    <---->    GP11

To ensure the code can output text just needs something like the following:

#include <stdio.h>

void main () {
  stdio_init_all();
  printf("PicoDexed...");
}

Then with both devices connected to my virtual Ubuntu Linux machine, I can run minicom (once installed – it isn’t installed by default):

$ sudo minicom -b 115200 -D /dev/ttyACM0

Here is the output.

Welcome to minicom 2.8

OPTIONS: I18n 
Port /dev/ttyACM0, 13:27:28
Press CTRL-A Z for help on special keys

PicoDexed...

Connecting PIO I2S audio
Copying mono to mono at 24000 Hz

Note, to exit minicom use CTRL-A then X.

Alternatively I could use PuTTY on Windows on the COM port associated with the “debugging” Pico.

At some point I’ll probably need to set up proper SWD debugging, but this should do for the time being.

I might also need to switch UARTs if I want to use UART 0 for MIDI, but apparently there are some defines that can be changed in the CMakeLists.txt file:

target_compile_definitions(picodexed PRIVATE
  PICO_DEFAULT_UART=0
  PICO_DEFAULT_UART_TX_PIN=0
  PICO_DEFAULT_UART_RX_PIN=1
)

PicoDexed design

It is time to start thinking seriously if I can turn this into something interesting or not, so borrowing from some of the design principles encapsulated in MiniDexed, I’ve now got a project that looks as follows:

• main.cpp -> Basic IO, initialisation and main update loop.
• picodexed.cpp -> The core synthesizer wrapper with the following key interface:
  • CPicoDexed::Init -> perform all the required synthesizer initialisation.
  • CPicoDexed::Process -> perform a single “tick” of the synthesizer functions, including updating the sample buffers from Dexed.
• mididevice.cpp, serialmidi.cpp, usbmidi.cpp -> placeholder classes that will eventually support MIDI message passing and a serial and USB MIDI interface. This borrows heavily from the way it is done in MiniDexed. These classes will support the following interface:
  • CSerialMIDI::Init -> Initialise the hardware (same for USB).
  • CSerialMIDI::Process -> poll the hardware (same for USB).
  • CMIDIDevice::MIDIMessageHandler -> will be called by the lower-level devices when a MIDI message is ready to be processed. Once parsed, it will trigger calls into the PicoDexed main synthesizer to update its state.
• soundevice.cpp -> Encapsulating the interface to the pico_audio library to use I2S audio, with the following key interface:
  • CSoundDevice::Init -> Set the sample rate and I2S interface pins.
  • CSoundDevice::Update -> Fill the sample buffer using the provided callback, which will be a call to the Dexed->getSamples code above.
• config.h -> contains some system-wide configuration items, such as sample rate and polyphony.

PicoDexed will include the functions required to control the synthesizer. Examples include keydown and keyup functions for when MIDI NoteOn and NoteOff messages, and so on. It also includes a means to set the MIDI channel and to load a voice.

I don’t know yet if the MIDI handling will be interrupt driven or polled. I need to read up on how the Pico SDK handles USB and serial data, but I suspect a polled interface should be fine for my purposes as long as it doesn’t hold up the sample calculations, buffer filling, and sample playing.

With my first pass of this code, there is no external interface – it is still playing a test chord only. But at least most of the structure is now in place to hook it up to MIDI.

The “to do” list so far:

• Ideally find a way to better manage the Synth_Dexed changes. I should submit a PR to Holger, the creator of Synth_Dexed and discuss some conditional compilation steps.
• Hook up USB MIDI so that the Pico can act as a MIDI device and play the synth that way.
• Hook up serial MIDI too.
• Implement volume. Without the filter there is currently no volume changing.
• Implement some basic voice and bank loading.
• Connect up some more core MIDI functionality for program change, BANKSEL, channel volume, master volume, and so on.
• Think about how best to utilise the second core – in theory it should be possible to expand it to 16-note polyphony by using both cores. Or an alternative might be two instances of Synth_Dexed running, so making a second tone generator.

MIDI/Serial Handling

Rather than jump into USB, I’ve opted to get serial MIDI working first. The serial port handling I’ve implemented in serialmidi.cpp borrows heavily from the “advanced” UART example: https://github.com/raspberrypi/pico-examples/tree/master/uart/uart_advanced

It is interrupt driven and shares a simple circular buffer with the main Read function based on the implementation described here: https://embedjournal.com/implementing-circular-buffer-embedded-c/.

The basic design of the serial MIDI interface is as follows:

Interrupt Handler:
  Empty the serial hardware of data writing it to the circular buffer

Init function:
  Initialise the UART as per the uart_advanced example
  Install the interrupt handler and enable interrupts

Process function:
  Call the MIDI device data parser to piece together any MIDI messages
  Call the MIDI device msg handler to handle any complete MIDI messages

Read function:
  Read the next byte out of the circular buffer

There is a common MIDI device that the serial MIDI device inherits from (and that I plan to also use with USB MIDI support when I get that far). This has the following essential functionality:

MIDIParser:
  Read a byte from the transport interface (e.g. the serial MIDI Read)
  IF starting a new message THEN
    Initialise MIDI msg structures
    IF a single byte message THEN
      Fill in MIDI msg structures for single-byte message
      return TRUE
    IF there is a valid Running Status byte stored THEN
      IF this now completes a valid two-byte msg THEN
        Fill in MIDI msg structures for a two-byte message
        return TRUE
  Otherwise process as a two or three byte message
  IF message now complete THEN
    Fill in MIDI msg structures
    return TRUE
  return FALSE

MIDI Message Parser:
  IF MIDI msg already processed THEN return
  IF on correct channel or OMNI THEN
    Based on received MIDI command:
      Extract parameters
      Call appropriate picoDexed function
  Mark MIDI msg as processed.

I had a fun bug where in the serial handling, I was writing to a byte one-out in the circular buffer which meant that the MIDI handling largely worked, but only when using a controller with ActiveSensing – basically the reception of the extra byte “pushed through” the previous message. But it was a bit unresponsive, and occasionally a note of a chord would sound after the others.

I spent the better part of a day instrumenting the code, attempting to work out where the delays might be coming from. Eventually I got so fed up with the active sensing reception clouding my analysis (and triggering my scope when I didn’t want it to) that I filtered it out in the serial interrupt routine – so as early as I could.

This the made the delay a whole pile worse! That was the point I realised it was continually essentially one message behind. As a consequence I had another look at the buffer handling and that was when I realised the mistake.

Multicore support

My initial thought on the above problem was that it was a performance issue – that the MIDI handling wasn’t responsive enough. So I pushed ahead and moved all the synthesis code over to the second core. This is something I wanted to do anyway as I always had the plan of splitting the functionality across the two cores.

To enable multicore support requires including pico_multicore in the list of libaries in the CMakeLists.txt file and then it should largely be a case of doing the following:

#include "pico/multicore.h"

void core1_entry (void)
{
  // stuff to do to initialise core 1

  while (1)
  {
    // Stuff to do repeatedly on core 1
  }
}

// Rest of "normal" (core 0) initialisation code
multicore_launch_core1 (core1_entry);

The question is where to enable this. Eventually I settled on implementing this in picoDexed itself to split out the ProcessSound function over to the second core. This required the following:

• PicoDexed::Init – initialise multi-core support and start the second core running.
• PicoDexed::Process – no longer calls ProcessSound.
• PicoDexed::core1_entry – now calls ProcessSound in an infinite loop.

In order to ensure that I don’t get Dexed into an inconsistent state, I’ve protected the calls into Dexed from the Dexed_Adaptor with spinlocks (mirroring what was happing in MiniDexed) as shown in the following extract:

class CDexedAdapter : public Dexed
{
public:
    CDexedAdapter (uint8_t maxnotes, int rate)
    : Dexed (maxnotes, rate)
    {
        spinlock_num = spin_lock_claim_unused(true);
        spinlock = spin_lock_init(spinlock_num);
    }

    void getSamples (int16_t* buffer, uint16_t n_samples)
    {
        spin_lock_unsafe_blocking(spinlock);
        Dexed::getSamples (buffer, n_samples);
        spin_unlock_unsafe(spinlock);
    }

private:
    int spinlock_num;
    spin_lock_t *spinlock;
}

Spinlocks are described in chapter 4.1.19 of the RPi C/C++ SDK and are part of the hardware_sync library.

In order to ensure that the spin_locks are not held too long, and to allow things like keyup/down events to be registered in a timely manner and not hold up core 0 whilst core 1 is calculating samples, I’ve now reduced the sample buffer to 64 bytes.

As core 1 is essentially free-running calculating samples now, I figured it wouldn’t make much difference how many samples are calculated in each “chunk” but going to 64 from 256 gives four times the number of break points in the cycle where other events can be processed between the spin_locks.

Once consequence of running multi-core seems to be that I can now push the polyphony up to 10 simultaneous notes without any artefacts.

If I can find a way to keep some of the sound generation on core 0 too, I might be able to increase that even further, although getting 6 additional sound engines running to get up to the magic 16 note polyphony might be stretching things still. The trick will be finding a way to trigger and mix samples from the sound generators in the two cores, as all of that current happens within Dexed itself.

Overclocking the Pico

There have been a number of experiments already in seeing how far a Pico can be pushed. There is a standard API call to set the system clock: set_sys_clock_khz(), although not all values can be accurately configured. But general wisdom seems to be that running the Pico at 250MHz isn’t a big deal…

Of course, at this point it is running outside of the “normal” spec, so the long term effects may well reduce the life of the Pico…

But by doing this, the Pico is now running twice as fast and so can now easily cope with 16 note polyphony at a sample rate of 24000, or up the sample rate to 48000 and stick with 8 note polyphony.

It might even raise the possibility of running two tone generators, one on each core! It really does open up a wide range of possibilities!

Closing Thoughts

I’m really pleased with the progress so far. I was starting to think there wouldn’t be a usable combination possible, but 10-note polyphony at a sample rate of 24000 isn’t too bad for a 133MHz CPU with no FPU.

I think my basic design goal would be for something usable with a MIDI controller. I’m not looking to implement a UI like there is with MiniDexed as part of this build. But I do need a bit more MIDI functionality first and I would like to find a way to squeeze some sample calculations out of core 0 when it isn’t handling MIDI.

I also want to get USB MIDI up and running too. I’m not sure if I want to push for both device and host USB support though. I’ll see how complicated it all is!

In the video, I’ve used my Raspberry Pi Pico MIDI Proto Expander. It just needs the addition of the Pimoroni audio board and it is ready to go!

Of course the key question is: would I recommend this to anyone? Answer: no! No way – get yourself a Raspberry Pi Zero V2 and run a full-blown set of 8 DX7s using MiniDexed 🙂

Still for me, this is a bit of fun a really good excuse to do something that’s been on my “to do” list for ages – start getting to grips with the Raspberry Pi Pico C/C++ SDK and the RP2040.

Kevin

https://diyelectromusic.wordpress.com/2024/02/04/raspberry-pi-pico-synth_dexed-part-3/

#dx7 #midi #picodexed #raspberryPiPico

PicoDexed MIDI Demonstration

https://makertube.net/w/1UFqJqAiupNYSm2NQJaUZN

PicoDexed MIDI Demonstration

https://makertube.net/w/1UFqJqAiupNYSm2NQJaUZN

PicoDexed MIDI Demonstration

https://makertube.net/w/1UFqJqAiupNYSm2NQJaUZN

PicoDexed MIDI Demonstration

https://makertube.net/w/1UFqJqAiupNYSm2NQJaUZN