#usb-midi — Public Fediverse posts

XIAO ESP32-C3 MIDI Synthesizer – Part 6

Expanding on my previous posts, I thought it might be interesting to see how I might be able to add some additional IO to the MIDI Synth. This is an exploration of some options there.

• Part 1 – Getting started and getting code running.
• Part 2 – Swapping the ESP32-C3 for a SAMD21 to get USB MIDI.
• Part 3 – Taking a deeper look at the SAM2695 itself.
• Part 4 – A USB MIDI Synth Module using the SAMD21 again as a USB MIDI Host.
• Part 5 – A Serial MIDI Synth Module using the original ESP32-C3.
• Part 6 – Pairs the Synth with a XIAO Expansion board to add display and potentiometers.

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

These are the key tutorials for the main concepts used in this project:

• Getting Started with the XIAO MIDI Synthesizer
• XIAO SAMD21, Arduino and MIDI
• XIAO SAMD21, Arduino and MIDI – Part 6

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

The Synth Grove Connector

One option to immediately explore for me was the Grove connector on the Synth – highlighted by the blue rectangle in the photo below. I’m thinking at this stage of the XIAO Expander Module (more here) and how that might give some options for easily hooking up to the Synth.

There one obvious issue with this, and one not so obvious issue.

First, of course, there is no access to this connector through the case. My initial thought was to simply remove the PCB from the case and use it as a stand-alone board. On initial inspection it seemed that there were two screws holding it down. Not so, a more thorough inspection (after remove the two screws and still not being able to remove it), revealed a third screw underneath the “light pipe” for the LEDs.

Unfortunately that light pipe is pretty well wedged into the case making removal particularly tricky. But without removing the light pipe, it isn’t possible to get to the screw at all.

I did wonder about making a hole in the 3D printed case. A better option might be to get hold of the published 3D print files and add a hole and make my own (they are available via the product page).

But both options would probably end up changing the original case somehow – even if printing my own, I still need to get the original PCB out somehow and that brings me back to the light pipe issue.

The second issue isn’t quite so obvious. In that photo we can see that the pins for the Grove connector are labelled as follows (top to bottom):

• NC
• TX
• 5V
• GND

The UART on the XIAO expander board, which I’d like to use, is labelled:

• RX7
• TX6
• 3V3
• GND

Checking in with the Synth schematic, the connector is wired as follows:

SYS_MIDI connects to the MIDI_IN pin of the SAM2695, so actually connecting “TX to TX” in this instance should be ok.

5V might be an issue though, as it really does look like (to me) that it really means 5V – it is the input to the TPL740F33 that generates the 3V3 power signal, as well as feeding the amplifier directly. The datasheet of the TPL740F33 does seem to imply that if receiving 3V3 it can still generate 3V3 so it might be ok? The amplifier obviously won’t be as powerful though running off 3V3.

Anyway, for now, instead I’ve just opted to use the GPIO again, wired into the expansion sockets with the XIAO removed.

At the XIAO expander end, I’ve used the additional pins rather than the Grove connector, as they support a 5V output.

The downsides to this approach:

• I’m not using the Grove connectors, which would have been really neat.
• I have no access to the four buttons on the XIAO MIDI Synth.

But I do now have access to two I2C Grove connectors, a GPIO Grove, and the RX part of the UART Grove too as well as the on-board display.

If a XIAO SAMD21 is used, then the previous code for USB to the Synth can be used directly – see XIAO ESP32-C3 MIDI Synthesizer – Part 2.

If the XIAO ESP32-C3 is used, then an additional serial MIDI connection is required. This can be connected to the Grove UART connector (using the RX pin, and leaving TX unconnected) or the RX pin of the additional 8-way pin header on the expansion board. Then the code from this will work directly: XIAO ESP32-C3 MIDI Synthesizer – Part 5.

Adding a Display and Program Control

I already have some code that has done this for a XIAO on an expansion board here XIAO SAMD21, Arduino and MIDI – Part 6.

But for this to work usefully with the Synth module, I need to adjust the routing so that MIDI goes from USB to serial, but the program change messages are also sent via serial to the synth module. That has already been address in previous parts, to I just need to merge the code with that from XIAO ESP32-C3 MIDI Synthesizer – Part 4.

This is the result.

There is a bit of jitter on the analog pot, but that is only because I’m using the original fairly simplified algorithm to detect changes. If I was fussed about it, I’d reuse the averaging class from Arduino MIDI Atari Paddles. And to be honest, a capacitor on the pot would probably go quite a long way too…

As a test, I also powered the device from the Grove UART port connecting it as follows:

• Expander GND – GND Synth
• Expander 3V3 – 5V IN Synth
• Expander TX – RX/D6 Synth
• Expander RX – N/C

And this all worked fine. So I think a Grove to Grove lead would work fine if I had access to the Synth’s Grove port.

This does mean that the exact same code can work with the M5 Synth module using a Grove to Grove lead. The downside of this, even though it is a lot simpler in connectivity terms, is that there is now external audio out like there is on the XIAO Synth.

For completeness the same code can be used with the XIAO ESP32-C3 and serial MIDI, see the photo at the start of this blog.

To turn off all USB handling in the code, the following must be commented out:

//#define HAS_USB
//#define SER_TO_USB
//#define MIDI_USB_PCCC

For other parts of the code, the Arduino abstraction for A0 maps over to the ESP32-C3 fine. The only thing to watch out for is the increased analog resolution from 10 to 12 bits, but a call to analogReadResolution(10) drops that back to the expected 10 bits.

Oh and the Serial port to use is different:

• XIAO SAMD21: Serial1
• XIAO ESP32-C3: Serial0

Find it on GitHub here.

Closing Thoughts

If I can be bothered, it would be nice to actually display the General MIDI voice name on the display. The SAM2695 also has its MT-32 mode, so having some means of selecting that might be interesting too.

And so far I’ve largely only messed about with driving it on a single MIDI channel, so there is a lot more that could be done there.

Kevin

#controlChange #esp32c3 #midi #programChange #SAM2695 #samd21 #usbMidi #xiao

The CME H4 MIDI WC is a self-sufficient USB MIDI host with a pair of DIN MIDI INs and OUTs as well as USB C and A.

The crucial thing is that you can extend the USB A with a hub to expand it to up to eight ports. You need to connect to a PC to set up that telephone exchange, but after that y're good to go.

Sure, I guess you could do all this with a Raspberry Pi, but it's actually cheaper than either that or one of those Kentons. Three thumbs up!

#midi #usb #usbmidi #midithru #omg

@paulrickards Wow! Good call... That's it.

• USB HID Current Required: 250mA

• USB MIDI Current Required: 100mA

I wonder if there's a workaround...

#raspberrypipico #usbhid #usbmidi #microcontrollers

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

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

A Yamaha DX7 On A USB Dongle

#musicalhacks #dx7 #midi #raspberrypi #usbmidi #yamaha #hackaday

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