pico-simple-stupid-synth

This repository contains the code for a Raspberry Pico RP2040 based very simple polyphonic synthesizer. The Pico will appear as a USB-MIDI device on any USB host computer that supports MIDI over USB.

Software Features

The synthesizer only supports a clean square shape as sole wave form, since this is the simplest and fastest wave form to synthesize. But it can do this for roughly 32 polyphonic voices in parallel. Actually, the exact limit of voices varies with environment properties such as the overall traffic on the MIDI lines and other. Also, lots of high pitches will produce slightly more MCU load than low pitches.

Connecting to a USB Host

MIDI data is transferred via USB. That is, just connect the Pico with your PC or notebook via USB, and the Pico, when programmed as synthesizer, will appear as a USB-MIDI device named Pico Simple Stupid Synth. You can play any MIDI files, using your favorite MIDI player, just by selecting this MIDI device.

Connecting to an Audio Output Device

The synthesizer outputs its generated audio as digital stereo signal via an I²S interface (not to be confused with I²C) with 16 bits sample size and a sample frequency of 48kHz. Specifically,

• the I²S BLCK clock signal is provided at GPIO 26 (pin 31),
• the I²S LRCLK word signal is provided at GPIO 27 (pin 32), and
• the I²S data signal is provided at GPIO 28 (pin 34).

To hear the sound, you will need to connect an I²S audio device to these pins. Check out for some I²S audio amplifier breakout or board or any device that features an I²S input. As an example, Fig. 1 shows an I²S audio device connected to the Pico.

Fig. 1: Assembly on Breadboard with I²S Audio Converter

If you are planning to build an I²S amplifier by your own, you may want to have a look at Sect. 3.4.2 “PCM/I2S audio” of the book “Hardware design with RP2040” published by Raspberry Pi Ltd, available at here, that presents a schematic diagram for a complete digitial I²S PCM audio circuit (though note that in the schematics, the assumed order of the GPIO pin numbers differs from our wiring).

Compiling

It is highly recommended to compile the project on a Raspberry Pi computer. Compiling should also work on any other Linux machine or maybe even under Windows, but I have not verified that.

The build files assume that the files of this repository are located side by side with the Pico SDK as provided by Raspberry Pi Ltd. See The C/C++ SDK for instruction how to install the SDK. The probably easiest way to install the SDK on a Raspberry Pi is to use the setup script that is mentioned in Section SDK Setup.

Assuming you have installed the Pico SDK in your home directory ~/pico, the directory layout structure should look as follows:

├── ~/pico
|   ├── openocd
|   ├── pico-examples
|   ├── pico-extras
|   ├── pico-playground
|   ├── picoprobe
|   ├── pico-sdk
|   ├── pico-simple-stupid-synth

Ensure you have version 1.5 of the Pico SDK or higher. Earlier releases of the Pico SDK include a version of the TinyUSB library that is likely not to work.

For compiling, run the script build.sh from within the pico-simple-stupid-synth directory.

Deploying

After successful compiling, you should find the file ~/pico/pico-simple-stupid-synth/build/simple-stupid-synth.elf. While pressing the BOOTSEL button on your pico, connect it via USB to your computer. You should now see a new (pseudo) file system, usually called RPI-RP2. If the file system does not automatically appear, you may need to manually mount it (it will usually be accessible under the /dev/sda1 device file, of course unless you have already some other device that has already grabbed this name).

To deploy, just copy the compiled file ~/pico/pico-simple-stupid-synth/build/simple-stupid-synth.elf onto the Pico file system. The file system should then automatically disappear again, and the synthesizer immediately starts.

There is the script deploy.sh that tries to detect device path and mount path of the connected Pico. Just execute this script after connecting your Pico for deploying the compiled file. The script should work for any Debian-like distribution, but it has not been tested widely, so chances are high that it will not work in a less typical configuration environment. In this case, try to deploy manually as described above.

Running

The Pico should now appear as a MIDI device named Pico Simple Stupid Synth (if not, try disconnecting and reconnecting your USB cable). You can play any MIDI files, using your favorite MIDI player, just by selecting this MIDI device as MIDI ouput device in your MIDI player.

Known Issues

The code is prepared to also use PWM audio instead of I²S audio, such that an amplifier or high-impedance headphones can be connected just via a resistance to the PWM outputs. However, due to various bugs / limitations, PWM audio does not yet work:

• PWM stereo audio does not seem to work at all. Even the sound_wave.c example in the pico-playground, when modified to produce stereo output with identical left and right channel samples, will get stuck shortly after calling audio_pwm_set_enabled and clog the complete system.

• PWM mono audio basically works (as the sound_wave.c example demonstrates), but stops producing any output on the associated GPIO pin, as soon as the TinyUSB library is set up for creating a MIDI USB port.

• While I²S audio presumably works for a wide range of sample frequencies, according to my observation PWM mono audio behaves as if it was stuck at a fixed sample rate of around 57000 Hz. In contrast to I²S audio, the frequency parameter that is set in the audio_format structure does not seem to have any influence on the sample rate of PWM audio.

• Also note that the implementation of PWM audio in audio_pwm.c launches MCU core 1 to initiate DMA transfers. That is, if any other part of the software also tries to deploy core 1, a clash will occur, if no further provisions are taken. Maybe a clash like this is somehow related to the reason why PWM mono does not work any more as soon as setting up a MIDI port with the TinyUSB library?

License & Third-Party Code

The source code of the Pico Simple Stupid Synth is available under the conditions of the GNU General Public Licence Version 2. Note that this repository additionally contains files tusb_config.h and usb_descriptors.c based on third-party code, copyrighted by Ha Thach (tinyusb.org), licensed under the MIT License. The files pico_extras_import.cmake and pico_sdk_import.cmake are copied from the Pico SDK and are copyrighted by Raspberry Pi (Trading) Ltd.