A Synthesizer written in Rust / Embassy for the Raspberry Pi Pico 2 microcontroller.
A minimal but capable "hardware" synth that can be easily bodged together from cheap components (£8.66 per unit for the core components, see below). Once you've wired it up and flashed it you can connect a USB MIDI keyboard and an amp or headphones, and it's ready to play. The software in this repo gives you a very basic synth, but if you want something custom you can modify synth.rs to taste.
- I'm fairly inexpert at electronics, Rust and audio synthesis, maybe this simple guide will be helpful for other inexperts. If you're looking for something advanced or expert, look elsewhere!
- Some of the code is LLM-assisted. In particular the default
synth.rswas pretty much all spat out by an LLM.
Here's some instructions for building it on a solderless breadboard, like this:
You'll need:
- A Raspberry Pi Pico 2 with headers, currently £4.49 here
- a PCM5102 I2S module. These are available on Amazon quite cheaply, eg. right now I see a pack of three of them for £8.02. Look for something like the purple board to the right of the above photo
- A micro USB OTG adapter. I can see these for £2.99 for two on Amazon.
You'll also need a bunch of other stuff like the breadboard, dupont wires, soldering iron + solder, a midi USB keyboard, USB and jack cables, headphones or an amp to plug it into... if we handwave all of that away, as well as delivery costs and the fact we're bulk-buying, that gives us a cost of £8.66 per unit, for the main components anyway.
Connect the Pico to the PCM5102 I2S module. This will give us a stereo audio output via its onboard 3.5mm jack socket. The module comes with separate header pins you'll have to solder on, or you could just solder wires directly to the board.
Connect it up as follows:
BCK -> GPIO18 (aka pin 24)
LCK -> GPIO19 (aka pin 25)
DIN -> GPIO20 (aka pin 26)
GND -> 0v
VIN -> 5v
Because we're using the onboard USB port in host mode to connect a USB MIDI keyboard, we can't use it for power in.
In the breadboard photo, 5v power is supplied via the grey and purple dupont wires at the top. You can easily get 5v by cannibalizing a USB A cable. Then wire up the pico to the power rails as follows:
VBUS (pin 40) -> 5v
VSYS (pin 39) -> 5v
GND (pin 38) -> 0v
- The wiring above is for USB host mode, where the Pico powers a USB device. With this wiring you MUST NOT connect the Pico's USB port directly to a USB host, eg. your computer, as that'll mean both hosts are trying to power each other, and hardware damage may ensue. Instead we're going to connect a USB device via OTG, powered from the Pico's USB socket.
- I'm not actually sure connecting VBUS and VSYS is wholly safe / recommended. All I can say is it seems to work here with no issues, and the Pico is a pretty cheap device anyway. And this guy seems to think it's OK.
That's the audio output stuff connected, but we'll also need a way to actually play it:
Pretty simple, plug an OTG adapter into the Pico's USB port, then plug a MIDI USB keyboard into that. It works here with my Akai LPK25, hopefully it'll work with yours too.
This repo contains Rust software that reads from the USB MIDI keyboard and outputs notes via the PCM5102. You can also send MIDI controller events to modify the waveform, ADSR and filter.
The default synth code is pretty boring. It reads a buffer of incoming midi messages and outputs notes to an audio buffer. It should be pretty hackable if you want to make it do something more interesting.
At some point I'll provide a UF2 firmware file you can flash directly to the device. But for now you'll need to build the Rust code yourself before flashing the Pico.
So, you'll need Rust and Cargo if you don't have them already.
and the thumbv8m.main-none-eabihf target:
rustup target add thumbv8m.main-none-eabihfI recommend using a debug probe to flash the device. Certainly if you plan on doing any firmware modification. Given which, you'll also want probe-rs.
If you use a debug probe, in theory you should be able to just connect it up and
do cargo run from the repo's root folder. If you do that it'll build with
debug output, which will cause timing glitches. For "release mode", try
DEFMT_LOG=off cargo run --release.
I may well have forgotten things, in which case please file a github issue!
Side note: A bit inconveniently, embassy-rs doesn't support host mode usb or gapless audio "out-of-the-box", so I had to patch it for those. Thanks to holly-hacker and dobrowolski-lukasz for doing the actual work. Fortunately it turns out it's easy to update the cargo deps to automatically use crates from the patched branch, so I've done that now.
- Provide a UF2 flash file. So far I've been using a debug probe to flash the device, I guess people might want to flash it without building all the Rust stuff
- Audio input (so we can use it for effects as well as synthesis)
- Remaining in the "cheap amazon components" vibe, this could be a PCM1808 module.
- But, it'd be nice if there was an I2S module that did both audio input + output, maybe based on something like the TLV310AIC3104. I find it a bit surprising nobody seems to make one. Maybe I should try to make one.
- I think, very theoretically, it could be possible to build static / no-std
LV2 binaries for
thumbv8m.main-none-eabihfand run them on the device. Maybe via bFLT, which would be simpler to deal with than ELF on the device side. It'd be cool to be able to make hardware versions of all the thousands of open source LV2 pluginsl Maybe connecting them together in the device software, a bit like the much beefier MOD devices. But I'm not expecting this to be easy, or necessarily even doable at all.