Cinderwave

An open-hardware acid groovebox that fits on a $4 microcontroller.

Cinderwave is a monophonic synth voice and 16-step sequencer in the TB-303 idiom — band-limited oscillators, a resonant zero-delay-feedback filter, per-step accent and slide — running on a Raspberry Pi Pico (RP2040). The entire DSP core is platform-independent C++17 with no dynamic allocation, no exceptions, and no dependencies, so the exact same code that drives the hardware also renders a .wav on your desktop.

   ┌───────────┐   ┌──────────┐   ┌──────────────┐   ┌─────────┐
   │ 16-step   │──▶│ Oscillator│──▶│ SVF filter   │──▶│ VCA     │──▶ out
   │ sequencer │   │ saw/sqr/  │   │ (cutoff env  │   │ (amp    │
   │ note/gate/│   │ tri/sine  │   │  + resonance)│   │  env)   │
   │ accent/   │   │ PolyBLEP  │   └──────▲───────┘   └────▲────┘
   │ slide     │   └───────────┘   filter env          amp env
   └───────────┘

Why it's fun

• Band-limited oscillators — PolyBLEP saw and square, so the high notes don't alias into mush on a cheap DAC.
• Cytomic TPT state-variable filter — the good resonant filter, stable and self-oscillating across the whole cutoff range.
• Real acid behavior — accent boosts amplitude and opens the filter; slide portamentos between steps without retriggering the envelope.
• One codebase, two worlds — flash it to a Pico, or cmake && ctest it on your laptop and bake the demo pattern to audio.

Quick start (no hardware needed)

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j
ctest --test-dir build --output-on-failure     # DSP unit tests
./build/cinderwave_render out.wav              # bake the demo acid line to WAV

cinderwave_render writes ~8 seconds of the built-in demo pattern (a driving 130 BPM bassline with accents and slides) to a 16-bit mono WAV.

Build the hardware

Full bill of materials, GPIO pin map, and two audio-output options (a budget PWM + RC filter, or a PCM5102A I2S DAC) live in hardware/BUILD.md.

Minimum viable Cinderwave:

Flash the firmware

cd firmware/rp2040
export PICO_SDK_PATH=/path/to/pico-sdk
cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build -j

Hold BOOTSEL while plugging in the Pico, then drop build/cinderwave_firmware.uf2 onto the RPI-RP2 drive. It boots straight into the demo pattern; the three pots take over cutoff, resonance, and tempo, and the button toggles playback.

Layout

include/cinderwave/   Public API — the frozen DSP contract (headers only)
src/                  DSP core: oscillator, envelope, filter, voice, sequencer, synth
host/                 Desktop WAV renderer (uses the identical core)
firmware/rp2040/      Pico SDK target: PWM audio + pot/button controls
tests/                Dependency-free unit tests (built and run in CI)
hardware/             Build guide, BOM, wiring, pin map

Continuous integration

Every push builds the core, runs the unit tests, renders the demo WAV, and cross-compiles the RP2040 .uf2 — both artifacts are uploaded from the run. See .github/workflows/ci.yml.

License

MIT — see LICENSE.