Live site: yorev89.github.io/Poly-Synth
A homebrew polyphonic synthesizer built from scratch, based on a three-MCU architecture. This repository contains all firmware files and hardware resources.
| MCU | Role |
|---|---|
| STM32F411 | Synthesis engine, I2S audio (DIN MIDI input wired, currently disabled) |
| ESP32 #1 | Web UI, WiFi access point |
| ESP32 #2 | BLE MIDI keyboard interface |
- 8-voice polyphony with envelope-based voice scaling and click-free voice stealing
- Dual oscillators (Osc1 + Osc2) with 4 waveforms each: Sine, Sawtooth, Square, Triangle
- Band-limited wavetables (one per octave, 2048 samples): alias-free high notes
- Sub oscillator: Sine or Square, selectable at -1 or -2 octaves
- White noise generator with adjustable level
- Ring modulator — continuously blendable from additive mix to full ring modulation
- Osc2 independent level and detune control (0–100 cents)
- Portamento (legato glide) with adjustable time (0–500ms), triggers on legato playing only
- Pitch bend with adjustable range
- Volume ADSR with analog-style exponential curves; attack, decay and release 1 ms–3 s each (exponential control scale; release/decay time = time to fall 60 dB)
- Filter ADSR envelope (same curves and ranges) with adjustable amount, on/off
- Global resonant low-pass: 2-pole (12 dB/octave) state-variable filter
- Cutoff and resonance controls; filter envelope and LFO modulation (sweeps from 60 Hz up to 18 kHz)
- Two independent LFOs
- Rate: 0.1–20 Hz each
- Waveforms: Sine, Triangle, Square, Sawtooth Up, Sawtooth Down
- Modulation targets per LFO: pitch (vibrato), filter cutoff, amplitude (tremolo), Osc2 detune
- Stereo output: each voice is panned by pitch (low notes left, high right)
- Stereo delay (10–400ms) with time, feedback, and mix controls
- 4 delay presets (Slapback, Short, Medium, Rhythmic) plus Off; ping-pong mode
- Modulated stereo chorus
- 16 factory presets: Init/Default, Fat Bass, TB-303 Acid, Bell Chime, Synth Lead, Analog Pad, Robot Voice, String Ensemble, Metallic Pad, Pluck Bass, Wobble Bass, Sitar, Ambient Wash, Glide Bass, 808 Bass, Piano (tuned for MIDI Play)
- 12 user-saveable preset slots (stored in ESP32 non-volatile memory)
- Presets store every setting, including resonance, delay time/feedback, ping-pong and a per-preset output level; factory preset levels can be adjusted and saved from the web page (changing a level requires the volume passcode)
- WiFi access point — no app required, connect directly from any browser
- 4-tab web interface: Sound, Modulation, FX, Presets; the page always shows the synth's current settings (opening or reloading it never changes the sound)
- BLE MIDI keyboard interface — auto-discovers BLE MIDI devices, with preferred device priority and automatic fallback to any BLE MIDI device
- Web-settable volume ceiling for venue or parental control, protected by a 4-digit passcode enforced by the web controller
- MIDI keyboard volume (CC7) respects the web-set ceiling
- MIDI Play: a laptop app plays standard MIDI files on the synth over the web
controller's USB port (no Wi-Fi, ~1 ms timing), with sustain-pedal emulation.
See
MIDI-Play/
- Subsonic protection: 30 Hz output high-pass, and the sub-oscillator fades out below 45 Hz, so very low notes can't overdrive amplifiers and speakers
| File | Board | Also needs (same folder) |
|---|---|---|
Firmware/STM32/stm32_Synth_Engine.ino |
STM32F411 | wavetables.h (generated oscillator tables) |
Firmware/ESP32-Web/esp32_WEB_Controller.ino |
ESP32 #1 | secrets.h (your own, see below) |
Firmware/ESP32-BLE/esp32_BLE.ino |
ESP32 #2 | — |
MIDI-Play/midiplay.py |
Laptop (Python 3) | pip install mido pyserial |
The three firmware files are built with the Arduino IDE.
stats— toggles a once-per-second load report: worst-case render time against the 2.9 ms deadline, late buffers, active voicesclock— prints the clock configuration and the actual sample rate
The web controller reads its Wi-Fi network name, Wi-Fi password and default
volume passcode from secrets.h, which is deliberately not in this
repository. Copy secrets.example.h to secrets.h in the same folder and set
your own values (Wi-Fi password at least 8 characters, passcode 4 digits).
Without secrets.h the sketch still builds using the public defaults, and
warns you at compile time and on the serial monitor.
- Core: STM32duino (STM32 MCU based boards)
- Board: Generic STM32F4 series, Board part number: BlackPill F411CE (uses the board's 25 MHz crystal for accurate tuning)
- Tools → Optimize: Fastest (-O3). This is required: with the default "Smallest (-Os)" the synthesis loop cannot keep up with 8 voices and the audio distorts.
Hardware complete and verified. Firmware is playable and in active development. Phase 2 bug fixing is complete (tuning, CPU load, clicks, voice stealing, pitch bend, release tails, BLE MIDI parsing, volume lock); the freed processor time went into stereo output, a modulated chorus, ping-pong delay, a resonant filter, alias-free oscillators, analog-style envelopes and rebuilt presets. MIDI Play (phase 1) plays MIDI files from a laptop. Next: hardware upgrades (first candidate: the 5-pin DIN MIDI input).