STM32H755 Granular Synthesizer

A real-time granular synthesizer running on the Cortex-M7 core of an STM32H755 dual-core MCU. Audio streams in and out over USB (UAC2 + MIDI composite device), with a browser-based control panel for parameter tweaking and monitoring.

What It Does

PC audio (YouTube, a DAW, anything) routes through the STM32 via USB Audio. The firmware captures incoming audio into a circular source buffer, then a 32-voice granular engine chops it into tiny overlapping grains with controllable density, size, spray, speed, and stereo spread. An FX chain (filter, ping-pong delay, Freeverb reverb) processes the output before sending it back to the PC over USB. A web UI provides sliders, transport controls, file playback, recording, and real-time performance telemetry.

Hardware

• Board: NUCLEO-H755ZI-Q (STM32H755ZIT6, Cortex-M7 @ 480 MHz)
• USB Audio/MIDI: CN13 (user USB port) — not the ST-LINK port
• Display: SSD1306 128x64 OLED via I2C1 (PB8/PB9)
• Power: SMPS only — do NOT configure for LDO or the board bricks

Features

• Granular engine: 32 simultaneous grains, Hann windowing, zero-crossing grain start, bipolar speed (-2x to +2x with reverse), speed scatter/randomize
• FX chain: Biquad LPF → stereo ping-pong delay (500ms max) → Freeverb
• USB composite device: UAC2 stereo 48 kHz/16-bit (in + out) + MIDI
• Web control panel: Real-time sliders, keyboard shortcuts, file playback, recording, preset import/export, performance telemetry
• OLED display: Grain visualization, CPU usage, active grain count
• Telemetry: 10 Hz MIDI CC stream with CPU %, active grains, peak/RMS levels, buffer stats

Architecture

PC Audio ──→ USB OUT ──→ Source Buffer (AXI SRAM, 250K samples)
                              │
                              ▼
                     32-voice Granular Engine
                              │
                              ▼
                    FX: LPF → Delay → Reverb
                              │
                              ▼
              USB IN ──→ PC Speakers (via PulseAudio loopback)
                              │
                              ▼
                     Web UI ←── MIDI CC telemetry

Memory Layout

Source Files

Build & Flash

Requires Docker (the toolchain runs in a container).

# Build and flash
docker compose run --rm dev /workspace/scripts/flash.sh

# USB must be on CN13 (user USB), not the ST-LINK port

The flash script builds with CMake + Ninja using arm-none-eabi-gcc, then flashes via OpenOCD.

Audio Routing

After flashing, set up PulseAudio loopback routing and the web server:

./scripts/audio-setup.sh           # set up routing + start web server
./scripts/audio-setup.sh --teardown # undo everything

This configures:

1. STM32 as the default PulseAudio sink (all PC audio routes through it)
2. Loopback from STM32 source back to PC speakers
3. Web server on http://localhost:8000

Web UI

Open http://localhost:8000 in Chrome (requires Web MIDI API support).

• Auto-connects to the Granular Synth MIDI device
• Transport: Record, Test Tone, File Playback, Loop
• Parameter sliders with click-to-edit values
• Keyboard shortcuts: Space (record), T (tone), P (play), S (stop), L (loop), Esc (stop all)
• Performance panel: CPU %, active grains, L/R level meters
• Preset import/export (JSON)

MIDI CC Map (Channel 1)

[README truncated]

Recent Activity

• 4cbaf8f reverb working nicely (2026-03-12)
• 1e14c82 added readme (2026-03-12)
• 1721a43 added fx see to work coul ddo with more testing (2026-03-11)
• 0c0c91b reverse grain playback with bidirectional speed slider.. (2026-03-11)
• d6af94b grain visulaiser working beautifully (2026-03-11)

Languages

• C: 51%
• HTML: 24%
• Assembly: 19%
• Shell: 4%
• CMake: 1%