Raspberry Pi Real-Time Audio Mixer

A professional multi-channel real-time audio mixer running on a Raspberry Pi 4B. Sub-10ms round-trip latency with 16-channel USB audio, MIDI controller support, touchscreen UI, multi-track recording, live streaming, and a web control surface.

Features

• 16-channel mixer with 3-band EQ, compressor, gate, gain, pan, mute, solo per channel
• 4 aux sends, 2 subgroups, 2 VCA groups, master bus with dim and monitor
• MIDI controller support — CC, NRPN, learn mode, clock sync, JACK MIDI bridge
• Multi-track recording — per-channel WAV capture with write-ahead buffering, punch in/out
• Backing track player — sync'd playback with metronome, loop/one-shot/segue modes
• Live streaming — GStreamer H.264 to YouTube, Twitch, Facebook, or custom RTMP
• Session manager — save/load mixer states, setlists with transitions, auto-save
• Touchscreen UI — Kivy-based multi-touch interface for 5"/7" displays
• Web control surface — FastAPI REST API + WebSocket real-time updates
• OSC server — Open Sound Control for DAW integration (Ableton, Reaper, etc.)
• Plugin host — LV2, VST2, NAM neural amp models via Carla rack
• Fader automation — record, playback, scenes with interpolation
• SD card image builder — automated build script produces ready-to-flash images

Architecture

┌──────────────────────────────────────────────────────────────┐
│                     User Interfaces                          │
│  ┌─────────────┐  ┌──────────────┐  ┌────────────────────┐  │
│  │ Touch UI    │  │ Web UI       │  │ MIDI Controllers   │  │
│  │ (Kivy/HDMI) │  │ (FastAPI)    │  │ (USB/ALSA seq)     │  │
│  └──────┬──────┘  └──────┬───────┘  └────────┬───────────┘  │
│         │                │                    │              │
├─────────┼────────────────┼────────────────────┼──────────────┤
│         │       REST/WS  │        OSC         │ MIDI/CC/NRPN │
│  ┌──────▼────────────────▼────────────────────▼──────────┐   │
│  │              Mixer Engine (Python)                     │   │
│  │  ┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐  │   │
│  │  │DSP Engine│ │Recording │ │ Streaming│ │ Session  │  │   │
│  │  │+ Carla   │ │(WAV)     │ │(GStream) │ │ Manager  │  │   │
│  │  └────┬─────┘ └──────────┘ └──────────┘ └──────────┘  │   │
│  │       │ OSC                                             │   │
│  │  ┌────▼─────────────────────────────────────────────┐  │   │
│  │  │              Carla Rack (LV2/VST/NAM)            │  │   │
│  │  └────────────────────┬─────────────────────────────┘  │   │
│  └───────────────────────┼────────────────────────────────┘   │
│                          │ JACK                               │
│                    ┌─────▼──────┐                              │
│                    │  JACK2 RT  │  SCHED_FIFO prio 70         │
│                    └─────┬──────┘                              │
│                          │ ALSA hw:USB                        │
├──────────────────────────┼────────────────────────────────────┤
│  Kernel                  │                                    │
│  ┌───────────────────────▼──────────────────────────────┐     │
│  │  PREEMPT_RT 6.12.y  │  snd-usb-audio  │  xHCI USB   │     │
│  └──────────────────────────────────────────────────────┘     │
└──────────────────────────────────────────────────────────────┘

Quick Start

Option 1: Download SD Card Image (Recommended)

1. Download the latest rpi-audio-mixer-*.img.xz from Releases
2. Flash to a 16GB+ SD card:

   xz -d rpi-audio-mixer-*.img.xz
   sudo dd if=rpi-audio-mixer-*.img of=/dev/sdX bs=4M status=progress conv=fsync
   

3. Insert SD card into RPi 4B, connect USB audio interface, power on
4. Follow the first-boot setup wizard on HDMI display
5. Open http://pi-mixer.local:8080 in your browser

Option 2: Build from Source

# Install build dependencies
sudo apt install wget xz-utils dosfstools e2fsprogs qemu-user-static

# Quick build (stock kernel, 10-15 min)
./build/build.sh --skip-kernel

# Full build with PREEMPT_RT kernel (30-45 min)
./build/build.sh

See docs/build-guide.md for detailed instructions.

Option 3: Development Setup

# Install dependencies
sudo apt install jackd2 carla python3-pip
pip install fastapi uvicorn python-jack-client numpy

# Run tests
python -m pytest tests/ -v

# Start the API server (without full hardware)
python -m src.network.run

Project Structure

├── src/
│   ├── mixer/          DSP engine, channel strips, routing, OSC, automation
│   ├── midi/           MIDI engine, learn mode, clock sync, device discovery
│   ├── session/        Session manager, setlists, snapshots
│   ├── recording/      Multi-track WAV recorder with punch in/out
│   ├── backing/        Backing track player, metronome, transport
│   ├── streaming/      GStreamer pipeline, platform presets, camera
│   ├── network/        REST API, WebSocket, OSC server, auth
│   ├── plugin/         Plugin manager, LV2/VST/NAM support, scanner
│   └── ui/             Kivy touchscreen UI, widgets, screens
├── tests/              735 tests (pytest)
├── build/              SD card image builder + first-boot scripts
├── scripts/            Utility scripts (Carla presets, LV2 lint, NAM build)
└── docs/               User and developer documentation

Documentation

Document	Description
docs/build-guide.md	Step-by-step SD card image build instructions
docs/user-manual.md	Comprehensive mixer operation guide
docs/hardware-compatibility.md	Tested USB interfaces, MIDI controllers, displays
docs/developer-guide.md	Architecture, contributing, source build
docs/troubleshooting.md	Common issues and solutions
build/README.md	Image builder reference

System Requirements

• Computer: Raspberry Pi 4 Model B (4GB+ RAM recommended)
• SD Card: 16GB+ Class A2 (32GB+ recommended for recordings)
• Display: HDMI touchscreen (official 7" or Waveshare 5") or headless via web UI
• Audio: USB audio interface (class-compliant UAC2)
• Network: Ethernet recommended; WiFi supported
• Power: 5V/3A USB-C power supply

Performance

Metric	Value
Round-trip latency	<10ms (128 frames @ 48kHz)
Channels	16 mono / 8 stereo
Sample rate	48 kHz (44.1k and 96k supported)
Bit depth	32-bit float (internal), 16/24-bit WAV
CPU usage (idle)	~5% (all 16 channels driven)
Recording tracks	Up to 16 simultaneous WAV
Streaming	1080p30 H.264 @ 4-8 Mbps

Status

All planned phases complete:

• P1: System Research & OS Selection — RPiOS Lite + PREEMPT_RT
• P2: ALSA + JACK2 Low-Latency Configuration
• P3: Core Mixer Engine, Backing Tracks, Multi-Track Recording
• P4: MIDI Controller Support with Learn Mode
• P5: Web App + Touchscreen UI (Kivy)
• P6: Streaming Pipeline + Session Manager
• P7: SD Card Image Builder + Documentation

License

MIT