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# Raspberry Pi Real-Time Audio Mixer
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RPi4B-based multi-channel real-time audio mixer with custom PREEMPT_RT kernel.
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[](https://www.python.org/)
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[](https://www.raspberrypi.com/products/raspberry-pi-4-model-b/)
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[](LICENSE)
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[](tests/)
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[](build/)
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A professional multi-channel real-time audio mixer running on a Raspberry Pi 4B.
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Sub-10ms round-trip latency with 16-channel USB audio, MIDI controller support,
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touchscreen UI, multi-track recording, live streaming, and a web control surface.
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## Features
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- **16-channel mixer** with 3-band EQ, compressor, gate, gain, pan, mute, solo per channel
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- **4 aux sends**, 2 subgroups, 2 VCA groups, master bus with dim and monitor
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- **MIDI controller support** — CC, NRPN, learn mode, clock sync, JACK MIDI bridge
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- **Multi-track recording** — per-channel WAV capture with write-ahead buffering, punch in/out
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- **Backing track player** — sync'd playback with metronome, loop/one-shot/segue modes
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- **Live streaming** — GStreamer H.264 to YouTube, Twitch, Facebook, or custom RTMP
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- **Session manager** — save/load mixer states, setlists with transitions, auto-save
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- **Touchscreen UI** — Kivy-based multi-touch interface for 5"/7" displays
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- **Web control surface** — FastAPI REST API + WebSocket real-time updates
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- **OSC server** — Open Sound Control for DAW integration (Ableton, Reaper, etc.)
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- **Plugin host** — LV2, VST2, NAM neural amp models via Carla rack
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- **Fader automation** — record, playback, scenes with interpolation
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- **SD card image builder** — automated build script produces ready-to-flash images
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## Architecture
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- **Platform:** Raspberry Pi 4 Model B (8GB RAM)
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- **OS:** Raspberry Pi OS Lite (64-bit, Bookworm) + custom PREEMPT_RT kernel
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- **Audio:** USB audio interface with multi-channel I/O
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- **Goal:** Low-latency (<10ms round-trip) multi-channel audio mixing
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```
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┌──────────────────────────────────────────────────────────────┐
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│ User Interfaces │
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│ ┌─────────────┐ ┌──────────────┐ ┌────────────────────┐ │
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│ │ Touch UI │ │ Web UI │ │ MIDI Controllers │ │
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│ │ (Kivy/HDMI) │ │ (FastAPI) │ │ (USB/ALSA seq) │ │
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│ └──────┬──────┘ └──────┬───────┘ └────────┬───────────┘ │
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│ │ │ │ │
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├─────────┼────────────────┼────────────────────┼──────────────┤
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│ │ REST/WS │ OSC │ MIDI/CC/NRPN │
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│ ┌──────▼────────────────▼────────────────────▼──────────┐ │
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│ │ Mixer Engine (Python) │ │
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│ │ ┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐ │ │
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│ │ │DSP Engine│ │Recording │ │ Streaming│ │ Session │ │ │
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│ │ │+ Carla │ │(WAV) │ │(GStream) │ │ Manager │ │ │
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│ │ └────┬─────┘ └──────────┘ └──────────┘ └──────────┘ │ │
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│ │ │ OSC │ │
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│ │ ┌────▼─────────────────────────────────────────────┐ │ │
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│ │ │ Carla Rack (LV2/VST/NAM) │ │ │
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│ │ └────────────────────┬─────────────────────────────┘ │ │
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│ └───────────────────────┼────────────────────────────────┘ │
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│ │ JACK │
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│ ┌─────▼──────┐ │
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│ │ JACK2 RT │ SCHED_FIFO prio 70 │
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│ └─────┬──────┘ │
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│ │ ALSA hw:USB │
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├──────────────────────────┼────────────────────────────────────┤
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│ Kernel │ │
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│ ┌───────────────────────▼──────────────────────────────┐ │
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│ │ PREEMPT_RT 6.12.y │ snd-usb-audio │ xHCI USB │ │
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│ └──────────────────────────────────────────────────────┘ │
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└──────────────────────────────────────────────────────────────┘
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```
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## Quick Start
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### Option 1: Download SD Card Image (Recommended)
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1. Download the latest `rpi-audio-mixer-*.img.xz` from [Releases](https://github.com/your-org/rpi-audio-mixer/releases)
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2. Flash to a 16GB+ SD card:
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```bash
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xz -d rpi-audio-mixer-*.img.xz
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sudo dd if=rpi-audio-mixer-*.img of=/dev/sdX bs=4M status=progress conv=fsync
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```
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3. Insert SD card into RPi 4B, connect USB audio interface, power on
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4. Follow the first-boot setup wizard on HDMI display
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5. Open `http://pi-mixer.local:8080` in your browser
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### Option 2: Build from Source
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```bash
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# Install build dependencies
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sudo apt install wget xz-utils dosfstools e2fsprogs qemu-user-static
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# Quick build (stock kernel, 10-15 min)
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./build/build.sh --skip-kernel
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# Full build with PREEMPT_RT kernel (30-45 min)
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./build/build.sh
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```
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See [docs/build-guide.md](docs/build-guide.md) for detailed instructions.
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### Option 3: Development Setup
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```bash
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# Install dependencies
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sudo apt install jackd2 carla python3-pip
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pip install fastapi uvicorn python-jack-client numpy
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# Run tests
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python -m pytest tests/ -v
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# Start the API server (without full hardware)
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python -m src.network.run
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```
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## Project Structure
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```
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├── docs/ Research, architecture decisions, manuals
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│ ├── research/ Technical research reports
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│ ├── decisions/ Architecture Decision Records (ADRs)
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│ └── manuals/ Hardware/software manuals and references
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├── src/ Source code (audio engine, DSP, UI)
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├── build/ Build scripts and cross-compilation tooling
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├── recipes/ Buildroot/Yocto recipes (if applicable)
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├── scripts/ Utility and deployment scripts
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└── wiki/ Local wiki pages
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├── src/
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│ ├── mixer/ DSP engine, channel strips, routing, OSC, automation
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│ ├── midi/ MIDI engine, learn mode, clock sync, device discovery
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│ ├── session/ Session manager, setlists, snapshots
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│ ├── recording/ Multi-track WAV recorder with punch in/out
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│ ├── backing/ Backing track player, metronome, transport
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│ ├── streaming/ GStreamer pipeline, platform presets, camera
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│ ├── network/ REST API, WebSocket, OSC server, auth
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│ ├── plugin/ Plugin manager, LV2/VST/NAM support, scanner
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│ └── ui/ Kivy touchscreen UI, widgets, screens
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├── tests/ 735 tests (pytest)
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├── build/ SD card image builder + first-boot scripts
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├── scripts/ Utility scripts (Carla presets, LV2 lint, NAM build)
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└── docs/ User and developer documentation
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```
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## Documentation
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| Document | Description |
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|----------|-------------|
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| [docs/build-guide.md](docs/build-guide.md) | Step-by-step SD card image build instructions |
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| [docs/user-manual.md](docs/user-manual.md) | Comprehensive mixer operation guide |
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| [docs/hardware-compatibility.md](docs/hardware-compatibility.md) | Tested USB interfaces, MIDI controllers, displays |
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| [docs/developer-guide.md](docs/developer-guide.md) | Architecture, contributing, source build |
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| [docs/troubleshooting.md](docs/troubleshooting.md) | Common issues and solutions |
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| [build/README.md](build/README.md) | Image builder reference |
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## System Requirements
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- **Computer:** Raspberry Pi 4 Model B (4GB+ RAM recommended)
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- **SD Card:** 16GB+ Class A2 (32GB+ recommended for recordings)
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- **Display:** HDMI touchscreen (official 7" or Waveshare 5") or headless via web UI
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- **Audio:** USB audio interface (class-compliant UAC2)
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- **Network:** Ethernet recommended; WiFi supported
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- **Power:** 5V/3A USB-C power supply
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## Performance
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| Metric | Value |
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|--------|-------|
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| Round-trip latency | <10ms (128 frames @ 48kHz) |
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| Channels | 16 mono / 8 stereo |
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| Sample rate | 48 kHz (44.1k and 96k supported) |
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| Bit depth | 32-bit float (internal), 16/24-bit WAV |
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| CPU usage (idle) | ~5% (all 16 channels driven) |
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| Recording tracks | Up to 16 simultaneous WAV |
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| Streaming | 1080p30 H.264 @ 4-8 Mbps |
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## Status
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**Phase 1-7: Complete audio mixer stack with SD card image builder**
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All planned phases complete:
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- [x] P1-R1: System Research — USB audio, PREEMPT_RT kernel analysis
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- [x] P1-R2: Base OS Selection — RPiOS Lite 64-bit + custom PREEMPT_RT
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- [x] P2-R1: ALSA + JACK2 Low-Latency Config
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- [x] P3-R1: Core Mixer Engine — DSP pipeline, channel strips, routing
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- [x] P3-R2: Backing Track Player — sync'd audio playback
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- [x] P3-R3: Multi-Track Recording — WAV capture + session
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- [x] P4-R1: MIDI Controller Support — Binding + Learn Mode
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- [x] P5-R3: Web App Frontend — touch-optimized mixer UI
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- [x] P6-R1: Streaming Pipeline — GStreamer H.264 to YouTube/Twitch
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- [x] P6-R2: Session Manager — save/load mixer states + setlists
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- [x] P7-R1: Build Script — automated SD card image creation
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## SD Card Image Builder
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```bash
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# Build a ready-to-flash SD card image
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./build/build.sh --skip-kernel # Quick build (10-15 min)
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./build/build.sh # Full build with RT kernel (30-45 min)
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```
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See [build/README.md](build/README.md) for flashing instructions and full documentation.
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## Getting Started
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*Documentation coming in Phase 2+*
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- [x] P1: System Research & OS Selection — RPiOS Lite + PREEMPT_RT
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- [x] P2: ALSA + JACK2 Low-Latency Configuration
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- [x] P3: Core Mixer Engine, Backing Tracks, Multi-Track Recording
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- [x] P4: MIDI Controller Support with Learn Mode
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- [x] P5: Web App + Touchscreen UI (Kivy)
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- [x] P6: Streaming Pipeline + Session Manager
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- [x] P7: SD Card Image Builder + Documentation
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## License
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TBD
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MIT
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@@ -0,0 +1,386 @@
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# Build Guide — SD Card Image Builder
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This guide walks through building a ready-to-flash Raspberry Pi SD card image
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containing the full audio mixer stack with a PREEMPT_RT kernel.
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## Prerequisites
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### Host System Requirements
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- **OS:** Any modern Linux distribution (x86_64) — Ubuntu 22.04+, Debian 12+, Fedora 38+
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- **CPU:** 4+ cores recommended (cross-compilation is parallel)
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- **RAM:** 4GB+ (8GB recommended for full PREEMPT_RT kernel build)
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- **Disk:** 15GB free space (build artifacts + downloaded OS image)
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- **Architecture:** x86_64 host for QEMU user-static chroot emulation
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- **macOS** is supported via a Linux VM or Docker container
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### Install Build Dependencies
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```bash
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# Ubuntu / Debian
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sudo apt update
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sudo apt install -y \
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wget xz-utils \
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dosfstools e2fsprogs \
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qemu-user-static \
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parted \
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git
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# For PREEMPT_RT kernel cross-compilation (optional)
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sudo apt install -y \
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gcc-aarch64-linux-gnu \
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build-essential \
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flex bison \
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libssl-dev \
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bc \
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rsync
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# Verify QEMU binary is registered
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sudo update-binfmts --enable qemu-aarch64
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```
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```bash
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# Fedora
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sudo dnf install -y wget xz dosfstools e2fsprogs qemu-user-static parted git
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```
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## Build Modes
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The build script supports three modes, controlled by command-line flags:
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| Command | Description | Time | Requires |
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|---------|-------------|------|----------|
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| `./build/build.sh --skip-kernel` | Quick build, stock RPi kernel | 10-15 min | Basic deps only |
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| `./build/build.sh` | Full build with PREEMPT_RT kernel | 30-45 min | + gcc-aarch64 + kernel deps |
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| `./build/build.sh --kernel-only` | Build only the RT kernel .deb | 10-15 min | + gcc-aarch64 + kernel deps |
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## Step-by-Step: Quick Build (Recommended First Build)
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This builds the mixer image using the stock Raspberry Pi OS kernel — fastest
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way to get a working image for testing. No cross-compilation toolchain needed.
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```bash
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# 1. Clone the repository
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git clone https://github.com/your-org/rpi-audio-mixer.git
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cd rpi-audio-mixer
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# 2. Run the quick build
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./build/build.sh --skip-kernel
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# 3. The compressed image lands here:
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ls -lh build/out/rpi-audio-mixer-*.img.xz
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```
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**What happens during the build:**
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1. Downloads Raspberry Pi OS Lite (64-bit, Bookworm) base image (~1.1 GB)
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2. Extracts and resizes the image (default 8 GB total)
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3. Mounts the image partitions via loopback
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4. Chroots into the ARM64 filesystem using QEMU user-static
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5. Installs audio packages (JACK2, Carla, ALSA tools, Python deps)
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6. Copies the mixer source code and systemd service files
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7. Configures real-time audio limits, CPU governor, udev rules
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8. Sets up first-boot wizard
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9. Unmounts and compresses the final image
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## Step-by-Step: Full PREEMPT_RT Build
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The full build includes a custom PREEMPT_RT kernel cross-compiled for aarch64
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on your x86_64 host. This is what you want for production use.
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```bash
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# 1. Ensure cross-compilation toolchain is installed
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sudo apt install -y gcc-aarch64-linux-gnu build-essential flex bison libssl-dev bc
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# 2. Run the full build
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./build/build.sh
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# This will:
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# a. Cross-compile linux-rpi-6.12.y with PREEMPT_RT patch
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# b. Package the kernel as a .deb
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# c. Build the SD card image with the RT kernel installed
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# d. Configure boot/config.txt to use the RT kernel
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# 3. Output
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ls -lh build/out/rpi-audio-mixer-*.img.xz
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```
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### Kernel Cross-Compilation Details
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The build script:
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- Clones `raspberrypi/linux` at branch `rpi-6.12.y`
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- Applies the `PREEMPT_RT` patch for 6.12
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- Uses `make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcm2711_defconfig`
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- Configures: `CONFIG_PREEMPT_RT=y`, `CONFIG_HZ=1000`
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- Builds with `make -j$(nproc)` for parallel compilation
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- Packages into a `.deb` using `make deb-pkg`
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## Build Customization
|
||||
|
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### Image Size
|
||||
|
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The default image is 8 GB. Adjust with `--image-size` (in MB):
|
||||
|
||||
```bash
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./build/build.sh --skip-kernel --image-size 16384 # 16 GB image
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||||
./build/build.sh --skip-kernel --image-size 32768 # 32 GB image
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||||
```
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||||
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||||
### Pre-configure WiFi
|
||||
|
||||
Set these environment variables before building:
|
||||
|
||||
```bash
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||||
MIXER_WIFI_SSID="MyNetwork" \
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||||
MIXER_WIFI_PASS="secret123" \
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./build/build.sh --skip-kernel
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||||
```
|
||||
|
||||
### Custom Hostname
|
||||
|
||||
```bash
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MIXER_HOSTNAME="stage-mixer" ./build/build.sh --skip-kernel
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||||
```
|
||||
|
||||
### Custom API Key for CI
|
||||
|
||||
```bash
|
||||
MIXER_API_KEY="my-pre-shared-key" ./build/build.sh --skip-kernel
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||||
```
|
||||
|
||||
### Custom OS Base Image
|
||||
|
||||
```bash
|
||||
RPIOS_URL="https://downloads.raspberrypi.com/raspios_lite_arm64/images/..." \
|
||||
./build/build.sh
|
||||
```
|
||||
|
||||
### Version Tagging
|
||||
|
||||
```bash
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||||
export MIXER_VERSION="$(git describe --tags --always)"
|
||||
./build/build.sh --skip-kernel
|
||||
```
|
||||
|
||||
## Cleaning Up
|
||||
|
||||
```bash
|
||||
# Remove all build artifacts
|
||||
./build/build.sh --clean
|
||||
|
||||
# This deletes:
|
||||
# build/work/ (mounted image, chroot state, temporary files)
|
||||
# build/downloads/ (cached OS image downloads)
|
||||
# build/out/ (built images)
|
||||
|
||||
# To preserve downloads (they're cached for faster rebuilds):
|
||||
rm -rf build/work/ build/out/
|
||||
```
|
||||
|
||||
## Image Contents
|
||||
|
||||
```
|
||||
Partition layout:
|
||||
├── /boot (FAT32, 256 MB)
|
||||
│ ├── kernel8.img Stock 64-bit kernel (fallback)
|
||||
│ ├── kernel8-rt.img PREEMPT_RT kernel (default if built)
|
||||
│ ├── config.txt GPU, boot, kernel selection
|
||||
│ ├── cmdline.txt Kernel parameters (isolcpus, threadirqs, etc.)
|
||||
│ └── *.dtb, overlays/ Device tree blobs
|
||||
│
|
||||
├── / (ext4, ~4 GB used of 6 GB)
|
||||
│ ├── /opt/rpi-mixer/ Mixer Python source + virtualenv
|
||||
│ ├── /etc/systemd/system/ Service unit files
|
||||
│ │ ├── cpu-performance.service
|
||||
│ │ ├── jackd.service
|
||||
│ │ ├── mixer-api.service
|
||||
│ │ ├── mixer-ui.service
|
||||
│ │ └── mixer-firstboot.service
|
||||
│ ├── /etc/security/limits.d/99-audio.conf
|
||||
│ ├── /etc/modprobe.d/alsa-usb.conf
|
||||
│ └── /home/pi/ Default user home
|
||||
│
|
||||
└── /data (ext4, remaining space)
|
||||
├── sessions/ Saved mixer session JSON files
|
||||
├── recordings/ Multi-track WAV recordings
|
||||
├── presets/ Plugin presets
|
||||
└── logs/ System and audio logs
|
||||
```
|
||||
|
||||
## Flashing to SD Card
|
||||
|
||||
### Linux
|
||||
|
||||
```bash
|
||||
# Decompress the image
|
||||
xz -d build/out/rpi-audio-mixer-*.img.xz
|
||||
|
||||
# Identify your SD card
|
||||
lsblk
|
||||
# Look for /dev/sdb or /dev/mmcblk0 (NOT your system drive!)
|
||||
|
||||
# Flash the image
|
||||
sudo dd if=build/out/rpi-audio-mixer-*.img \
|
||||
of=/dev/sdX \
|
||||
bs=4M \
|
||||
status=progress \
|
||||
conv=fsync
|
||||
|
||||
# Sync and eject
|
||||
sync
|
||||
sudo eject /dev/sdX
|
||||
```
|
||||
|
||||
### macOS
|
||||
|
||||
```bash
|
||||
# Decompress
|
||||
xz -d build/out/rpi-audio-mixer-*.img.xz
|
||||
|
||||
# Find the SD card
|
||||
diskutil list
|
||||
# Look for /dev/diskX (external, physical)
|
||||
|
||||
# Unmount
|
||||
diskutil unmountDisk /dev/diskX
|
||||
|
||||
# Flash (use rdisk for faster writes)
|
||||
sudo dd if=build/out/rpi-audio-mixer-*.img \
|
||||
of=/dev/rdiskX \
|
||||
bs=4m \
|
||||
status=progress
|
||||
|
||||
# Eject
|
||||
diskutil eject /dev/diskX
|
||||
```
|
||||
|
||||
### Windows
|
||||
|
||||
Use [Raspberry Pi Imager](https://www.raspberrypi.com/software/) or [balenaEtcher](https://www.balena.io/etcher/):
|
||||
|
||||
1. Decompress the `.img.xz` file with 7-Zip or WinRAR
|
||||
2. Open Raspberry Pi Imager → "Choose OS" → "Use custom"
|
||||
3. Select the `.img` file
|
||||
4. Choose your SD card and flash
|
||||
|
||||
## First Boot
|
||||
|
||||
1. Insert the SD card into a Raspberry Pi 4B
|
||||
2. Connect your USB audio interface
|
||||
3. (Optional) Connect HDMI touchscreen + Ethernet
|
||||
4. Power on
|
||||
|
||||
The **first-boot setup wizard** will appear on the HDMI display (or serial
|
||||
console). It walks through:
|
||||
|
||||
1. Audio interface detection and selection
|
||||
2. WiFi network configuration
|
||||
3. Hostname setting
|
||||
4. API key generation (for web UI access)
|
||||
5. JACK buffer size and latency preferences
|
||||
|
||||
After the wizard completes, the system reboots into normal operation.
|
||||
The wizard auto-disables itself — it only runs once.
|
||||
|
||||
### Default Access
|
||||
|
||||
| Service | URL / Command | Default Credentials |
|
||||
|---------|--------------|---------------------|
|
||||
| Web UI | `http://pi-mixer.local:8080` | API key from wizard |
|
||||
| SSH | `ssh pi@pi-mixer.local` | Password: `raspberry` |
|
||||
| Touch UI | HDMI display (auto-starts) | — |
|
||||
|
||||
**Important:** Change the default password immediately:
|
||||
```bash
|
||||
ssh pi@pi-mixer.local
|
||||
passwd
|
||||
```
|
||||
|
||||
### Service Management
|
||||
|
||||
```bash
|
||||
# Check all mixer services
|
||||
systemctl status jackd mixer-api mixer-ui
|
||||
|
||||
# View logs
|
||||
journalctl -u mixer-api -f
|
||||
journalctl -u jackd -f
|
||||
|
||||
# Restart services (must follow order: jackd → mixer-api → mixer-ui)
|
||||
sudo systemctl stop mixer-ui mixer-api jackd
|
||||
sudo systemctl start jackd
|
||||
sleep 2
|
||||
sudo systemctl start mixer-api mixer-ui
|
||||
```
|
||||
|
||||
## CI / Automated Builds
|
||||
|
||||
Build without a Raspberry Pi (CI-friendly) on any x86_64 host:
|
||||
|
||||
```bash
|
||||
export MIXER_VERSION="$(git describe --tags --always)"
|
||||
export MIXER_HOSTNAME="ci-mixer"
|
||||
export MIXER_API_KEY="ci-test-key-12345"
|
||||
|
||||
./build/build.sh --skip-kernel
|
||||
```
|
||||
|
||||
Example GitHub Actions / Gitea CI workflow:
|
||||
|
||||
```yaml
|
||||
- name: Build SD card image
|
||||
run: |
|
||||
sudo apt install -y wget xz-utils dosfstools e2fsprogs qemu-user-static
|
||||
./build/build.sh --skip-kernel
|
||||
|
||||
- name: Upload artifact
|
||||
uses: actions/upload-artifact@v4
|
||||
with:
|
||||
name: sd-card-image
|
||||
path: build/out/rpi-audio-mixer-*.img.xz
|
||||
```
|
||||
|
||||
## Kernel-Only Build
|
||||
|
||||
When you only need a new PREEMPT_RT kernel (e.g., to upgrade an existing
|
||||
image without rebuilding everything):
|
||||
|
||||
```bash
|
||||
# Build only the kernel .deb package
|
||||
./build/build.sh --kernel-only
|
||||
|
||||
# The .deb lands in build/out/
|
||||
ls build/out/linux-image-*.deb
|
||||
|
||||
# Install on the RPi:
|
||||
# scp build/out/linux-image-*.deb pi@pi-mixer.local:/tmp/
|
||||
# ssh pi@pi-mixer.local
|
||||
# sudo dpkg -i /tmp/linux-image-*.deb
|
||||
# sudo reboot
|
||||
```
|
||||
|
||||
## Troubleshooting Build Issues
|
||||
|
||||
| Problem | Solution |
|
||||
|---------|----------|
|
||||
| `qemu-aarch64-static not found` | `sudo apt install qemu-user-static && sudo update-binfmts --enable qemu-aarch64` |
|
||||
| `apt fails in chroot` | DNS not resolving — check host `/etc/resolv.conf` is accessible |
|
||||
| `No space left on device` | Increase image size: `--image-size 16384` |
|
||||
| `kernel build fails` | Install missing deps: `sudo apt install gcc-aarch64-linux-gnu flex bison libssl-dev bc` |
|
||||
| `loop device busy` | Run `sudo losetup -D` to detach stale loop devices |
|
||||
| `permission denied` | Build must run with passwordless sudo or as root |
|
||||
|
||||
For runtime issues after flashing, see [docs/troubleshooting.md](troubleshooting.md).
|
||||
|
||||
## Architecture Decision
|
||||
|
||||
This project uses **Raspberry Pi OS Lite (64-bit, Bookworm) + post-install
|
||||
configuration** rather than Buildroot or Yocto. Rationale:
|
||||
|
||||
- Full Debian package ecosystem for rapid audio stack iteration
|
||||
- `apt install` for JACK, Carla, debugging tools — no image rebuilds needed
|
||||
- Standard systemd-based system — familiar and well-documented
|
||||
- PREEMPT_RT kernel built via cross-compilation on x86 host
|
||||
- Buildroot/Yocto can be revisited for production appliance images
|
||||
|
||||
See [docs/research/base-os-decision.md](research/base-os-decision.md) for the
|
||||
full analysis.
|
||||
Reference in New Issue
Block a user