# Carla Integration — Plugin Host for RPi4B Audio Mixer

**Date:** 2026-05-19
**Task:** P2-R2
**Parent:** P2-R1 (ALSA + JACK2 Low-Latency Config)
**Target Platform:** Raspberry Pi 4B, RPi OS Lite 64-bit (Bookworm), PREEMPT_RT 6.12.y

---

## Table of Contents

1. [Architecture Overview](#1-architecture-overview)
2. [Build Carla from Source](#2-build-carla-from-source)
3. [Build Neural Amp Modeler (NAM) LV2](#3-build-neural-amp-modeler-nam-lv2)
4. [Carla Configuration](#4-carla-configuration)
5. [Default Mixer Rack & Patchbay](#5-default-mixer-rack--patchbay)
6. [JACK Auto-Connection](#6-jack-auto-connection)
7. [Plugin Compatibility Testing](#7-plugin-compatibility-testing)
8. [Preset & Program Management](#8-preset--program-management)
9. [Performance Tuning for RPi4B](#9-performance-tuning-for-rpi4b)
10. [Operational Procedures](#10-operational-procedures)
11. [Troubleshooting](#11-troubleshooting)
12. [File Inventory](#12-file-inventory)

---

## 1. Architecture Overview

Carla sits as the plugin host layer between JACK and the audio applications:

```
┌──────────────────────────────────────────────────────────────┐
│  User Space                                                    │
│                                                                │
│  ┌────────────────────────────────────────────────────┐       │
│  │  Carla Plugin Host (carla)                          │       │
│  │  ┌──────────────────────────────────────────────┐   │       │
│  │  │  Rack Mode — 8-channel mixer                  │   │       │
│  │  │  ┌──────┐ ┌──────┐ ┌──────┐ ┌──────────┐    │   │       │
│  │  │  │Ch1   │ │Ch2   │ │Ch3   │ │AUX 1/2   │    │   │       │
│  │  │  │Gate→ │ │Gate→ │ │NAM→  │ │Reverb    │    │   │       │
│  │  │  │EQ→   │ │EQ→   │ │IR    │ │Delay     │    │   │       │
│  │  │  │Comp  │ │Comp  │ │      │ │          │    │   │       │
│  │  │  └──┬───┘ └──┬───┘ └──┬───┘ └────┬─────┘    │   │       │
│  │  │     │        │        │           │          │   │       │
│  │  │     └────────┼────────┼───────────┘          │   │       │
│  │  │              │        │                      │   │       │
│  │  │         ┌────▼────────▼────┐                 │   │       │
│  │  │         │  Master Limiter  │                 │   │       │
│  │  │         └────────┬────────┘                 │   │       │
│  │  └──────────────────┼──────────────────────────┘   │       │
│  │                     │                              │       │
│  │  ┌──────────────────▼──────────────────────────┐   │       │
│  │  │  Patchbay — dynamic signal routing           │   │       │
│  │  │  (system capture → plugin inputs)            │   │       │
│  │  │  (plugin outputs → system playback)          │   │       │
│  │  └──────────────────┬──────────────────────────┘   │       │
│  └─────────────────────┼──────────────────────────────┘       │
│                        │                                      │
│  ┌─────────────────────▼──────────────────────────────┐       │
│  │  JACK2 (jackd) — SCHED_FIFO prio 70-80             │       │
│  │  ALSA backend (hw:USB), 48kHz, 128f/3p, 8ms        │       │
│  └─────────────────────┬──────────────────────────────┘       │
├─────────────────────────┼──────────────────────────────────────┤
│  Kernel                 │                                      │
│  snd-usb-audio (nrpacks=1) → xHCI (IRQ prio 95)              │
│  PREEMPT_RT 6.12.y, cores 1-3 isolated for audio              │
└─────────────────────────┼──────────────────────────────────────┘
                          │
                 ┌────────▼────────┐
                 │  USB Audio I/F  │
                 │  (class UAC2)   │
                 └─────────────────┘
```

### Plugin Format Support

| Format | Carla Support | Build Requirement |
|--------|--------------|-------------------|
| LV2    | ✓ Native     | lv2-dev, lilv    |
| LADSPA | ✓ Native     | Built-in         |
| DSSI   | ✓ Native     | Built-in         |
| VST2   | ✓ Via bridge | VST2 SDK (optional) |
| VST3   | ✓ Via bridge | VST3 SDK (/opt/vst3sdk) |
| SF2/SFZ| ✓ Native     | libfluidsynth-dev |
| NAM    | ✓ LV2 host   | See §3 below     |

### Key Design Decisions

| Parameter | Value | Rationale |
|-----------|-------|-----------|
| Carla process mode | MultipleClients | Each plugin = separate JACK client for independent patching |
| Default channels | 8 (expandable) | 2 mic/line, 1 guitar, 2 aux, master |
| Plugin format priority | LV2 | Native Linux, no bridge overhead, best ARM support |
| GUI toolkit | PyQt5 (headless supported) | Carla can run without GUI on RPi4B |
| ARM optimization | Cortex-A72 + NEON + LTO | -march=armv8-a+crc+crypto -mtune=cortex-a72 |
| Transport sync | Disabled | Mixer doesn't need DAW transport |

---

## 2. Build Carla from Source

### Prerequisites

- Working JACK2 setup (see parent task P2-R1: `docs/audio-stack-config.md`)
- ~500MB free disk space for build
- Internet connection for dependency download
- **Run directly on the Raspberry Pi 4B** (not cross-compiled)

### Build

```bash
cd /home/pi/raspberry-pi-mixer

# One-command build
chmod +x scripts/carla-build.sh
./scripts/carla-build.sh --install
```

Estimated build time: 30-60 minutes on RPi4B (4 cores).

### What gets built

The script:
1. Installs all build dependencies (Qt5, JACK, ALSA, FFmpeg, FluidSynth, OSC, etc.)
2. Clones Carla from `https://github.com/falkTX/Carla.git`
3. Patches the Makefile to add ARM Cortex-A72 optimization flags (`-march=armv8-a+crc+crypto -mtune=cortex-a72`)
4. Builds with `WITH_LTO=true` for smaller, faster binary
5. Optionally installs to `/usr/local`

### Cortex-A72 Optimization

The Makefile patch adds ARM64-specific flags to `source/Makefile.mk`:

```makefile
ifeq ($(CPU_ARM64_OR_AARCH64),true)
BASE_OPTS += -march=armv8-a+crc+crypto -mtune=cortex-a72
endif
```

The original Makefile only has NEON flags for ARM32 (`Cortex-A7`). ARM64/aarch64 uses the compiler's default NEON support (always available on ARMv8-A).

### VST3 Support

VST3 plugin hosting requires the Steinberg VST3 SDK:

```bash
sudo git clone --recursive https://github.com/steinbergmedia/vst3sdk.git /opt/vst3sdk
```

Then rebuild Carla. Without the SDK, LV2, LADSPA, and DSSI still work.

### Verification

```bash
# Check binary
file /usr/local/bin/carla
# Expected: ELF 64-bit LSB executable, ARM aarch64

# Check features
carla --help 2>&1 | head -20

# Test with JACK
sudo audio-stack-start
carla &
```

---

## 3. Build Neural Amp Modeler (NAM) LV2

NAM uses the RTNeural library (real-time neural inference, header-only C++) wrapped in
an LV2 plugin. It does NOT require libtorch/PyTorch.

### Build

```bash
cd /home/pi/raspberry-pi-mixer
chmod +x scripts/nam-build.sh
./scripts/nam-build.sh --install
```

### Critical: Model Sizes Matter

| Model Size | Parameters | RPi4B Feasibility | xrun Risk at 128f |
|------------|-----------|-------------------|-------------------|
| Standard   | ~500K+    | ✗ No              | Guaranteed xruns |
| Feather    | ~100-200K | ⚠ Marginal       | Occasional       |
| Nano       | ~30-50K   | ✓ Yes             | Low              |
| Micro      | ~10-20K   | ✓ Yes             | Very low         |

**For RPi4B, use nano or feather models from [Tone3000](https://www.tone3000.com/search?sizes=feather).**

```bash
# Download models
mkdir -p ~/nam-models
# Then download .nam files from Tone3000 and place them in ~/nam-models/
```

### NAM + IR Chain in Carla

The default rack preset places NAM in Channel 3 followed by an IR loader
for cabinet simulation:

```
system:capture_3 → [Ch3 NAM] → [Ch3 IR Loader] → Master → system:playback_1/2
```

---

## 4. Carla Configuration

### Configuration File

Carla's settings are stored at `~/.config/falkTX/Carla.conf` (JSON-format).

Key settings for our mixer setup:

```json
{
  "Main": {
    "AudioDriver": "JACK",
    "ProcessMode": "MultipleClients",
    "TransportMode": "Disabled"
  },
  "JACK": {
    "AutoConnectPorts": false,
    "RackExportPorts": true
  },
  "LV2": {
    "UIbridges": false,
    "BridgedWine": false
  }
}
```

The `scripts/carla-jack-autoconnect` script handles port wiring instead of
Carla's built-in auto-connect, giving us precise control.

### Environment Variables

```bash
# For headless operation (RPi4B without display)
export CARLA_BRIDGE_DUMMY=1       # Disable plugin UIs
export QT_QPA_PLATFORM=offscreen  # Qt offscreen rendering

# Performance
export JACK_NO_AUDIO_OUT=0        # Don't mute JACK when jackd starts
```

### Carla Startup with the Rack

```bash
# Start the full audio stack
sudo audio-stack-start

# Launch Carla with the default mixer rack
carla config/carla-8ch-default.carxp &

# Auto-wire JACK ports
./scripts/carla-jack-autoconnect --watch
```

---

## 5. Default Mixer Rack & Patchbay

### Rack Layout (`config/carla-8ch-default.carxp`)

```
Plugin Chain (signal flow top-to-bottom):
─────────────────────────────────────────
Channel 1:  Gate → 12-Band EQ → Compressor
Channel 2:  Gate → 8-Band EQ  → Compressor
Channel 3:  NAM  → IR Loader (guitar)
AUX 1:      Hall Reverb
AUX 2:      Vintage Delay
Master:     Limiter (ceiling -1.0 dBFS)
─────────────────────────────────────────
```

All plugins use Calf Studio Gear LV2 suite for CPU-efficient DSP on ARM.

### Patchbay Rules (`config/carla-patchbay.xml`)

```
system:capture_1 → Ch1 Gate input
system:capture_2 → Ch2 Gate input
system:capture_3 → Ch3 NAM input
Master Limiter out 1 → system:playback_1
Master Limiter out 2 → system:playback_2
```

### Generating Custom Racks

```bash
# 4-channel minimal rack
python3 scripts/carla-preset-gen.py --channels 4

# Patchbay only
python3 scripts/carla-preset-gen.py --patchbay
```

### Adding/Removing Plugins

Edit the rack in Carla's GUI, or modify `scripts/carla-preset-gen.py` and regenerate.
The preset uses standard LV2 URIs:
- `http://calf.sourceforge.net/plugins/Gate`
- `http://calf.sourceforge.net/plugins/Equalizer12Band`
- `http://calf.sourceforge.net/plugins/Equalizer8Band`
- `http://calf.sourceforge.net/plugins/Compressor`
- `http://calf.sourceforge.net/plugins/Reverb`
- `http://calf.sourceforge.net/plugins/VintageDelay`
- `http://calf.sourceforge.net/plugins/Limiter`
- `http://github.com/mikeoliphant/neural-amp-modeler-lv2`

---

## 6. JACK Auto-Connection

### One-shot Connection

```bash
./scripts/carla-jack-autoconnect
```

Connects all currently available Carla ports according to routing rules.

### Persistent Auto-Connect Daemon

```bash
./scripts/carla-jack-autoconnect --watch
```

Monitors JACK for new Carla plugin ports and wires them automatically.
Run this in a tmux session or as a systemd user service:

```ini
# ~/.config/systemd/user/carla-autoconnect.service
[Unit]
Description=Carla JACK Auto-Connect
After=jackd.service

[Service]
ExecStart=/home/pi/raspberry-pi-mixer/scripts/carla-jack-autoconnect --watch
Restart=on-failure

[Install]
WantedBy=default.target
```

```bash
systemctl --user daemon-reload
systemctl --user enable --now carla-autoconnect.service
```

### Dry Run

```bash
./scripts/carla-jack-autoconnect --dry-run
```

Shows what would be connected without making changes.

### Disconnect All Carla Ports

```bash
./scripts/carla-jack-autoconnect --disconnect-all
```

---

## 7. Plugin Compatibility Testing

### LV2 Lint Validation

The `lv2lint` tool checks LV2 plugin bundles for spec compliance:

```bash
chmod +x scripts/lv2lint-check.sh

# Check all installed LV2 plugins
./scripts/lv2lint-check.sh

# Check only NAM
./scripts/lv2lint-check.sh --nam-only

# Check a specific plugin
./scripts/lv2lint-check.sh /usr/lib/lv2/calf.lv2

# List all installed plugins
./scripts/lv2lint-check.sh --list
```

### What gets checked

| Check | Description |
|-------|-------------|
| Manifest parsing | Valid `manifest.ttl` exists |
| Binary architecture | Must be ARM aarch64 (not x86) |
| Shared library deps | All `.so` dependencies resolvable |
| URI validity | Unique, well-formed plugin URI |
| Port definitions | Valid audio/CV/control port specs |
| Parameter ranges | min/max/default values defined |
| atom:Path support | Critical for NAM model selection |
| Carla extensions | Carla-specific features if present |

### ARM-Specific Checks

- Architecture verification: ensures no x86 binaries are present
- NEON SIMD detection: checks if plugins use NEON optimization
- CPU feature requirements: verifies no SSSE3/AVX instructions

### Runtime Testing Procedure

```bash
# 1. Start JACK
sudo audio-stack-start

# 2. Run Carla with test rack
carla config/carla-8ch-default.carxp &

# 3. Auto-connect
sleep 2 && ./scripts/carla-jack-autoconnect

# 4. Check for xruns
watch -n 1 "jack_cpu_load; echo '---'; tail -5 /tmp/jackd.log | grep -i xrun || echo 'no xruns'"

# 5. Verify audio path
jack_lsp -c | grep -E "(Carla|system)"

# 6. Test each plugin bypass
# Send MIDI CC or use OSC to toggle plugin bypass
```

---

## 8. Preset & Program Management

### CLI Interface

```bash
python3 scripts/carla-preset-manager.py [command]
```

| Command | Description |
|---------|-------------|
| `list` | List all saved presets |
| `save <name>` | Save current Carla state as preset |
| `load <name>` | Load a preset (overwrites current state) |
| `export <name> <file>` | Export preset + models as .tar.gz bundle |
| `import <bundle>` | Import a preset bundle |
| `set-model <plugin> <path>` | Set NAM model path via OSC |

### Preset Storage

- **User presets:** `~/carla-presets/*.carxp`
- **System presets:** `config/*.carxp` (in repo)
- **Metadata:** `~/carla-presets/*.meta.json`
- **Models:** `~/nam-models/*.nam`

### Workflow: Save and Restore

```bash
# Tweak your rack in Carla GUI, then:
python3 scripts/carla-preset-manager.py save "my-live-set"

# Later, restore:
python3 scripts/carla-preset-manager.py load "my-live-set"
# Restart Carla to apply
```

### Workflow: Export a Full Bundle

```bash
# Export preset with model references
python3 scripts/carla-preset-manager.py export "my-live-set" /tmp/my-set.tar.gz

# Transfer to another RPi4B
scp /tmp/my-set.tar.gz pi@other-rpi:/tmp/
ssh pi@other-rpi "python3 ~/raspberry-pi-mixer/scripts/carla-preset-manager.py import /tmp/my-set.tar.gz"
```

---

## 9. Performance Tuning for RPi4B

### CPU Governor

```bash
# Must be 'performance' for audio
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# Expected: performance
```

### CPU Isolation

From parent task — cores 1-3 isolated for audio via `isolcpus=1-3` in cmdline.txt.
Carla plugins run on isolated cores. Check with:

```bash
taskset -cp $(pgrep carla)
# Expected: affinity list includes 1-3
```

### Carla Process Priority

After Carla starts:

```bash
# Set Carla to SCHED_FIFO priority (JACK client)
sudo chrt -f -p 80 $(pgrep carla)
```

### NAM Performance Tuning

For the NAM plugin specifically:

1. **Use nano/feather models** — standard models cause xruns at any buffer
2. **Increase buffer if needed** — edit `/etc/jackdrc` and change `-p 128` to `-p 256`
3. **Disable GUI updates** — Carla GUI redraws consume CPU
4. **Bypass unused plugins** — Carla's bypass is true DSP bypass (zero CPU)

### Monitoring

```bash
# Real-time CPU load
jack_cpu_load

# Per-process DSP load
top -H -p $(pgrep carla)

# Xrun count
grep -c xrun /tmp/jackd.log
```

### Expected Performance

| Configuration | CPU Load (idle) | CPU Load (8ch active) | Headroom |
|--------------|-----------------|-----------------------|----------|
| No NAM, all Calf | ~8% | ~22% | 78% |
| 1x Nano NAM | ~12% | ~35% | 65% |
| 1x Feather NAM | ~18% | ~52% | 48% |
| 1x Standard NAM | ~45% | ~95% | **xruns** |

---

## 10. Operational Procedures

### Startup Sequence

```bash
#!/bin/bash
# Full audio stack startup with Carla

# 1. Start JACK + ALSA
sudo audio-stack-start

# 2. Wait for JACK to stabilize
sleep 3

# 3. Set CPU governor
echo performance | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

# 4. Launch Carla with default rack
carla config/carla-8ch-default.carxp &
CARLA_PID=$!

# 5. Set realtime priority
sleep 2
sudo chrt -f -p 80 $CARLA_PID 2>/dev/null || true

# 6. Auto-connect JACK ports
./scripts/carla-jack-autoconnect &
AUTOCONNECT_PID=$!

echo "Audio stack running. Carla PID: $CARLA_PID"
```

### Shutdown Sequence

```bash
#!/bin/bash
# Graceful shutdown

# 1. Disconnect all Carla ports
./scripts/carla-jack-autoconnect --disconnect-all

# 2. Stop Carla
pkill -f carla || true

# 3. Stop JACK
sudo audio-stack-stop
```

### Headless Operation

For RPi4B without a display, Carla can operate headlessly:

```bash
# Run Carla without GUI
QT_QPA_PLATFORM=offscreen carla config/carla-8ch-default.carxp &

# Control via OSC
carla-control --osc-port 22752 set_parameter "Ch1 EQ" 0 0.5
```

### OSC Remote Control

Carla exposes all parameters via OSC on port 22752 (configurable):

```bash
# List plugins
oscsend localhost 22752 /Carla/refresh

# Set parameter (plugin_id, param_index, value)
oscsend localhost 22752 /Carla/set_parameter i 0 i 0 f 0.707

# Bypass plugin
oscsend localhost 22752 /Carla/set_active i 0 i 0
```

---

## 11. Troubleshooting

### Carla won't start

```bash
# Check JACK is running
pgrep jackd && echo "JACK OK" || echo "Start JACK first: sudo audio-stack-start"

# Check Carla binary
ldd $(which carla) | grep "not found"
# If any libraries missing, rebuild with: ./scripts/carla-build.sh --clean --install

# Run with debug output
carla --debug 2>&1 | tail -20
```

### No audio (silent, no xruns)

```bash
# Check JACK connections
jack_lsp -c | grep system

# Run auto-connect
./scripts/carla-jack-autoconnect

# Check plugin bypass state — all bypassed = silence
# In Carla GUI: check each plugin's bypass button
```

### Xruns with NAM

```bash
# 1. Switch to nano model
# 2. Increase JACK buffer: edit /etc/jackdrc, change -p 128 to -p 256
sudo systemctl restart jackd

# 3. Try feather models with buffer 256
# 4. If still xrunning, skip NAM for live use — use it only for re-amping
```

### Plugin not loading in Carla

```bash
# Refresh LV2 plugin cache
rm -f ~/.cache/carla/carla-plugin-cache
carla --refresh &

# Check lv2ls
lv2ls 2>&1 | grep -i <plugin-name>

# Validate the plugin
./scripts/lv2lint-check.sh /usr/lib/lv2/<plugin-name>.lv2
```

### VST3 plugins not showing

```bash
# VST3 SDK was not installed during build
ls /opt/vst3sdk && echo "SDK found" || echo "Install SDK and rebuild"
# Rebuild: ./scripts/carla-build.sh --clean --install
```

### High CPU / thermal throttling

```bash
# Check CPU temp
vcgencmd measure_temp

# Check if throttled
vcgencmd get_throttled

# If throttling:
# - Reduce plugin count
# - Bypass unused plugins
# - Add active cooling
# - Use nano NAM models only
```

---

## 12. File Inventory

```
raspberry-pi-mixer/
├── scripts/
│   ├── carla-build.sh              # Build Carla from source (ARM Cortex-A72 optimized)
│   ├── nam-build.sh                # Build NAM LV2 plugin for ARM
│   ├── carla-preset-gen.py         # Generate default rack + patchbay presets
│   ├── carla-jack-autoconnect      # Auto-wire JACK ports to Carla plugins
│   ├── carla-preset-manager.py     # Save/load/export presets
│   ├── lv2lint-check.sh            # LV2 plugin validation suite
│   ├── audio-stack-start           # (from P2-R1) Full audio stack startup
│   ├── audio-stack-stop            # (from P2-R1) Full audio stack shutdown
│   ├── jack-latency-test           # (from P2-R1) Round-trip latency measurement
│   └── jack-route-default          # (from P2-R1) Default JACK routing
├── config/
│   ├── carla-8ch-default.carxp     # Default 8-channel mixer rack preset
│   ├── carla-patchbay.xml          # Patchbay connection rules
│   ├── jackdrc                     # (from P2-R1) JACK server config
│   ├── alsa-usb.conf               # (from P2-R1) ALSA USB config
│   └── ...                         # (other P2-R1 configs)
└── docs/
    ├── audio-stack-config.md        # (from P2-R1) Full audio stack docs
    └── carla-integration.md         # THIS FILE — Carla integration docs
```

### Lines of Code (new in this task)

| File | Lines | Purpose |
|------|-------|---------|
| scripts/carla-build.sh | 196 | Carla build automation |
| scripts/nam-build.sh | 184 | NAM LV2 build automation |
| scripts/carla-preset-gen.py | 196 | Preset generation |
| scripts/carla-jack-autoconnect | 208 | JACK port auto-wiring |
| scripts/carla-preset-manager.py | 246 | Preset save/load/export |
| scripts/lv2lint-check.sh | 217 | Plugin validation |
| config/carla-8ch-default.carxp | auto-generated | Default rack |
| config/carla-patchbay.xml | auto-generated | Patchbay rules |
| docs/carla-integration.md | this file | Documentation |
| **Total** | **~1,247 lines** | 7 scripts + 2 configs + 1 doc |

---

## Verification Checklist

- [ ] Carla builds successfully on RPi4B (`./scripts/carla-build.sh --install`)
- [ ] `carla --help` works and shows JACK audio driver
- [ ] NAM LV2 builds successfully (`./scripts/nam-build.sh --install`)
- [ ] `lv2ls | grep neural` shows NAM plugin
- [ ] `./scripts/lv2lint-check.sh` passes with 0 errors
- [ ] JACK starts: `sudo audio-stack-start`
- [ ] Carla launches with rack: `carla config/carla-8ch-default.carxp`
- [ ] Auto-connect wires ports: `./scripts/carla-jack-autoconnect`
- [ ] Audio passes through: `jack_lsp -c` shows Carla ↔ system connections
- [ ] No xruns at idle: `jack_cpu_load` < 10%
- [ ] NAM loads a nano model without xruns (128f buffer)
- [ ] Preset save/load works: `./scripts/carla-preset-manager.py save test && load test`
- [ ] CPU governor is 'performance': `cat /sys/.../scaling_governor`
- [ ] Carla runs at SCHED_FIFO prio 80: `chrt -p $(pgrep carla)`