# Audio Configuration — PCM1808+PCM5102 I2S HAT

> **HAT:** Custom PCM1808 ADC + PCM5102 DAC I2S audio HAT  
> **RPi:** 4B, Raspberry Pi OS (Bookworm, 64-bit)  
> **Kernel:** rpi-6.6.y (upstream Raspberry Pi kernel)  
> **Last updated:** 2026-06-07

---

## Device Tree Overlay

The PCM1808 ADC has **no upstream overlay** in the rpi-6.6.y kernel tree. A custom overlay is provided at:

```
hardware/pcm1808-pcm5102-overlay.dts
```

### Build & Install

```bash
# Install device-tree-compiler if not present
sudo apt-get install device-tree-compiler

# Compile overlay
dtc -@ -I dts -O dtb -o pcm1808-pcm5102-overlay.dtbo \
    hardware/pcm1808-pcm5102-overlay.dts

# Copy to boot overlays directory
sudo cp pcm1808-pcm5102-overlay.dtbo /boot/firmware/overlays/
# On older Pi OS: /boot/overlays/
```

This compiles to `/boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo`.

### Enable Overlay

Edit `/boot/firmware/config.txt` (or `/boot/config.txt` on older installs):

```ini
# ── Custom I2S HAT: PCM1808 ADC + PCM5102 DAC ─────────
dtoverlay=pcm1808-pcm5102-overlay

# Disable onboard audio to free I2S pins
dtparam=audio=off

# Optional: force I2S pins if overlay doesn't set them
# dtparam=i2s=on
```

### Verify Overlay Loads

```bash
# Check overlay is loaded
dmesg | grep -i "pcm1808\|pcm5102\|i2s"

# Check ALSA sees both devices
aplay -l
arecord -l

# Expected output for playback:
#   card 0: pcm1808pcm5102hat [pcm1808-pcm5102-hat], device 0: ... []
#   Subdevices: 1/1

# Expected output for capture:
#   card 0: pcm1808pcm5102hat [pcm1808-pcm5102-hat], device 0: ... []
#   Subdevices: 1/1

# Check /proc device tree
cat /proc/device-tree/sound/name  # should show "pcm1808-pcm5102-hat"
```

---

## ALSA Configuration

### Default Card Setup

Create `/etc/asound.conf` (system-wide) or `~/.asoundrc` (per-user):

```conf
# ── PCM1808+PCM5102 I2S HAT ─────────────────────────

# Default playback device
pcm.!default {
    type asym
    playback.pcm {
        type hw
        card 0
        device 0
    }
    capture.pcm {
        type hw
        card 0
        device 0
    }
}

# Hardware device aliases
pcm.i2s_playback {
    type hw
    card 0
    device 0
}

pcm.i2s_capture {
    type hw
    card 0
    device 0
}

# Software volume control (PCM1808/PCM5102 have no HW volume)
pcm.softvol_playback {
    type softvol
    slave.pcm "i2s_playback"
    control {
        name "Master Playback Volume"
        card 0
    }
    min_dB -50.0
    max_dB 0.0
}

# 48kHz upsampler for 44.1kHz sources (like YouTube/Spotify)
pcm.rate_convert {
    type plug
    slave {
        pcm "softvol_playback"
        rate 48000
        format S24_3LE
        channels 2
    }
}

# Default: use rate conversion + softvol
pcm.!default {
    type plug
    slave.pcm "rate_convert"
}

# Control device
ctl.!default {
    type hw
    card 0
}
```

### Test Audio

```bash
# --- Playback test ---
# Sine wave at 440Hz (A4)
speaker-test -t sine -f 440 -l 1 -c 2

# Play a WAV file
aplay /usr/share/sounds/alsa/Front_Center.wav

# --- Capture test ---
# Record 3 seconds from ADC
arecord -d 3 -f cd -t wav test_record.wav

# Play back the recording
aplay test_record.wav

# --- Full loopback test ---
# Play tone while recording (connect DAC output to ADC input)
# Check latency: <10ms at 48kHz/128 frames
arecord -d 5 -f S24_3LE -r 48000 -c 2 loopback_test.wav &
speaker-test -t sine -f 440 -l 2
wait
aplay loopback_test.wav
```

---

## JACK Audio Configuration

The pedal DSP runs under JACK. The existing JACK config in `main.py` uses `hw:1,0` (USB). Swap to `hw:0,0` for I2S:

### JACK Start

```bash
# Kill PulseAudio (conflicts with JACK on RPi)
pulseaudio --kill

# Start JACK at 48kHz/128 frames (recommended target: ~5.3ms buffer)
jackd -R -d alsa \
  -d hw:0,0 \
  -r 48000 \
  -p 128 \
  -n 3

# For low-latency mode (risky, check xruns):
jackd -R -d alsa -d hw:0,0 -r 48000 -p 64 -n 2
```

### Update main.py Config

In `/home/oplabs/projects/pi-multifx-pedal/main.py`, change:

```python
# FROM (USB Audio):
# config = {
#     "input_device": "hw:1,0",
#     "output_device": "hw:1,0",
# }

# TO (I2S HAT):
config = {
    "input_device": "hw:0,0",       # PCM1808 ADC
    "output_device": "hw:0,0",      # PCM5102 DAC
    "sample_rate": 48000,
    "period_size": 128,              # 2.6ms buffer
    "num_periods": 3,                # triple-buffered
}
```

---

## Integration with NAM DSP

The I2S HAT uses the **same block size** as the NAM DSP engine (256 samples):

| Parameter | Value | Note |
|-----------|-------|------|
| Sample rate | 48,000 Hz | Both JACK and NAM |
| Block size | 256 frames | NAM internal block |
| JACK period | 128 frames | 2 × periods per block |
| Round-trip (target) | <8ms | 128fr × 2 + codec delay + DSP |

At 48kHz/128 frames × 3 periods = ~8ms round-trip, well within the 10ms target for NAM DSP. The 256-sample NAM block fits exactly into 2 JACK periods.

---

## Troubleshooting

### No card 0 detected

```bash
# Check if I2S is enabled in pin controller
dmesg | grep -i i2s

# Check loaded overlays
vcgencmd get_config dtov
# Should show: pcm1808-pcm5102-overlay

# Rebuild overlay if kernel was updated
sudo dtc -@ -I dts -O dtb -o /boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo \
    /home/oplabs/projects/pi-multifx-pedal/hardware/pcm1808-pcm5102-overlay.dts
```

### No capture device

PCM1808 has no upstream overlay — the custom overlay provides the `pcm1808` codec binding. If capture isn't showing:

```bash
# Verify the overlay node compiled
dtc -I dtb -O dts /boot/firmware/overlays/pcm1808-pcm5102-overlay.dtbo | grep -i pcm1808

# Check I2S pin muxing is correct
raspi-gpio get 18 19 20 21
# Expect: GPIO 18: level=0 func=ALT5 (I2S BCLK)
#          GPIO 19: level=0 func=ALT5 (I2S LRCLK)
#          GPIO 20: level=0 func=ALT5 (I2S DIN)
#          GPIO 21: level=0 func=ALT5 (I2S DOUT)
```

### PCM5102 hiss

- Add 330Ω + 10µF RC filter on DAC output (R5, C10 in BOM)
- Ensure XSMT (pin 17) is pulled to 3.3V, not floating
- Check 3.3V rail noise with oscilloscope (<50mV ripple at audio frequencies)

### JACK xruns

- Increase period size: `-p 256` (5.3ms, safer)
- Increase num periods: `-n 3` (triple buffer)
- Isolate RPi 4B from USB power noise
- Check `cat /proc/asound/card0/pcm0p/sub0/status` for xrun count

---

## Pinout Reference

| Signal | BCM GPIO | Physical Pin | ALT Function | Direction |
|--------|----------|:------------:|:------------:|:---------:|
| BCLK   | GPIO18   | Pin 12       | ALT5 (I2S)   | RPi → HAT |
| LRCLK  | GPIO19   | Pin 35       | ALT5 (I2S)   | RPi → HAT |
| DIN    | GPIO20   | Pin 38       | ALT5 (I2S)   | HAT → RPi |
| DOUT   | GPIO21   | Pin 40       | ALT5 (I2S)   | RPi → HAT |
| 3.3V   | —        | Pins 1, 17   | Power        | RPi → HAT |
| 5V     | —        | Pins 2, 4    | Power        | RPi → HAT |
| GND    | —        | Pins 6, 9, 14, 25, 30, 34, 39 | Ground | — |

The I2S HAT uses a **stacking header** (2×20 female socket) to pass through all 40 GPIO pins so footswitches, LEDs, and display are accessible from the top of the HAT.

---

## Focusrite Scarlett 2i2 — 4CM (Four Cable Method)

> **Interface:** Focusrite Scarlett 2i2 (3rd gen) USB audio interface
> **Mode:** `stereo_4cm` in `audio.mode`
> **Device:** `hw:1,0` (USB card index, may vary)

### Wiring

```
Guitar ─────→ Input 1 (ADC channel 0) ─→ hw:1,0 capture_0 ─→ Pipeline input_0
Amp FX Send ─→ Input 2 (ADC channel 1) ─→ hw:1,0 capture_1 ─→ Pipeline input_1

Pipeline output_0 (pre-amp) ─→ Output 1 (DAC channel 0) ─→ hw:1,0 playback_0 ─→ Amp Input (Send)
Pipeline output_1 (post-amp) ─→ Output 2 (DAC channel 1) ─→ hw:1,0 playback_1 ─→ Amp FX Return
```

### JACK Config

```bash
# Kill PulseAudio first — it grabs USB audio devices
pulseaudio --kill

# Start JACK at 48kHz/128 frames, 2 capture + 2 playback channels
jackd -R -d alsa -d hw:1,0 -r 48000 -p 128 -n 3 -C 2 -P 2
```

### Application Config

Set these fields in `~/.pedal/config.yaml`:

```yaml
audio:
  hat_type: "focusrite_2i2_3gen"
  profile: "standard"
  mode: "stereo_4cm"
  input_device: "hw:1,0"
  output_device: "hw:1,0"
  jack_enabled: true
  auto_connect: true
```

### Port Auto-Connect

When `auto_connect: true` and mode is `stereo_4cm`, the system creates these JACK connections:

| Source | Destination | Signal |
|--------|-------------|--------|
| `system:capture_1` | `fx_in:input_0` | Guitar → pre-amp DSP |
| `system:capture_2` | `fx_in:input_1` | FX Send → post-amp DSP |
| `fx_out:output_0` | `system:playback_1` | Pre-amp DSP → amp input |
| `fx_out:output_1` | `system:playback_2` | Post-amp DSP → FX return |

### Channel Mapping Helper

At runtime, call `AudioSystem.channel_mapping_help("stereo_4cm")` to get these mappings as structured data.

### Notes

- The Focusrite 2i2 registers as ALSA card index **1** when the I2S HAT is card 0 (Linux arranges by driver probe order). If HDMI audio or other USB devices are present, the index may shift — use `aplay -l` / `arecord -l` to confirm.
- 2-in/2-out at 24-bit/48kHz. JACK runs at 24-bit by default over ALSA.
- At 48kHz/128 frames × 3 periods, round-trip latency is ~6-8ms — well within the 10ms target for real-time guitar DSP.