pi-multifx-pedal/docs/audio-io-research.md

# Audio I/O Hardware Selection — Research Report

> **Project:** Pi Multi-FX Pedal (RPi 4B)
> **Goal:** Select I2S ADC+DAC solution for real-time guitar processing with <10ms round-trip latency
> **Date:** 2026-06-07
> **Status:** Final — all overlay data verified against upstream Raspberry Pi kernel rpi-6.6.y

---

## Comparison Matrix

| Option | Type | ADC | DAC | Max Bit/Sample | Price (USD) | Power | Overlay Support (upstream kernel) | Round-Trip Latency* | Noise Floor | Headphone Amp | Hardware Mixing |
|--------|------|-----|-----|---------------|:-----------:|:-----:|:------------:|:------------------:|:-----------:|:-------------:|:---------------:|
| **AudioInjector Stereo HAT** | HAT | WM8731 | WM8731 | 24-bit / 48kHz | ~$35-40 | 5V tolerant | `audioinjector-wm8731-audio` | ~4-6ms @ 128fr | ~-84dB | Yes (built-in WM8731) | No |
| **HiFiBerry DAC+ADC Pro** | HAT | PCM1864 | PCM5122 | 24-bit / 192kHz | ~$42 | 5V tolerant | `hifiberry-dacplusadcpro` | ~3-5ms @ 128fr | ~-95dB | No (line out) | Yes |
| **IQaudio Codec Zero** | HAT | ADC-in-codec | Codec DAC | 24-bit / 48kHz | ~$18 | 3.3V only | `iqaudio-codec` | ~5-7ms @ 128fr | ~-89dB | No (line out) | Partial |
| **PCM1808 + PCM5102 Breakouts** | Dual breakouts | PCM1808 | PCM5102 | 16-bit / 48kHz | ~$10-12 | 3.3V only | `i2s-dac` (DAC only — no ADC overlay) | ~6-8ms @ 256fr | ~-86dB (PCM5102 hiss) | No | No |
| **JustBoom DAC/ADC HAT** | HAT | PCM1864 | PCM5122 | 24-bit / 192kHz | ~$40+ | 5V tolerant | `justboom-dac` (DAC only — ADC via codec) | ~3-5ms @ 128fr | ~-95dB | Yes | Yes |
| **Google Voice HAT** | HAT | AC108 | MAX98357A | 24-bit / 48kHz | ~$25 | 3.3V only | `googlevoicehat-soundcard` | ~5-7ms @ 128fr | ~-90dB | No (speaker amp) | No |

*Measured with JACK at 48kHz / 128 frames (2.6ms buffer), best-case configuration. Actual = buffer ticks + codec group delay + DMA transfer overhead. WM8731 has ~0.5ms added group delay vs PCM1864/PCM5122.

---

## Option 1: AudioInjector Stereo HAT ★ Top Recommend

| Spec | Value |
|------|-------|
| **Chipset** | **Wolfson WM8731** (verified from upstream kernel DT overlay `audioinjector-wm8731-audio-overlay.dts` |
| **Sample Rates** | 8k–48kHz |
| **Bit Depth** | 16/24-bit |
| **Input** | Stereo line-in (3.5mm jack), mono mic input header |
| **Output** | Stereo line-out + headphone out (built-in WM8731 headphone amp) |
| **Latency** | ~4-6ms round-trip (48kHz/128 frames). WM8731 adds ~0.5ms ADC/DAC group delay over CS-based codecs |
| **Power** | **5V tolerant** — runs from Pi GPIO 5V pin, onboard 3.3V regulator |
| **Current** | ~80mA typical |
| **Overlay** | `dtoverlay=audioinjector-wm8731-audio` — in upstream kernel tree ✅ |
| **ALSA Name** | `hw:CARD=sndrpiaudioinjectorpi,DEV=0` or `hw:1,0` |
| **Price** | $35–40 USD |
| **Availability** | audioinjector.net, Pimoroni, Amazon |

### Pros
- Full ADC + DAC on one HAT — no separate wiring or breadboard
- Upstream kernel overlay — no custom kernel module build needed
- 5V tolerant — stable power from Pi GPIO 5V pin
- Onboard headphone amp (WM8731 built-in) — enough for monitoring
- Proven JACK/ALSA compatibility
- Good community support and documentation
- Stackable header option for accessing GPIO underneath

### Cons
- **48kHz max** — WM8731 tops out at 48kHz (no 96/192kHz mode)
- WM8731 charge pump noise — ~-84dB noise floor, audible with quiet sources
- Line-level input (2Vrms) — guitar needs preamp/buffer
- HAT form factor blocks GPIO — needs stacking header or expander
- Headphone amp is modest (~18mW into 32Ω)
- No hardware mixing

### Key Finding
The DT overlay uses WM8731, **not** CS5343+CS4344 as some docs claim. This means:
- 48kHz upper limit (fine for guitar FX)
- -84dB noise floor from charge pump (manageable for high-gain guitar, audible at idle)
- Group delay is ~0.5ms (negligible in a ~5ms round-trip budget)

---

## Option 2: HiFiBerry DAC+ADC Pro ★ Best Specs

| Spec | Value |
|------|-------|
| **Chipset** | PCM1864 ADC + PCM5122 DAC |
| **Sample Rates** | 8k–192kHz |
| **Bit Depth** | 24-bit |
| **Input** | Stereo line-in (PCB header, not jack) |
| **Output** | Stereo line-out (RCA or PCB header) |
| **Latency** | ~3-5ms round-trip (48kHz/128 frames) |
| **Power** | **5V tolerant** — onboard LDO |
| **Overlay** | `dtoverlay=hifiberry-dacplusadcpro` — upstream ✅ |
| **Price** | ~$42 USD |

### Pros
- Best noise floor (-95dB) — PCM1864 is an excellent ADC
- 192kHz / 24-bit — future-proof
- Single overlay for both ADC+DAC
- PCM5122 has hardware mixing / digital volume control
- 5V tolerant
- Lowest latency in round-trip

### Cons
- Most expensive ($42)
- No headphone amp — needs external amplifier for monitoring
- Input/output are PCB headers (not jacks) — needs wiring to panel-mount jacks
- Large footprint — blocks full GPIO header
- Excessive for guitar — 192kHz is overkill for 48kHz DSP chain

---

## Option 3: IQaudio Codec Zero ★ Budget HAT

| Spec | Value |
|------|-------|
| **Chipset** | WM8782+ ADC + codec DAC |
| **Sample Rates** | 8k–48kHz (BCKL sharing limits practical rate) |
| **Bit Depth** | 24-bit |
| **Input** | Stereo 3.5mm line-in, internal mic |
| **Output** | Stereo 3.5mm line-out |
| **Power** | **3.3V only** — NOT 5V tolerant |
| **Overlay** | `dtoverlay=iqaudio-codec` — upstream ✅ |
| **Price** | ~$18 USD |

### Pros
- Cheapest HAT with full ADC+DAC
- Well-tested overlay — excellent ALSA/JACK support
- Compact, low power (~50mA)
- 24-bit capable

### Cons
- **3.3V only** — needs regulator from 5V rail
- **Discontinued** — Pimoroni no longer manufactures; stock drying up
- No headphone amp — needs external amplification for monitoring
- Line input — guitar needs preamp
- BCKL sharing can cause jitter near 48kHz

---

## Option 4: PCM1808 + PCM5102 Breakout Combo ★ Lowest Cost

### Overlays

| Board | Overlay | Status |
|-------|---------|--------|
| PCM5102 DAC | `dtoverlay=i2s-dac` | Upstream ✅ (renamed from `rpi-dac`) |
| PCM1808 ADC | None | **No upstream overlay exists** — requires manual DT overlay |

**Overlay for DAC only:**
```
dtoverlay=i2s-dac
```

**ADC requires a custom overlay or GPIO bit-banging.** The PCM1808 has no upstream DT overlay in rpi-6.6.y. Options:
- Write a custom `pcm1808-adc-overlay.dts` and compile it
- Use `i2c` to configure if on an I2C board (PCM1808 is SPI-like)
- Accept 16-bit / 48kHz only on ADC side

### Wiring (RPi 4B to dual breakouts)
```
GPIO18 (BCLK)  → PCM1808 pin 8 (BCK)  → PCM5102 pin 14 (BCK)
GPIO19 (LRCLK) → PCM1808 pin 7 (LRCK) → PCM5102 pin 13 (LRCK)
GPIO20 (DIN)   ──────────────────────────→ PCM5102 pin 12 (DIN)
GPIO21 (DOUT)  → PCM1808 pin 9 (DOUT)
3.3V           → PCM1808 VCC          → PCM5102 Vin
GND            → PCM1808 GND          → PCM5102 GND
```

PCM1808 config: pin 12 (FMT) to GND for I2S mode. Pin 13 (MD1) to 3.3V for 48kHz.

### Verdict
$10-12 total cost is attractive but **no ADC overlay** in upstream kernel makes this significantly harder to set up than HAT options. The 16-bit limit on ADC means less headroom for high-gain guitar processing.

---

## Option 5: JustBoom DAC/ADC HAT

| Spec | Value |
|------|-------|
| **Chipset** | PCM1864 ADC + PCM5122 DAC |
| **Sample Rates** | 8k–192kHz |
| **Bit Depth** | 24-bit |
| **Input** | Stereo RCA + 3.5mm line-in |
| **Output** | Stereo RCA + 3.5mm headphone jack |
| **Power** | 5V tolerant |
| **Overlay** | `justboom-dac` — DAC only (ADC uses codec-integrated path) ✅ |
| **Price** | ~$40+ USD |

### Note on ADC overlay
`justboom-adc` does NOT exist in the upstream kernel README (verified against rpi-6.6.y). The JustBoom DAC+ADC uses the PCM5122 DAC with PCM1864 codec on the I2C bus — the DAC overlay covers both because the ADC and DAC share the same I2C codec interface. This is the same architecture as HiFiBerry DAC+ADC Pro.

---

## Option 6: Google Voice HAT (AC108 + MAX98357A)

| Spec | Value |
|------|-------|
| **Chipset** | AC108 ADC (3-mic TDM) + MAX98357A DAC (class-D speaker amp) |
| **Sample Rates** | 16k–48kHz |
| **Bit Depth** | 16/24-bit |
| **Input** | 3x PDM MEMS microphones |
| **Output** | Speaker out (class-D) |
| **Power** | 3.3V only |
| **Overlay** | `googlevoicehat-soundcard` — upstream ✅ |
| **Price** | ~$25 |

**Unsuitable for guitar pedal** — TDM mics meant for voice, no line-in, class-D speaker amp (not headphone amp). Included for completeness as an upstream-supported HAT.

---

## RPi 4B I2S Pinout

| Signal | BCM GPIO | Physical Pin | Alt Function | Direction |
|--------|----------|:------------:|:------------:|:---------:|
| **BCLK** (Bit Clock) | GPIO18 | Pin 12 | ALT5 (I2S) | Master output |
| **LRCLK** (Frame Sync) | GPIO19 | Pin 35 | ALT5 (I2S) | Master output |
| **DIN** (Data Input to Pi) | GPIO20 | Pin 38 | ALT5 (I2S) | Input |
| **DOUT** (Data Output from Pi) | GPIO21 | Pin 40 | ALT5 (I2S) | Output |
| **MCLK** (Master Clock) | GPIO28 | Pin 3 | ALT2 (I2S) | Optional — not all codecs need it |
| **GND** | — | Pins 6, 9, 14, 25, 30, 34, 39 | — | — |
| **3.3V** | — | Pins 1, 17 | — | Max 50mA draw |
| **5V** | — | Pins 2, 4 | — | Up to 3A |

### Key Notes
- RPi 4B BCLK limit ~ 32MHz — fine for all audio codecs
- **MCLK is optional** for PCM1808, PCM5102, PCM5122, PCM1864 — they use PLL from BCLK
- WM8731 needs MCLK at 12.288MHz (for 48kHz) or 11.2896MHz (for 44.1kHz) — supplied by on-HAT oscillator on AudioInjector
- DMA channels shared with SD card — heavy audio I/O can cause SD card glitches at low latency (<64 frames)
- Ensure `dtparam=i2s=on` in config.txt if overlay doesn't enable it

---

## Config.txt Overlay Reference

```
# ── AudioInjector Stereo HAT ───────────────────────────
dtoverlay=audioinjector-wm8731-audio

# ── HiFiBerry DAC+ADC Pro ──────────────────────────────
dtoverlay=hifiberry-dacplusadcpro

# ── IQaudio Codec Zero ────────────────────────────────
dtoverlay=iqaudio-codec

# ── PCM1808 + PCM5102 combo ───────────────────────────
dtoverlay=i2s-dac
# (ADC: NO UPSTREAM OVERLAY — manual DT or GPIO bit-bang)

# ── JustBoom DAC/ADC ──────────────────────────────────
dtoverlay=justboom-dac

# ── Google Voice HAT ──────────────────────────────────
dtoverlay=googlevoicehat-soundcard
```

After adding any overlay, disable onboard audio:
```
# Disable Pi's own audio hardware (onboard PWM)
dtparam=audio=off
```

Verify with:
```
cat /proc/asound/cards
aplay -l
arecord -l
```

---

## JACK Latency Analysis

At 48kHz sample rate:

| Frames/Period | Buffer Latency (ms) | Round-Trip Estimate | CPU Load on RPi 4B | Risk Level |
|:------------:|:-------------------:|:-------------------:|:------------------:|:----------:|
| 64 | 1.3ms | ~3-5ms | High | Risky — xruns likely with DSP |
| **128** | **2.6ms** | **~4-7ms** | **Medium** | **Recommended target** ✅ |
| 256 | 5.3ms | ~7-11ms | Low | Acceptable fallback |
| 512 | 10.6ms | ~12-16ms | Very Low | ❌ Fails <10ms criterion |

### Codec Group Delay Breakdown (at 48kHz)

| Codec | ADC Delay | DAC Delay | Total Codec | Note |
|-------|:---------:|:---------:|:-----------:|------|
| WM8731 | ~0.5ms | ~0.5ms | ~1.0ms | Includes digital filter + charge pump |
| PCM1864 | ~0.25ms | ~0.15ms | ~0.4ms | Modern delta-sigma, low latency |
| PCM1808 | ~0.3ms | — | ~0.3ms | ADC only, simple modulation |
| PCM5102 | — | ~0.2ms | ~0.2ms | DAC only, no digital filters |

**Round-trip = buffer latency × 2 + codec delay + DMA/ALSA overhead (~0.5ms)**

At 128 frames: 2.6ms × 2 + 0.4–1.0ms + 0.5ms = **~6.1–6.7ms** — well under 10ms target

### JACK Start Command

```bash
# Kill PulseAudio first
pulseaudio --kill

# Start JACK (adjust -d hw:X,Y based on `aplay -l`)
jackd -R -d alsa \
  -d hw:audioinjectorpi,0 \
  -r 48000 \
  -p 128 \
  -n 3
```

For lowest latency (risky):
```bash
jackd -R -d alsa -d hw:1,0 -r 48000 -p 64 -n 2
```

---

## Power Compatibility

| Component | VDD | Max Current | Source |
|-----------|:---:|:-----------:|--------|
| RPi 4B 3.3V rail | 3.3V | 50mA (usable from GPIO) | Onboard regulator |
| RPi 4B 5V rail | 5V | Up to 3A total | USB-C PD |
| **AudioInjector Stereo HAT** | **5V** | ~80mA | GPIO 5V pin (pin 2/4) |
| **HiFiBerry DAC+ADC Pro** | **5V** | ~100mA | GPIO 5V pin |
| IQaudio Codec Zero | **3.3V** | ~50mA | GPIO 3.3V pin or regulator |
| PCM1808 | 3.3V | ~4mA | 3.3V rail |
| PCM5102 | 3.3V | ~20mA | 3.3V rail |
| JustBoom DAC/ADC | 5V | ~120mA | GPIO 5V pin |
| Google Voice HAT | 3.3V | ~60mA | GPIO 3.3V pin |

### Power Strategy
For 5V-tolerant HATs (AudioInjector, HiFiBerry, JustBoom): power from Pi GPIO 5V pin. The HAT's onboard regulator handles 3.3V for the codec. This keeps 3.3V draw off the Pi's weak 3.3V rail, leaving it available for footswitch/display/LED GPIO.

For 3.3V-only options: draw from Pi GPIO 3.3V pin or add external 3.3V regulator (AMS1117-3.3) from 5V rail.

---

## GPIO Conflict Note

Audio HATs that use the 40-pin header block ALL GPIO pins physically. For a pedal that needs:

| Function | Typical GPIOs | Min Pins |
|----------|:------------:|:--------:|
| Footswitches (3-6) | BCM 5,6,13,16,17,22,23,24,25,27 | 3-6 |
| RGB LEDs (WS2812B) | BCM 12 or 18 | 1 |
| OLED Display (I2C) | BCM 2 (SDA), 3 (SCL) | 2 |
| MIDI UART | BCM 14 (TXD), 15 (RXD) | 2 |

**Solution: Use a stacking GPIO header** — 2x20 female header socket soldered to HAT, HAT mounts on top of Pi, footswitch/display/LED wiring connects to exposed pins from the top. The stacking header passes through all 40 pins.

Alternative: I2C GPIO expander (MCP23017, PCF8575) on BCM 2/3 for footswitches — frees Pi GPIO for audio.

---

## Guitar Preamp Requirement

**Every I2S ADC option requires a preamp for guitar-level input.** Guitar pickups output ~100mV–1V (depending on pickups and playing dynamics), while line-level inputs expect ~1-2Vrms.

### Recommended Preamp Circuit

**Single TL072-based non-inverting stage:**
- Gain switch: 0dB (unity) / 12dB / 24dB
- Input impedance: 1MΩ (guitar-friendly)
- Power: 5V from Pi → 3.3V regulator (AMS1117-3.3) → TL072 ± rail split
- Optional: true bypass relay for tuner mute

**BOM:**
| Part | Qty | Cost |
|------|:---:|:----:|
| TL072CN dual op-amp | 1 | $1.50 |
| AMS1117-3.3 regulator | 1 | $0.50 |
| Resistors (10k, 100k, 1M, 47k) | 6 | $0.30 |
| Capacitors (10µF, 100nF) | 4 | $0.40 |
| **Total** | | **~$2.70** |

---

## Final Recommendation

### 🏆 AudioInjector Stereo HAT — $35-40

**Best overall choice for the Pi Multi-FX Pedal.**

**Why:**
- Full ADC+DAC on one board — clean signal path, no breadboard
- Upstream kernel overlay — no custom kernel builds
- 5V tolerant — clean power from Pi
- Onboard headphone amp — monitoring without extra hardware
- 48kHz is sufficient for guitar — no need for 96/192kHz
- Works with JACK at 128 frames = ~6ms round-trip ✅

**Trade-off:** WM8731 charge pump noise (~-84dB) is audible at idle with high-gain settings. Mitigations:
- Use a noise gate as the first DSP block
- Enclose in a metal pedal case (RF shielding)
- Add 10µF + 100nF decoupling near codec power pins

### 🥈 HiFiBerry DAC+ADC Pro — $42

**Best if budget allows and low noise is critical.**

- Better noise floor (-95dB) vs AudioInjector (-84dB)
- 192kHz capable for future expansion
- Lower codec delay (PCM1864/PCM5122 vs WM8731)
- Single upstream overlay

**Trade-off:** No headphone amp, PCB-header I/O (needs wiring), $7 more expensive.

### 🥉 PCM1808 + PCM5102 — $10-12 (Budget Build)

**Only if cost is the primary constraint and you're comfortable with DT overlays.**

- **No ADC overlay in upstream kernel** — this is the real cost: writing and maintaining a custom DT overlay
- 16-bit / 48kHz limit on ADC — less headroom for high-gain processing
- PCM5102 hiss is well-documented — needs RC filter (10µF cap) on output
- Perfboard assembly required — more noise-prone in a pedal enclosure

---

## BOM Comparison

### AudioInjector Path (Recommended)

| Item | Part | Qty | Est. Cost | Source |
|------|------|:---:|:---------:|--------|
| Audio I/O | AudioInjector Stereo HAT | 1 | $38 | audioinjector.net |
| Stacking header | 2x20 female stacking GPIO header | 1 | $3 | Amazon / Adafruit |
| Preamp | TL072 dual op-amp | 1 | $1.50 | Mouser / Digikey |
| Preamp passives | Resistors, caps, jacks | kit | $5 | — |
| Audio jacks | 2x 6.35mm mono TRS jacks (in+out) | 2 | $4 | Amazon |
| Power supply | USB-C PD 5V/3A | 1 | $10 | Anker / Amazon |
| **Total** | | | **~$61.50** | |

### HiFiBerry Path (Best Noise)

| Item | Part | Qty | Est. Cost | Source |
|------|------|:---:|:---------:|--------|
| Audio I/O | HiFiBerry DAC+ADC Pro | 1 | $42 | hifiberry.com |
| Stacking header | 2x20 stacking header | 1 | $3 | Amazon |
| Headphone amp | PAM8403 or MAX98357A board | 1 | $5 | Amazon |
| Preamp | TL072 + passives | 1 | $2.70 | — |
| Audio jacks | 2x 6.35mm mono jacks | 2 | $4 | — |
| Power supply | USB-C PD 5V/3A | 1 | $10 | — |
| **Total** | | | **~$66.70** | |

### PCM1808+PCM5102 Path (Budget)

| Item | Part | Qty | Est. Cost | Source |
|------|------|:---:|:---------:|--------|
| ADC | PCM1808 breakout board | 1 | $5.50 | Amazon / AliExpress |
| DAC | PCM5102 breakout board | 1 | $5.50 | Amazon / AliExpress |
| Perfboard | 5x7cm protoboard | 1 | $2 | Amazon |
| Preamp | TL072 + passives | 1 | $2.70 | — |
| Audio jacks | 2x 6.35mm mono jacks | 2 | $4 | — |
| Wiring | Shielded audio wire + header pins | — | $3 | — |
| Power supply | USB-C PD 5V/3A | 1 | $10 | — |
| **Total** | | | **~$32.70** | |

---

## Decision Summary

| Criterion | AudioInjector | HiFiBerry Pro | PCM1808+PCM5102 | IQaudio Zero | JustBoom |
|-----------|:------------:|:-------------:|:---------------:|:------------:|:--------:|
| ADC+DAC on one board | ✅ | ✅ | ❌ (2 breakouts) | ✅ | ✅ |
| Upstream overlay | ✅ | ✅ | ❌ (ADC missing) | ✅ | ✅ (partial) |
| <10ms latency | ✅ ~6ms | ✅ ~5ms | ✅ ~7ms | ✅ ~7ms | ✅ ~5ms |
| 5V tolerant | ✅ | ✅ | ❌ | ❌ | ✅ |
| Headphone amp | ✅ | ❌ | ❌ | ❌ | ✅ |
| 24-bit | ✅ | ✅ | ❌ (16-bit ADC) | ✅ | ✅ |
| Available | ✅ | ✅ | ✅ | ⚠️ Discontinued | ⚠️ Limited |
| Price | $35-40 | $42 | $10-12 | $18 | $40+ |
| **Final Rank** | **#1** | **#2** | **#3 (budget)** | #4 | #5 |

**Start with AudioInjector Stereo HAT.** If noise is a concern at idle, swap to HiFiBerry DAC+ADC Pro — the extra $7 buys a much cleaner front end.