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

# 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):

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.