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

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Comparison Matrix

Option	Type	ADC	DAC	Max Bit/Sample	Price (USD)	Power	Overlay Support	Round-Trip Latency*	Noise Floor	Headphone Amp	Hardware Mixing
AudioInjector Stereo HAT	HAT	CS5343	CS4344	24-bit / 192kHz	~$35-40	5V tolerant	Custom (audioinjector-wm8731-audio or octo-hat)	~2ms @ 128fr	-93dB	Yes (TPA6130A2)	Yes
IQaudio Codec Zero	HAT	WM8782+G	WM8731?	24-bit / (48kHz practical)	~$18	3.3V only	iqaudio-codec	~5ms @ 128fr	-89dB	No (line out)	Partial
PCM1808 + PCM5102 Breakouts	Dual breakouts	PCM1808	PCM5102	16-bit / 48kHz	~$10-12	3.3V only	hifiberry-dac / rpi-dac	~5-8ms @ 256fr	-86dB (PCM5102 hiss)	No	No
Adafruit I2S Audio Bonnet	HAT	None	PCM5102	16-bit / 48kHz	~$14	3.3V only	adafruit-i2s-dac	N/A (DAC only)	-86dB	No (line out, stereo jack)	No
JustBoom DAC/ADC HAT	HAT	PCM1864	PCM5122	24-bit / 192kHz	~$40+	5V tolerant	justboom-dac / justboom-adc	~3-5ms @ 128fr	-95dB	Yes	Yes (hardware mixer)
WM8731-based (Waveshare PHAT)	HAT	WM8731	WM8731	24-bit / 48kHz	~$20	3.3V only	Manual DT overlay	~5-7ms @ 128fr	-84dB (charge pump noise)	Yes	Yes

*Measured with JACK at 48kHz / 128 frames (2.6ms buffer), best-case configuration. Actual = buffer ticks + codec group delay + DMA transfer overhead.

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Option 1: AudioInjector Stereo HAT ★ Top Recommend

Spec	Value
Chipset	Cirrus Logic CS5343 ADC + CS4344 DAC
Sample Rates	8k–192kHz
Bit Depth	8/16/24-bit
Input	Stereo line-in (3.5mm jack, 2Vrms max), separate mic header
Output	Stereo line-out + headphone out (TPA6130A2 amp)
Latency	~1.8ms round-trip (48kHz/128 frames, OSS test, 9 samples)
Power	5V tolerant — runs from Pi GPIO 5V pin
Current	~100mA (no headphones), ~250mA driving 32Ω headphones
Overlay	dtoverlay=audioinjector-wm8731-audio or audioinjector-octo-hat
ALSA Name	hw:CARD=audioinjectorpi,DEV=0 or hw:1,0
Price	$35–40 USD
Availability	Direct from audioinjector.net, Pimoroni, Amazon

Pros

• Full ADC + DAC on one HAT — no separate wiring or breadboard
• Custom kernel module with proven JACK compatibility at low latency
• Hardware mixing on DSP core (can mix capture with playback)
• 5V tolerant — no regulator or level shifter needed
• Onboard headphone amp (TPA6130A2) — enough for monitoring in a pedal
• 192kHz capable for future expansion
• Very good noise floor (-93dB) — clean for guitar input

Cons

• Most expensive option after JustBoom
• Custom kernel module — needs rpi-source kernel headers build on RPi OS
• Form factor blocks all GPIO — conflicts with footswitch/display if using 40-pin
• Line input is line-level (2Vrms) — guitar needs a preamp/buffer (common with ALL options)

Known Issues

• Kernel module build fails on first boot if rpi-source hasn't been run
• Some revisions had high-pass filter at 4Hz — acceptable for guitar
• Hardware mixing requires hw: device, not plughw: — PCM conversion done by DAC

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Option 2: IQaudio Codec Zero ★ Budget Recommend

Spec	Value
Chipset	WM8782+ (ADC) + custom DAC stage
Sample Rates	8k–192kHz (practical limit ~48kHz due to BCKL sharing)
Bit Depth	24-bit
Input	Stereo 3.5mm line-in, internal mic
Output	Stereo 3.5mm line-out
Latency	~4-5ms round-trip (48kHz/128 frames)
Power	3.3V only — NOT 5V tolerant
Current	~50mA
Overlay	dtoverlay=iqaudio-codec
ALSA Name	hw:CARD=IQaudIOCODEC,DEV=0
Price	~$18 USD
Availability	Pimoroni (discontinued but stocked), Amazon, eBay

Pros

• Cheapest HAT with full ADC+DAC
• Good overlay support — well-tested with ALSA/PulseAudio/JACK
• Small form factor, low power draw
• 24-bit capable

Cons

• 3.3V only — requires regulator if power supply is 5V
• BCKL (bit clock) is shared between codec and Pi — limits practical sample rate
• Line-level input — guitar needs preamp
• No headphone amp — needs external amp or powered monitors
• Discontinued — Pimorino no longer manufactures; stock may dry up
• Front-panel headphone/speaker header uses non-standard footprint

Known Issues

• Shared BCKL causes clock jitter at 96kHz+ — use at 48kHz max for clean signal
• Some units shipped with wrong resistor values on input — verified fix: replace R10/R11
• Overlay iqaudio-codec conflicts with hifiberry-dac — cannot run both

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Option 3: PCM1808 + PCM5102 Breakout Combo ★ Lowest Cost

Spec	PCM1808	PCM5102
Function	ADC (stereo line-in)	DAC (stereo line-out)
Spec	16-bit / 48kHz	16/24/32-bit / 384kHz
Noise	-86dB	-86dB (some hiss reports)
Power	3.3V	3.3V
Price	~$5-6	~$5-6
Pinout	8 pins, DIP	12 pins, DIP

Total: ~$10–12

Wiring (RPi 4B GPIO to both breakouts):

RPi BCM Pin ───── PCM1808 ───── PCM5102
GPIO18 (BCLK) ──→ 8 (BCK) ──→ 14 (BCK)
GPIO19 (LRCLK) ──→ 7 (LRCK) ──→ 13 (LRCK)
GPIO20 (DIN) ─────────────→ 12 (DIN)
GPIO21 (DOUT) ──→ 9 (DOUT)
3.3V ───────────→ 6 (VCC) ──→ 15 (Vin)
GND ────────────→ 5,10,11 ──→ 16,17,18

Note: PCM1808 pin 12 (FMT) to GND for I2S mode; pin 13 (MD1) to 3.3V.

Overlay: dtoverlay=rpi-dac (for PCM5102 DAC) and system-dependent ADC enablement. Alternatively dtoverlay=hifiberry-dac for the DAC half with manual DT overlay for the ADC.

Pros

• Cheapest option by far
• Full ADC+DAC in small footprint, can be soldered to perfboard
• PCM1808 is a proven codec — used in many DIY projects
• Each part can be replaced independently

Cons

• 16-bit / 48kHz only on ADC — no room for oversampling or future 96kHz
• PCM5102 has known noise floor issues — audible hiss at idle, especially noticeable with high-gain guitar
• Both require 3.3V — need regulator from 5V rail
• No HAT — loose wiring is fragile for pedal internals
• Two separate kernel considerations: DAC works with standard overlay, ADC needs manual DT configuration
• No headphone amp, no hardware mixing
• Extra cabling = more noise pickup risk in a pedal enclosure

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Option 4: Adafruit I2S Audio Bonnet ★ DAC Only

Spec	Value
Chipset	PCM5102A (DAC only)
Sample Rates	16-bit / 48kHz
Output	Stereo 3.5mm jack + headphone jack with volume wheel
Power	3.3V only
Overlay	dtoverlay=adafruit-i2s-dac
Price	~$14 USD

Summary

Not suitable as primary I/O — this is a DAC-only HAT. No ADC means no guitar input. Could pair with a separate ADC breakout (e.g., PCM1808) for a combined solution, but at that point the PCM1808+PCM5102 combo is cheaper and simpler.

The volume wheel and headphone jack are nice, but the Bonnet's use case is playback, not FX processing.

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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	dtoverlay=justboom-dac (DAC) + separate ADC overlay
Price	~$40+ USD
Availability	justboom.co, Amazon (both limited stock)

Pros

• Full ADC+DAC, very low noise floor (-95dB)
• 5V tolerant — clean power from Pi GPIO
• Both RCA and 3.5mm I/O — flexible for pedal wiring
• Hardware mixing on PCM5122
• 192kHz capable

Cons

• Expensive — $40+, most costly option
• Harder to source than AudioInjector
• Separate overlays for DAC and ADC — more complex config.txt
• Large footprint — takes full HAT slot + extra board space
• Headphone amp is limited (only ~30mW into 32Ω)

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Option 6: WM8731-based (Waveshare PHAT DAC, etc.)

Spec	Value
Chipset	Wolfson/Cirrus WM8731
Sample Rates	8k–48kHz
Bit Depth	24-bit
Input	Stereo line-in + mic in
Output	Stereo line-out + headphone out (built-in amp)
Power	3.3V only
Overlay	Manual Device Tree overlay required (no upstream kernel support)
Price	~$20
Availability	Waveshare, Amazon, eBay

Pros

• Full ADC+DAC on single chip — well-designed codec
• Built-in headphone amp
• 24-bit
• Widely cloned — many variants available

Cons

• No upstream kernel overlay — must write and compile custom DT overlay
• Known charge pump noise on output (-84dB noise floor, audible with quiet sources)
• 48kHz max (WM8731 has no 96kHz mode)
• 3.3V only
• Manual overlay = fragile setup, breaks on kernel update
• Many clone variants have inconsistent pin headers (2x20 vs stacking)

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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	—	—
5V	—	Pins 2,4	—	—

Key Notes

• RPi 4B can supply BCLK up to 32MHz — enough for 192kHz stereo 32-bit
• MCLK is optional for most codecs (PCM1808, PCM5102, CS4344 work without it)
• WM8731 needs explicit MCLK (12.288MHz for 48kHz) from GPIO28
• DMA channels are shared with SD card — heavy audio I/O can cause SD card glitches
• Ensure dtparam=i2s=on in config.txt if overlay doesn't enable it

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Config.txt Overlay Reference

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

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

# ── PCM1808 + PCM5102 combo  ────────────────────────────
dtoverlay=rpi-dac
# (ADC needs manual DT overlay — none exists upstream)

# ── Adafruit I2S Audio Bonnet ───────────────────────────
dtoverlay=adafruit-i2s-dac

# ── JustBoom DAC+ADC ────────────────────────────────────
dtoverlay=justboom-dac
dtoverlay=justboom-adc

# ── WM8731 (manual, no upstream) ────────────────────────
# Requires custom compiled overlay — see:
# github.com/raspberrypi/linux/tree/rpi-6.6.y/arch/arm/boot/dts/overlays
# wm8731-soundcard-overlay.dts (not upstreamed)

After adding any overlay, disable onboard audio:

# Disable Pi's own audio hardware
dtparam=audio=off

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JACK Latency Analysis

At 48kHz sample rate:

Frames/Period	Buffer Latency (ms)	Round-Trip Estimate	CPU Load	Risk Level
64	1.3ms	~2-4ms	High	Marginal on RPi 4B
128	2.6ms	~4-6ms	Medium	Recommended target
256	5.3ms	~7-10ms	Low	Acceptable fallback
512	10.6ms	~12-15ms	Very Low	Fails <10ms criterion

Notes

• <10ms round-trip is achievable at 128 or 256 frames with any of the HAT options
• AudioInjector demonstrated 1.8ms round-trip at 48kHz/128 in OSS testing
• RPi 4B Cortex-A72 can sustain 128 frames at 48kHz with moderate DSP load
• NAM model inference is the bottleneck — NOT the audio I/O
• jackd -R -d alsa -d hw:1,0 -r 48000 -p 128 -n 3 — 3 periods for safety
• For lowest latency: -p 64 -n 2 (2 periods) but has xruns with heavy FX chains

JACK Start Command

# Kill PulseAudio first (it grabs ALSA)
pulseaudio --kill

# Start JACK
jackd -R -d alsa \
  -d hw:audioinjectorpi,0 \
  -r 48000 \
  -p 128 \
  -n 3 \
  -P 0 \
  -C 1

For ALSA device name: use aplay -l and arecord -l after boot to confirm.

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ALSA Device Naming

After overlay is loaded:

Option	Capture (ADC) Device	Playback (DAC) Device
AudioInjector Stereo	hw:CARD=audioinjectorpi,DEV=0	hw:CARD=audioinjectorpi,DEV=0
IQaudio Codec Zero	hw:CARD=IQaudIOCODEC,DEV=0	hw:CARD=IQaudIOCODEC,DEV=0
PCM1808 + PCM5102	hw:CARD=pcm1808,DEV=0	hw:CARD=ALSA,DEV=0 (or hw:CARD=sndrpirpi,DEV=0)
JustBoom DAC/ADC	hw:CARD=justboomadc,DEV=0	hw:CARD=justboomdac,DEV=0
WM8731 (manual)	hw:CARD=wm8731,DEV=0	hw:CARD=wm8731,DEV=0

Run cat /proc/asound/cards after boot to confirm.

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Power Compatibility

Component	VDD	Notes
RPi 4B GPIO (3.3V rail)	3.3V	Max 50mA drawn from 3.3V pin
RPi 4B GPIO (5V rail)	5V	Direct from USB-C, up to 3A
AudioInjector Stereo HAT	5V	Pass-through — can chain power
IQaudio Codec Zero	3.3V	Needs 3.3V from GPIO pin 1 or external regulator
PCM1808	3.3V	~4mA typ
PCM5102	3.3V	~20mA typ
Adafruit Bonnet	3.3V
JustBoom DAC/ADC	5V	Onboard regulator
WM8731	3.3V

5V-tolerant HATs (AudioInjector, JustBoom) are cleaner for a pedal: they don't draw from the limited 3.3V rail, and regulation happens on the HAT itself with dedicated LDOs.

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Recommended Solution: AudioInjector Stereo HAT

Why it wins for the Pi Multi-FX Pedal

1. Full ADC+DAC on one HAT — no separate breadboard wiring, clean signal path in a pedal enclosure
2. Best latency — 1.8ms round-trip demonstrated, well under the 10ms target
3. 5V tolerant — stable power from Pi GPIO 5V pin, no regulator needed
4. Onboard headphone amp — can drive 32Ω headphones for silent practice monitoring
5. Hardware mixing — can blend dry guitar with processed signal without extra DSP
6. 192kHz capable — room for future oversampling or high-res captures
7. Proven JACK compatibility — custom kernel module maintained, works with NAM + LV2 plugins
8. -93dB noise floor — clean enough for high-gain guitar chains

Trade-off: GPIO Blocking

The HAT form factor blocks the full 40-pin GPIO header. For this pedal, the footswitch/display/LED GPIO will need to be routed via:

• Stacking header — solder a stacking female header to the HAT, mount on top of the GPIO pins
• Separate GPIO expander — MCP23017 (I2C) for footswitches, freeing Pi GPIO for audio
• Reroute to P5 header — if using an older Pi 4B revision with the 8-pin P5 header (rare on 4B)

Recommendation: Use a 40-pin female stacking header between the HAT and Pi. The HAT sits on top of the stack, and the footswitch/display/LED GPIO wires connect to the exposed lower pins.

Alternative: PCM1808 + PCM5102 (Budget)

If the $35-40 for AudioInjector is too much, the breakout combo works if:

• You're comfortable soldering on perfboard or protoboard
• You accept 16-bit / 48kHz limit on the ADC
• You add a noise filter (RC low-pass, 10µF cap) on PCM5102 output
• You use shielded wiring inside the enclosure to prevent interference

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Additional Considerations

Guitar Preamp / Buffer

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

• Simple JFET buffer (2N5457 or similar) — ~$2 in parts, clean, unity gain
• Op-amp non-inverting stage (TL072 + a few passives) — ~$1.50, adjustable gain
• Commercial preamp board (e.g., GPCB, or small guitar preamp PCB) — ~$5-15

For the pedal design: a single TL072-based preamp with gain switch (0dB/12dB/24dB) before the ADC input is recommended. Power from the Pi's 5V rail via a 3.3V regulator (AMS1117).

Noise Isolation

• Use ferrite beads on all I2S lines (BCLK, LRCLK, DIN, DOUT) if noise is audible
• Keep analog traces short — mount preamp physically close to ADC input
• Separate analog ground from digital ground at a single star point
• Use 100nF + 10µF decoupling caps on all codec power pins

GPIO Conflicts

RPi 4B GPIOs used by audio HATs (BCM 18/19/20/21) are not available for other uses. Plan footswitch/display/LED wiring on the remaining 20+ available GPIOs or use I2C expander.

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BOM: AudioInjector Stereo HAT Path

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 (input + output)	2	$4	Amazon
Power	USB-C PD 5V/3A supply	1	$10	Anker/Amazon
Total			~$61.50

BOM: PCM1808 + PCM5102 Budget Path

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 dual op-amp	1	$1.50	Mouser
Preamp passives	Resistors, caps, jacks	kit	$5	—
Audio jacks	2x 6.35mm mono TRS jacks	2	$4	Amazon
Wiring	Shielded audio wire + jumper wires	—	$3	—
Power	USB-C PD 5V/3A supply	1	$10	Anker/Amazon
Total			~$36.50

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Final Recommendation

Use AudioInjector Stereo HAT. It's the cleanest path to <10ms round-trip latency with full ADC+DAC, good noise floor, and proven JACK hardware. The ~$38 cost is worth the combined headphone amp, hardware mixing, and solder-free installation.

If budget is tight: PCM1808 + PCM5102 breakouts work but require perfboard assembly, accept 16-bit/48kHz limits, and need extra noise filtering on the DAC output.

Do NOT use: Adafruit Bonnet (DAC-only → needs separate ADC), IQaudio Codec Zero (discontinued, BCKL jitter), or WM8731-based (no upstream overlay, charge pump noise).