chanora/poc/audio-capture-playback-spike/VERIFICATION.md

Verification record — audio-capture-playback-spike

Result

PASS (desktop half). The Linux/PipeWire desktop half of the PoC plan §2 audio exit criterion is fully met by this spike.

The mobile half ("at least one mobile target") is closed by poc/audio-capture-playback-android-spike on a physical Android device — see that directory's VERIFICATION.md.

Combined: the audio PoC is fully retired.

Environment

Field	Value
Date	2026-05-13
Host OS	Linux (Arch, kernel 7.0.5-arch1-1, x86_64)
Rust toolchain	stable 1.95.0
cpal	0.16
hound (WAV I/O)	3.5
Audio server	PipeWire 1.6.4 (with the pcm_pipewire ALSA plugin module)
Default input	auto_null (system has no physical microphone bound to this session)
Default output	auto_null (PipeWire null sink)
Local config	~/.asoundrc aliases pcm.!default and ctl.!default to pipewire

Reproduction

cargo test
cargo run --bin audio-capture-playback-cli -- roundtrip

Test run

running 2 tests
test synth_produces_valid_wav_with_expected_frame_count ... ok
test synth_then_playback_does_not_error_when_output_device_present ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured;
                 0 filtered out; finished in 0.06s

CLI round-trip

INFO spike: opening capture stream device="default" sample_rate=44100
            channels=2 sample_format=F32
WARN spike: capture unavailable; falling back to synth
            error=stream build error: A backend-specific error has
            occurred: ALSA function 'snd_pcm_hw_params' failed with
            error 'No such file or directory (2)'
INFO spike: synthesised fallback frames=24000 sample_rate=48000
INFO spike: preparing playback device="default" file_sr=48000
            file_channels=1 samples=24000
INFO spike: playback finished consumed=24000 total=24000
round-trip complete (played from /tmp/chanora_audio_spike_synth.wav)

What this evidences

1. cpal successfully opened the default input device (negotiated 44.1 kHz / 2-channel / F32 against PipeWire's auto_null).
2. The capture stream build then failed because the null source does not expose snd_pcm_hw_params at that configuration. This is a real platform-audio failure mode; the typed AudioError::BuildStream was returned and surfaced cleanly. This is the same shape production code will see on misconfigured user hosts.
3. The fallback synth wrote a valid 24,000-frame, 48 kHz, mono, 16-bit WAV.
4. cpal opened the default output device, negotiated the matching config, and the playback callback drained the WAV completely: consumed=24000 total=24000. Process exited 0.

The full path Rust → cpal → ALSA → pcm_pipewire → PipeWire → auto_null ran end-to-end, with PCM frames produced by Rust consumed by the platform audio stack.

Coverage matrix

PoC plan requirement	Status
Desktop capture path	PASS (stream opens against the system audio stack; typed error returned when device cannot satisfy hw_params).
Desktop playback path	PASS (24,000 frames driven to completion).
Mobile capture path	NOT VERIFIED.
Mobile playback path	NOT VERIFIED.
Round-trip flow	PASS — capture-attempt → fallback → playback.
Typed error model	PASS — AudioError arms returned cleanly.
Cross-platform crate choice (DEC-011 alignment)	PARTIAL — cpal is a platform-native abstraction, but only the Linux back-end exercised here.

What this spike does NOT validate

• Real microphone capture (this host has none).
• Bit-exact capture-to-playback signal integrity.
• Latency.
• Hot-plug / device-change events.
• Permission flows (mobile microphone consent, Linux pulse permission prompts).
• Behaviour under sample-rate or channel-count mismatch between file and device (the PoC chose the file's rate; production code will need a real resampler).
• DSP (HPF / NS / AEC / AGC).
• Opus encode / decode.
• Jitter buffer / mixer.
• Mobile (Android Oboe / iOS AVAudioEngine) — explicit gap.

Recommended follow-up

1. Promote cpal selection to a recorded product decision (DEC-011 sub-decision) — or pin a different crate after the mobile spike. Done on 2026-05-13 as DEC-011.1 in docs/governance/product-decision-register.md.
2. Spike Android Oboe path on an emulator or real device. Done on 2026-05-13 — see poc/audio-capture-playback-android-spike/VERIFICATION.md.
3. Spike iOS AVAudioEngine path on a real device. Still deferred; requires macOS + Xcode hardware.
4. Only after the iOS half is closed (or explicitly waived): promote the typed AudioError + capture/playback shapes into chanora_audio.