# 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`](../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

```bash
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`.