Sylpheed/docs/AUDIO-VERIFICATION.md

Verifying audio without an audio device

Neither container has a sound card, so "does it actually play?" cannot be answered by listening. It can be answered by measurement, and the two things usually meant by that question need different measurements.

Separate them before reaching for a tool:

question	needs Godot?	needs a device?
Is the transcoded file faithful to the source?	no	no
Does Godot actually route it to an output?	yes	no
What does the game play on a menu move?	no (Canary)	a virtual one

1. Transcode fidelity — file against file

This is the question P4 actually raised, and it needs neither an engine nor a device. Decode both, subtract, and measure what is left.

# Source, for a reference level
ffmpeg -hide_banner -t 25 -i ADV.wmv \
  -af "aformat=channel_layouts=stereo,astats=measure_perchannel=none" -f null - 2>&1 \
  | grep "RMS level"

# The difference signal: source minus transcode
ffmpeg -hide_banner -t 25 -i ADV.wmv -t 25 -i ADV.ogv -filter_complex \
  "[0:a]aformat=channel_layouts=stereo[a];\
   [1:a]aformat=channel_layouts=stereo,volume=-1[b];\
   [a][b]amix=inputs=2:normalize=0,astats=measure_perchannel=none" -f null - 2>&1 \
  | grep "RMS level"

A faithful transcode puts the difference 40 dB or more below the source.

Three ways this measurement lies

Run it wrong and it reports a disaster that is not there. All three of these were hit on the first attempt:

• Alignment. A one-sample offset makes the difference nearly as loud as the source. Cross-correlate and compensate before subtracting, or the number is meaningless. A first run gave source −25.3 dB against difference −34.2 dB — only 9 dB down, which looks catastrophic and proves nothing.
• Channel layout. The source and the transcode do not have the same channel count. You are not comparing like with like unless both sides are downmixed the same way, and astats will give you a confident number regardless. See movie-audio-channels for which profile a given movie is in — that is a disc fact and lives in the RE corpus, not here.
• A file still being written. ffprobe reported the .ogv as 33 s against the source's 137 s — apparent catastrophic truncation, actually a transcode in progress. Check mtime and packet count before believing a duration, and write to a temp name and rename on completion so a reader cannot see a partial file at all.

⚠️ The downmix is an unrecorded decision, and it is not ours to make quietly. Nothing in the manifest says a fold happened or on what weighting; it is whatever ffmpeg defaulted to, and that default can change between versions. Centre-channel dialogue folds into L/R, so this changes how speech sits against music — an aesthetic judgement, not a container detail. Pin it explicitly and record it, exactly as MISSION §6 requires of the transcode command itself.

2. Engine routing — Godot writes a WAV instead of a device

Godot does not need a sound card to produce audio you can inspect. Put an AudioEffectRecord on the Master bus and it captures the mixed output from inside a headless run:

var bus := AudioServer.get_bus_index("Master")
var rec := AudioEffectRecord.new()
AudioServer.add_bus_effect(bus, rec)
rec.set_recording_active(true)
# ... play the scene ...
rec.set_recording_active(false)
rec.get_recording().save_to_wav("user://master.wav")

Then feed that WAV through §1 against the source. That closes the loop: it proves the asset is right and that the engine reached it, which no amount of file comparison can show on its own.

Confirm the dummy driver is what is actually in use rather than assuming it — AudioServer.get_driver_name() — and say so in the write-up, because "recorded under a dummy driver" is a weaker claim than "heard", and the difference matters.

3. A virtual device, when something insists on a real one

For anything that opens a device rather than a bus — the emulator, most obviously — a PulseAudio null sink is a real device that records to a file:

pactl load-module module-null-sink sink_name=cap sink_properties=device.description=cap
PULSE_SINK=cap <the application>
parec -d cap.monitor --file-format=wav /tmp/captured.wav

This is the route to capturing what the game plays — the menu move and confirm cues behind HANDOFF Q8 — rather than what we think it should play. It needs pulseaudio-utils in the image, so it is a rebuild, not something to reach for mid-iteration.

What none of this establishes

That it sounds right. Every method here shows correspondence to a source, not that the source is the audio the game plays at that moment, and not that levels are sane in a mix. A ten-second human listen still answers something no measurement above does — so when a result rests on one of these, say which one.