a8d2491366
The audio work was three parts and I shipped two. The transcode-fidelity method and the pinned 5.1 downmix landed; the null sink -- the only one that answers "what does the GAME play" -- I deferred to "the next natural rebuild window" and then rebuilt both images four times without doing it. pulseaudio-utils is now in both, with tools/audio-capture wrapping it: a null sink is a real device as far as an application is concerned, so Canary and Godot open it normally and parec records what they emit. This unblocks the decoder's Q8. The cue-to-event bindings are currently a name match against the authors' own identifiers -- a plausible guess, not a measurement -- and capturing what the game plays on a menu move converts them. `audio-capture run` reports the peak level and warns when the capture is silent, because silence is the failure that looks like success: a WAV of exactly the right duration, full of zeroes, because the application opened a different sink. A duration check alone passes it, which is how a confident wrong number gets made.
5.3 KiB
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
astatswill give you a confident number regardless. Seemovie-audio-channelsfor 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.
ffprobereported the.ogvas 33 s against the source's 137 s — apparent catastrophic truncation, actually a transcode in progress. Checkmtimeand 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.
pulseaudio-utils is in both images, and audio-capture wraps it:
audio-capture run /tmp/menu.wav -- run-canary # start sink, run, record
audio-capture start # or drive it by hand
PULSE_SINK=cap godot --path port
audio-capture record /tmp/out.wav &
This is the route to capturing what the game plays — the menu move and confirm cues behind HANDOFF Q8 — rather than what we believe it should play. Those bindings are currently a name match against the authors' own identifiers; a capture turns them into a measurement.
⚠️ audio-capture run reports the peak level and warns when the result is
silent, because silence is the failure that looks like success: a WAV of
exactly the right duration, full of zeroes, because the application opened a
different sink. A duration check alone would pass it.
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.