7e643c242c
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.
118 lines
5.3 KiB
Markdown
118 lines
5.3 KiB
Markdown
# Verifying audio without an audio device
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Neither container has a sound card, so "does it actually play?" cannot be
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answered by listening. It can be answered by measurement, and the two things
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usually meant by that question need different measurements.
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**Separate them before reaching for a tool:**
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| question | needs Godot? | needs a device? |
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|---|---|---|
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| Is the transcoded file faithful to the source? | no | no |
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| Does Godot actually route it to an output? | yes | no |
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| What does the *game* play on a menu move? | no (Canary) | a virtual one |
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## 1. Transcode fidelity — file against file
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This is the question P4 actually raised, and it needs neither an engine nor a
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device. Decode both, subtract, and measure what is left.
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```bash
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# Source, for a reference level
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ffmpeg -hide_banner -t 25 -i ADV.wmv \
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-af "aformat=channel_layouts=stereo,astats=measure_perchannel=none" -f null - 2>&1 \
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| grep "RMS level"
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# The difference signal: source minus transcode
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ffmpeg -hide_banner -t 25 -i ADV.wmv -t 25 -i ADV.ogv -filter_complex \
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"[0:a]aformat=channel_layouts=stereo[a];\
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[1:a]aformat=channel_layouts=stereo,volume=-1[b];\
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[a][b]amix=inputs=2:normalize=0,astats=measure_perchannel=none" -f null - 2>&1 \
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| grep "RMS level"
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```
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A faithful transcode puts the difference **40 dB or more below** the source.
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### Three ways this measurement lies
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Run it wrong and it reports a disaster that is not there. All three of these
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were hit on the first attempt:
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* **Alignment.** A one-sample offset makes the difference nearly as loud as the
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source. Cross-correlate and compensate *before* subtracting, or the number is
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meaningless. A first run gave source −25.3 dB against difference −34.2 dB —
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only 9 dB down, which looks catastrophic and proves nothing.
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* **Channel layout.** The source and the transcode do not have the same channel
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count. You are not comparing like with like unless both sides are downmixed
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the same way, and `astats` will give you a confident number regardless. See
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[`movie-audio-channels`][mac] for which profile a given movie is in — that is
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a disc fact and lives in the RE corpus, not here.
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* **A file still being written.** `ffprobe` reported the `.ogv` as 33 s against
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the source's 137 s — apparent catastrophic truncation, actually a transcode in
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progress. Check `mtime` and packet count before believing a duration, and
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write to a temp name and rename on completion so a reader cannot see a partial
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file at all.
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⚠️ **The downmix is an unrecorded decision, and it is not ours to make quietly.**
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Nothing in the manifest says a fold happened or on what weighting; it is whatever
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ffmpeg defaulted to, and that default can change between versions. Centre-channel
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dialogue folds into L/R, so this changes how speech sits against music — an
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aesthetic judgement, not a container detail. Pin it explicitly and record it,
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exactly as MISSION §6 requires of the transcode command itself.
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[mac]: https://git.mc02.dev/fabi/Syplheed-Reborn/src/branch/main/docs/re/structures/movie-audio-channels.md
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## 2. Engine routing — Godot writes a WAV instead of a device
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Godot does not need a sound card to produce audio you can inspect. Put an
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`AudioEffectRecord` on the **Master** bus and it captures the mixed output from
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inside a headless run:
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```gdscript
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var bus := AudioServer.get_bus_index("Master")
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var rec := AudioEffectRecord.new()
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AudioServer.add_bus_effect(bus, rec)
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rec.set_recording_active(true)
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# ... play the scene ...
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rec.set_recording_active(false)
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rec.get_recording().save_to_wav("user://master.wav")
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```
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Then feed that WAV through §1 against the source. That closes the loop: it
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proves the asset is right **and** that the engine reached it, which no amount of
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file comparison can show on its own.
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Confirm the dummy driver is what is actually in use rather than assuming it —
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`AudioServer.get_driver_name()` — and say so in the write-up, because "recorded
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under a dummy driver" is a weaker claim than "heard", and the difference matters.
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## 3. A virtual device, when something insists on a real one
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For anything that opens a device rather than a bus — the emulator, most
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obviously — a PulseAudio **null sink** is a real device that records to a file.
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`pulseaudio-utils` is in both images, and `audio-capture` wraps it:
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```bash
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audio-capture run /tmp/menu.wav -- run-canary # start sink, run, record
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audio-capture start # or drive it by hand
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PULSE_SINK=cap godot --path port
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audio-capture record /tmp/out.wav &
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```
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This is the route to capturing what the **game** plays — the menu move and
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confirm cues behind HANDOFF Q8 — rather than what we believe it should play.
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Those bindings are currently a name match against the authors' own identifiers;
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a capture turns them into a measurement.
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⚠️ `audio-capture run` reports the peak level and **warns when the result is
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silent**, because silence is the failure that looks like success: a WAV of
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exactly the right duration, full of zeroes, because the application opened a
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different sink. A duration check alone would pass it.
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## What none of this establishes
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That it *sounds right*. Every method here shows correspondence to a source, not
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that the source is the audio the game plays at that moment, and not that levels
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are sane in a mix. A ten-second human listen still answers something no
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measurement above does — so when a result rests on one of these, say which one.
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