f44ebced59
Brought in with a subtree merge rather than a copy, so the port's 31 commits survive as history rather than arriving as one anonymous import. Landed under godot-import/ and moved into the final layout in the next commit, which keeps git's rename detection able to follow each file across the move.
111 lines
4.9 KiB
Markdown
111 lines
4.9 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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```bash
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pactl load-module module-null-sink sink_name=cap sink_properties=device.description=cap
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PULSE_SINK=cap <the application>
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parec -d cap.monitor --file-format=wav /tmp/captured.wav
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```
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This is the route to capturing what the *game* plays — the menu move and confirm
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cues behind HANDOFF Q8 — rather than what we think it should play. It needs
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`pulseaudio-utils` in the image, so it is a rebuild, not something to reach for
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mid-iteration.
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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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