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fix(audio): renumber timestamps between apad and atrim in mixed branches (#3380)

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On FFmpeg 5.x through 8.0.x the samples `apad` appends carry timestamps
the following `atrim` misreads. A delayed branch then sounds at t=0
instead of its offset and, once four or more branches are mixed, the last
one disappears from the output entirely. No error is raised; the render
succeeds with wrong audio.

Reverting to `apad=whole_dur=` is not an option: #2769 moved off that
form because some builds reject the option outright ("Error applying
option 'whole_dur': Option not found"). Inserting `asetpts=N/SR/TB`
between the pad and the trim rebuilds the timestamps from the sample
count using only filters every build ships, so it fixes the misplacement
without giving up the portability that change bought.

Verified on FFmpeg 4.2.7, 7.0.2, an 8.x nightly and 8.1.1: the current
form is wrong on the middle two, the new form is correct on all four.

audioPadTrim.ts also pads with apad+atrim but has no adelay and is
correct on every version tested, so it is left alone.

Closes #3344
This commit is contained in:
Miguel Ángel
2026-08-20 21:17:50 -07:00
committed by GitHub
parent 477e09642b
commit c056289d83
3 changed files with 70 additions and 4 deletions
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packages/producer/src/services/audioExtractor.ts
+2 -1
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@@ -211,8 +211,9 @@ async function mixTracks(
const delayMs = Math.round(track.start * 1000);
const trimDuration = track.duration > 0 ? track.duration : totalDuration;
// See audioMixer.ts for why asetpts sits between apad and atrim.
filterParts.push(
`[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},apad,atrim=0:${totalDuration}[a${i}]`,
`[${i}:a]atrim=0:${trimDuration},volume=${track.volume},adelay=${delayMs}|${delayMs},apad,asetpts=N/SR/TB,atrim=0:${totalDuration}[a${i}]`,
);
});