AAC audio in an MP4/MOV whose sample rate differs from the mix rate
(a 44.1 kHz clip in the 48 kHz preview or export, or the reverse)
stuttered and sounded low-pitched in previews and in exported movies.
Cause: FilmCraft's MP4 reader, asked for audio at a rate other than
the file's, applied the edit list's priming offset (1024 samples for
AAC) twice when it resampled: each read started about 1024 source
samples early and extrapolated a ramp over its first ~940 samples
(peaks near 100 against a 0.5 tone). Export and the preview mix ask
for audio a frame or a device buffer at a time, so the ramp repeated
dozens of times a second. Same-rate reads take FilmCraft's copy path
and were exact. FilmCraft fixed this upstream in 5bf2203, 87 commits
past the pinned revision.
Fix: MediaPool reads footage audio at the source's own rate and
resamples it itself (linear, by absolute sample position, so a range
comes out the same however it is split into reads), for every
container.
Test: resampled_audio_read_in_frame_pieces_matches_ffmpeg (44.1 kHz
AAC read at 48 kHz and 48 kHz at 44.1 kHz in 29.97 fps frame pieces,
against ffmpeg's decode: max error 44 before, 0.002 after) and
movies_carry_resampled_footage_audio (a 29.97 fps comp with 44.1 kHz
AAC footage exported to MP4/AAC and MOV/PCM, decoded by ffmpeg: max
error 8.7 before, 0.03 / 0.002 after).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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