#11031: CMAF MP4 not skipping priming samples for audio tracks
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Reporter: derrod | Type: defect
Status: new | Priority: normal
Component: avformat | Version: git-master
Keywords: fmp4 cmaf | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Summary of the bug:
When muxing an audio stream to a CMAF file, edit lists get disabled unless
explicitly enabled (which prints a warning message). While they are not
necessary for video due to negative CTS offsets being enabled, audio still
requires them to skip "priming" samples produced particularly by AAC and
Opus encoders.
When muxing an audio stream that has priming samples (e.g. 2112 when using
the AudioToolbox AAC encoder) into a CMAF ISO file the edit list will be
missing and the first 2112 samples of silence will not be skipped,
resulting in A/V desync.
I do not have access to the CMAF spec so I cannot check what the current
intended solution is. The 2016 draft does say that an edit list "SHOULD"
be used to deal with audio data before presentation time 0 and names
priming as an example.
How to reproduce:
{{{
# base file
% ffmpeg -f lavfi -i sine -c:a aac_at -t 10 audio_test.mp4
# mux with CMAF enabled
% ffmpeg -i audio_test -c copy -movflags cmaf+delay_moov
audio_test_cmaf.mp4
# remux to regular MP4, which will play 2112 additional samples of silence
at the start (44 ms at 48 kHz)
% ffmpeg -i audio_test_cmaf.mp4 -c copy audio_test_remuxed.mp4
}}}
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Ticket URL: <https://trac.ffmpeg.org/ticket/11031>
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