[FFmpeg-trac] #11031(avformat:new): CMAF MP4 not skipping priming samples for audio tracks

[FFmpeg]

#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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