[FFmpeg-trac] #9375(undetermined:new): AAC decoder priming not discarded

[FFmpeg]

#9375: AAC decoder priming not discarded
-------------------------------------+-------------------------------------
             Reporter:  neXyon       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:  AAC aac      |               Blocked By:
  priming                            |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:

 AAC priming samples should be discarded and not transcoded with ffmpeg.
 This issue exists when using ffmpeg on the command line, but we also
 experience it using avcodec as library directly in Blender.

 How to reproduce:

 Take any wav file and transcode it to aac and back to wav:

 {{{
 % ffmpeg -i original.wav -acodec aac intermediate.aac
 % ffmpeg -i intermediate.aac final.wav
 }}}

 If you now compare the two wav files you'll see (besides some expected
 artifacts due to the compression of aac) that the final.wav has priming
 samples added at the beginning that shouldn't be there.

 Using the API:

 Using the API of ffmpeg, we would like to be able to discard the priming
 ourselves, but while AVPacket and AVFrame both have flags fields which can
 have either AV_PKT_FLAG_DISCARD or AV_FRAME_FLAG_DISCARD set, none of
 those are set for the respective packages. Is there another way to
 distinguish whether the start of an audio stream should be discard?

 A possible solution for a video file where we have a video and an aac
 audio stream is to drop any audio before the first PTS of the video
 stream. However, this is not a good solution since there could be audio
 before the video stream that should be played back (so the audio stream
 starts before the video stream intentionally).
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9375>
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