[FFmpeg-trac] #2325(undetermined:new): MP4 AAC Audio is delayed by 2ms when converted to PCM

[FFmpeg]

#2325: MP4 AAC Audio is delayed by 2ms when converted to PCM
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             Reporter:  brchapman    |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  important    |                Component:
              Version:  git-master   |  undetermined
             Keywords:  aac mov      |               Resolution:
  regression                         |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by cehoyos):

 Replying to [comment:8 brchapman]:
 > Just tried pulling 0332324, and everything lines up great! there's no
 2ms delay,

 > and the other ticket about a duplicate first frame I posted #2324 is
 also fixed!

 No, the output file is not valid.
 (You can easily change the FFmpeg source to allow writing VFR mov files,
 but they are not conforming to any specification.)

 > > > > Could you explain how you know that the delay is a bug?
 > > > When converting through After Effects, I don't get this delay.
 Everything lines up exactly in the output wave file.
 > >
 > > How does it "line up"? (I don't understand how the numbers should
 relate to the sound. I am certainly not claiming there is no issue - I
 don't know - but since we know already of three - very different -
 applications that decode the sample differently from After Effects, I
 wonder how you can be sure that it is correct.)
 >
 > I'm defining correct as placing the source mp4 and output wave file in a
 timeline together and checking if the waveforms match between them.  This
 is shown in the attached screenshot.  Since I'm not doing anything other
 than just reading a file in and encoding it to a different, I would expect
 the input and output sound to line up exactly.  Is this how you would
 expect it work?

 Your reasoning basically assumes that After Effects is right and FFmpeg,
 nero and QuickTime are wrong. While I am not saying this isn't the case,
 it is no proof imo.
 (There is a mov sample from a camera somewhere on this tracker that shows
 a "visible" noise (knocking on a table iirc), it would be interesting to
 test that sample with all applications, I unfortunately fail to find it
 atm.)

 > > > > Do you see the same problem if you extract the audio stream from
 the mov file with "ffmpeg -i test100.mp4 -acodec copy out.aac" ? Ie, is
 the problem in any way related to the container or only to aac?
 > > > No, I don't get the delay. It lines up perfectly.
 > >
 > > You mean you get the same delay if you use FFmpeg but no delay with
 After Effects if you try with the aac file - or do I misunderstand?
 > When I run the command  "ffmpeg -i test100.mp4 -acodec copy out.aac",
 the out.aac file audio matches the source mp4's audio exactly, without any
 delay.

 And if you transcode the out.aac file with FFmpeg and compare it in
 AfterEffects, you see the same delay as when transcoding the original mp4
 file, or am I wrong?

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2325#comment:9>
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