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

FFmpeg trac at avcodec.org
Wed Mar 6 02:56:42 CET 2013

#2325: MP4 AAC Audio is delayed by 2ms when converted to PCM
             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            |

Comment (by brchapman):

 Replying to [comment:9 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
 > 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?
 yes, if i first transcode the orignal
 % ffmpeg -i test100.mp4 -c:a copy test100.aac
 % ffmpeg -i test100.aac -c:a pcm_s16le test100_audio.wav
 test100_audio.wav is delayed.

 Also, if I encode test100.mp4 without the aac audio stream (ie with no
 audio) and then convert it:
 % ffmpeg -i test100_no_aac.mp4 -c:v prores test100_ffmpeg.mov
 I don't get the duplicate first frame bug in #2324
 Based on this I would guess that this would work:
 ffmpeg -i test100.mp4 -c:v prores -an test100_ffmpeg.mov
 However, it doesn't. The first frame is still duplicated.

Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2325#comment:10>
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