[FFmpeg-trac] #5910(undetermined:new): AAC to PCM conversion inserts extra silence in the beginning

FFmpeg trac at avcodec.org
Tue Nov 1 17:32:46 EET 2016

#5910: AAC to PCM conversion inserts extra silence in the beginning
             Reporter:               |                    Owner:
  jwilhelmsson                       |                   Status:  new
                 Type:  defect       |                Component:
             Priority:  normal       |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:  aac pcm      |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |

Comment (by jwilhelmsson):

 I figured.

 I don't know what the expected delay is, or if there is any, only the
 difference between what ffmpeg produces when extracting and converting and
 what Nuendo/Protools/Quicktime produces.

 As I wrote, the source material we receive is mp4 and/or mov files, but
 when we work with it we need to have the audio (in wav format) on a
 separate track so I need to extract and convert the aac audio streams.
 During this conversion, ffmpeg seems to insert silence in the beginning of
 the files, as follows (all numbers are approximate and measured by marking
 areas in Audacity):

 The "tj" files get 44 ms delay (about 2100 samples, which sounds close to
 the 2112 value).

 The "td" files get 108 ms delay.

 The "g" file gets 32 ms delay *at the start*, but 44 ms overall. This is
 easy to see since it starts with color bars and a tone which should start
 at the very beginning and has a clean end. Comparing the end points you
 get 44 ms, but the start is for some reason shorter.

 And as I've written, these delays appear regardless of whether the audio
 is extracted and converted from the original file, with {{{ffmpeg -i
 source.mp4 -c:a pcm_s24le -ar48k out.wav}}}, or if the audio is first
 extracted ({{{ffmpeg -i source.mp4 -c:a copy source.aac}}}) and then
 converted ({{{ffmpeg -i source.aac -c:a pcm_s24le -ar48k out.wav}}}).

Ticket URL: <https://trac.ffmpeg.org/ticket/5910#comment:17>
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