[FFmpeg-trac] #7963(ffmpeg:new): Concat demuxer adds time to video duration

[FFmpeg]

#7963: Concat demuxer adds time to video duration
-------------------------------------+-------------------------------------
             Reporter:  bgx90        |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  4.1
             Keywords:  concat       |               Blocked By:
  demuxer protocol concatenate       |
  duration mkv                       |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 I concatenate some ts files with the concat demuxer and create a mkv file
 using stream copying with the following command:
 {{{
 ffmpeg -f concat -safe 0 -fflags +genpts -i file_list.txt -c copy
 output.mkv
 }}}
 The result is a playable mkv, but the duration is 8 seconds longer than
 the duration of the original ts stream. Also, the picture periodically
 freezes for a very small fraction of a second, but I don't detect any
 pause in the audio; I suspect these stutters are the source of the extra 8
 seconds.  I have tried the command without the +genpts flag as well, but
 there is no change in the resulting mkv.

 Concatenating the ts files with the concat protocol results in an mkv with
 the correct duration and no pauses in the picture during playback.
 This command works as expected:
 {{{
 ffmpeg -fflags +genpts -i "concat:$file_list" -c copy output.mkv
 }}}

 Running ''ffprobe -show_format -show_streams'' on both of the files and
 comparing the outputs shows only the following differences:

 Concat demuxer mkv
 {{{
 duration=1449.811000
 size=280861150
 bit_rate=1549780
 }}}

 Concat protocol mkv
 {{{
 duration=1441.727000
 size=280861151
 bit_rate=1558470
 }}}


 I read in ticket [https://trac.ffmpeg.org/ticket/2801 2801] that the
 concat demuxer needs the files to be split cleanly on keyframes. Could
 this problem be caused by bad splitting of the ts files?

 I don't mind using the concat protocol instead of the concat demuxer. I
 only worry about the limit on command length.  It was suggested in ticket
 [https://trac.ffmpeg.org/ticket/5686 5686] that the cat command could be
 used in cases where the file list is too long and the concat protocol
 works but the concat demuxer does not.  Is using the cat command to
 concatenate ts files and giving the resulting file as input to ffmpeg the
 same as using the concat protocol, or does the concat protocol do
 something more than just concatenating files as the cat command does?

--
Ticket URL: <https://trac.ffmpeg.org/ticket/7963>
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