[FFmpeg-trac] #7939(undetermined:new): The concat demuxer changes a constant frame rate into variable one

FFmpeg trac at avcodec.org
Tue Jun 4 13:55:33 EEST 2019

#7939: The concat demuxer changes a constant frame rate into variable one
             Reporter:  RigacciOrg   |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  concat       |               Blocked By:
  h264_mp4toannexb codec_time_base   |
  avg_frame_rate                     |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 The file "clip_mpeg2.ts" is an MPEG-2 transport stream in constant frame
 rate (as reported by ''ffprobe -show_format -show_streams''):


 If I concatenate it twice, to produce an '''mp4''' file, I get a variable
 frame rate file! The concat command is:

 ffmpeg -f concat -safe 0 -i clip-list.txt -c copy clip-concat.mp4

 where the clip-list.txt contains just two lines with ''file

 In the resulting file, the '''codec_time_base''' is changed,
 '''r_frame_rate''' and '''avg_frame_rate''' are not exact:


 I suspect that the source file has some bits which triggers this bug,
 because I was used to concatenate clips, but I never encountered this

 May be it is related to ticket #5380, because the input file is produced
 using '''-bsf:v h264_mp4toannexb'''.

 If instead I use an '''mkv''' container as destination, the problem does
 not exist.

Ticket URL: <https://trac.ffmpeg.org/ticket/7939>
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