[FFmpeg-trac] #4768(ffmpeg:new): FFmpeg preserving CFR during TS to MP4 conversion

FFmpeg trac at avcodec.org
Tue Aug 11 16:58:48 CEST 2015

#4768: FFmpeg preserving CFR during TS to MP4 conversion
             Reporter:  zer0z   |                     Type:  enhancement
               Status:  new     |                 Priority:  normal
            Component:  ffmpeg  |                  Version:  1.0.10
             Keywords:          |               Blocked By:
             Blocking:          |  Reproduced by developer:  0
Analyzed by developer:  0       |
 Summary of the bug: Problem due to technical limitation of TS format,
 duration are rounded and this rounding is not detected during TS to MP4
 (not a bug, only a lack of feature). Instead FFmpeg should detect the CFR
 in TS to MP4 conversion and create a CFR stts atom. Instead of the actual
 stts (time table) atom which takes lot of space and which is not CFR
 stricly speaking.

 I convert MP4 to TS, which has a hard coded (spec) frequency of 90 000 Hz.
 ffmpeg converts it as it can, with some rounding (in my time time line:
 sometime 3754, sometime 3753, but the real duration should be 3753.75, and
 Ts accepts no decimal).

 Then I convert it back to MP4, and ffmpeg does not understand it is CFR,
 so it provides the same rounding. Resultant file ends up as VFR.

 How to reproduce:

 From ffmpeg wiki:
 If you have MP4 files, these could be losslessly concatenated by first
 transcoding them to mpeg transport streams. With h.264 video and AAC
 audio, the following can be used:
 ffmpeg -i input1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts
 ffmpeg -i input2.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts
 ffmpeg -i "concat:intermediate1.ts|intermediate2.ts" -c copy -bsf:a
 aac_adtstoasc output.mp4

 I convert 1.mp4, 2.mp4, 3.mp4 (all encoded at CFR) with above method to
 .ts. Then issue:
 ffmpeg -i "concat:1.ts|2.ts|3.ts" -c copy -bsf:a aac_adtstoasc merged.mp4

 Due to the above issue the result from concat will be VFR:

 Frame rate mode                          : Variable
 Frame rate                               : 23.976 fps
 Minimum frame rate                       : 23.968 fps
 Maximum frame rate                       : 23.981 fps

 If this can be looked into and hopefully resolved to preserve the Constant
 frame rate I would really appreciate it.

Ticket URL: <https://trac.ffmpeg.org/ticket/4768>
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