[FFmpeg-user] concat demuxer and 'strange' mp4s with wrong TBN

Gabriele Greco gabriele.greco at wyscout.com
Wed Aug 22 10:21:13 EEST 2018

I have problems to concatenate with the concat demuxer two mp4, that
individually play fine but the second one has wrong time base information
in the context:

I tried both with 3.4.1 version of ffmpeg and the SVN one.

Extracting relevant infos with ffprobe I found that:

file1.mp4 has the following video track:
    Stream #0:0(eng): Video: mpeg4 (Simple Profile) (mp4v / 0x7634706D),
yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 5000 kb/s, 30 fps, 30 tbr, 600 tbn,
30 tbc (default)

file2.mp4 has the following video track:
    Stream #0:0(und): Video: mpeg4 (Simple Profile) (mp4v / 0x7634706D),
yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 4979 kb/s, 30 fps, 30 tbr, 15360 tbn,
30 tbc (default)

As you can see TBN for the second file is wrong, don't ask me why, we get
videos of every format from our customers...

Both files play fine in VLC and other players since I think they use TBR (the
time base in AVCodecContext) or TBC (the guessed one) instead of TBN (the
time base in AVStream) for the playback, the problem is that the concat
demuxer uses TBN and this is clear by the fact that during the concat
operation my progress and bitrate jump when the input switch to the second

frame=10423 fps=2670 q=-1.0 size=  217600kB time=00:05:47.49
bitrate=5129.8kbits/s speed=  89x
frame=29581 fps=2997 q=-1.0 size=  617472kB time=00:16:26.04
bitrate=5129.9kbits/s speed=99.9x
frame=57845 fps=3131 q=-1.0 size= 1207296kB time=00:32:08.17
bitrate=5129.3kbits/s speed= 104x
frame=79035 fps=3176 q=-1.0 size= 1649664kB time=00:43:54.55
bitrate=5129.5kbits/s speed= 106x
[ ... here the concat demuxer switches to the second file .. ]
frame=91329 fps=3170 q=-1.0 size= 1901824kB time=21:38:57.30 bitrate=
199.9kbits/s speed=2.71e+03x
frame=101922 fps=3179 q=-1.0 size= 2123008kB time=24:09:36.66 bitrate=
200.0kbits/s speed=2.71e+03x
frame=112519 fps=3192 q=-1.0 size= 2344192kB time=26:40:19.43 bitrate=
200.0kbits/s speed=2.72e+03x
frame=151570 fps=3220 q=-1.0 size= 3155200kB time=35:55:44.18 bitrate=
199.8kbits/s speed=2.75e+03x
frame=171012 fps=3235 q=-1.0 Lsize= 3566188kB time=40:32:14.69 bitrate=
200.2kbits/s speed=2.76e+03x

file 'file1.mp4'
file 'file2.mp4'

usual call:
ffmpeg -f concat -safe 0 -i inputs.txt -c copy test.mp4

I tried also to add -fflags +genpts but it doesnt change anything:
ffmpeg -fflags +genpts -f concat -safe 0 -i inputs.txt -c copy test.mp4

there is a way, also patching sources, to force concat demuxer to use tbc
for his calculations instead of TBN if the TBN of one of the files is
clearly wrong?

*Gabriele Greco*

*Senior Software Engineer - Mobile Division*Mobile: *+39 3391941700*
Website:  www.wyscout.com
[image: Descrizione: Descrizione: Descrizione: Descrizione:


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