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When extracting audio and video separately from a mp4 and recombining again

ffmpeg -accurate_seek -ss 30 -t 10 -i input.mp4 -vn output-audio.mp4
ffmpeg -accurate_seek -ss 30 -t 10 -i input.mp4 -an output-video.mp4
ffmpeg -i output-audio.mp4 -i output-video.mp4 output-combine.mp4

the three mp4-s have different length according to ffprobe:

ffprobe output-video.mp4
Duration: 00:00:10.00, start: 0.000000, ...
ffprobe output-audio.mp4
Duration: 00:00:10.03, start: 0.033333, ...
ffprobe output-combine.mp4
Duration: 00:00:10.08, start: 0.033333, ...

when combining audio and video with -shortest

ffmpeg -i output-audio.mp4 -i output-video.mp4 test-combine.mp4

the length is

ffprobe output-combine.mp4
Duration: 00:00:10.04, start: 0.033333, ...

How can the start-offset (0.033333) be avoided so that all parts have the same length and the original part can be restored by combining audio and video?

  • Share full log of ffmpeg -i input.mp4 – Gyan May 13 '18 at 13:25
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Correction: instead of

ffmpeg -i output-audio.mp4 -i output-video.mp4 test-combine.mp4

it should be

ffmpeg -i output-audio.mp4 -i output-video.mp4 -shortest output-combine.mp4

Kind of answer:

ffmpeg -i output-video.mp4 -itsoffset 0.033333 -t 9.966666 -i output-audio.mp4  output-combine.mp4

seems to do the job, although the result differs in file-size slightly from an direct extraction (direct is bigger than combine) done with

ffmpeg -accurate_seek -ss 30 -t 10 -i input.mp4 output-direct.mp4

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