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    I think this question belongs on the ffmpeg forum, no? Commented Aug 15, 2015 at 23:29
  • I'm pretty sure ffmpeg is the tool for your needs. Also undf means undefined or unknown.
    – GiantTree
    Commented Aug 15, 2015 at 23:53
  • Please show the complete console output of: ffmpeg -i input.mkv.
    – llogan
    Commented Aug 16, 2015 at 2:36