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MKVMerge fails concatenating AV1 IVFs correctly unless default duration is set to the stream's framerate, resulting in audio desynchronization

  1. Version used at the time: 50.0.0

  2. Windows 10 2004 64-bit. Issue can also be reproduced on Ubuntu 20.4.1.

  3. How to get the bug

  • Split up the source file into multiple segments.
  • Encode those segments using the reference AV1 aomenc encoder.
  • Concatenate the streams together.
  • Framerate: 24000/1001 FPS
  1. Expected behaviour
  • Video stream is perfectly synchronized with the source audio.
  • Framerate in MediaInfo is noted to be the source framerate with CFR(24000/1001 FPS)
  1. What actually happened
  • Video stream is completely desynchronized compared to the source audio, and gets more desynced as time goes on.
  • Framerate in Mediainfo is noted to have a completely different framerate(24.199FPS) with VFR(variable framerate)
  1. Hypothesis of the cause
  • MKVMerge/MKVToolnix might not be able to parse the IVF information correctly, which creates an error and makes MKVMerge add in a bunch of duplicate frames, desyncing the video stream.
  1. Temporary way to fix it.
  • Fix the default duration framerate in MKVMerge to the video stream's framerate(in this case, 24000/1001 FPS).

Hopefully, this should not be too difficult to fix.