[FFmpeg-trac] #8443(undetermined:new): A/V drift when reencoding NTSC

FFmpeg trac at avcodec.org
Fri Dec 27 04:26:31 EET 2019

#8443: A/V drift when reencoding NTSC
             Reporter:               |                    Owner:
  madscientist159                    |
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |

Comment (by pdr0):

 I don't follow you - did you mean on a longer sample, or "output.mp4" ?

 What overlaid audio ? Are we looking at the same thing ?

 To be clear - I'm looking at the audio track waveform, on the actual
 track. Make sure view=> waveform drawing => waveforms on is checked. Not
 the showwaves waveform burned into the video.

 Since the audio tone occurs every second, and since the video framerate is
 23.976fps (~ 41.7ms / frame) - you cannot express whole seconds perfectly
 everytime (unlike something like 25.0 fps). So the audio waveform position
 might be slightly off a few ms each flash - you should expect that since
 it's not a integer multiple.

 But I'm not seeing drifting in that 30s sample. Drifting implies
 progressively worsening. This means the largest delta should be been first
 and last. I'm seeing the expected fwe ms variation in relation to the
 flash frame. If it's a bit to the left, it's a bit to the right a bit
 later. Progressive drift implies always 1 direction

 You need to check longer sample. If it eventually shift 1 frame by a
 certain time

 Another possible complication is blender might not be the best tool for
 this either; it usually deals with image sequences. It can have problems
 with long GOP formats with non linear seeks (sometimes frames get mixed

Ticket URL: <https://trac.ffmpeg.org/ticket/8443#comment:11>
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