[FFmpeg-trac] #6114(undetermined:new): Vsync behaviour when used along with -ss

[FFmpeg]

#6114: Vsync behaviour when used along with -ss
-------------------------------------+-------------------------------------
             Reporter:  Mlmlte       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  vsync        |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Summary of the bug:

 To encode some TV transport streams in x264 for archiving purposes, I've
 been using -vsync 1 so that the resulting mkv file gets muxed without
 delay on any of the two tracks (audio/video), which is fundamental since I
 am then able to cut my encodes on key frames without re-encoding, and
 everything plays fine on all of my devices.

 Without the -vsync 1 option, I get a random negative delay on the audio,
 as mediainfo shows (e.g. Delay relative to video: -1s 105ms) and if I then
 try to cut my encode, the audio is out of sync.

 When I use the -ss parameter to avoid encoding the whole transport stream
 file, it seems that the -vsync 1 does not have the same behavior since I
 get a similar negative delay on the audio track.

 However, dupe frames are indeed inserted at the beginning of the encode,
 so I'm not sure exactly what happens here.

 Is it a bug? Any way to get file with zero delay when using -ss parameter?

 In this case, the original file is a h264 transport stream with AC3 audio
 with -668ms delay.

 How to reproduce:

 ffmpeg.exe -ss 00:13:40 -i source.ts -vf -s 1280x720 -sws_flags lanczos
 -c:v libx264 -crf 20 -preset fast -c:a copy -sn -vsync 1 output.mkv

 -> Negative delay

 ffmpeg.exe -i source.ts -vf -s 1280x720 -sws_flags lanczos -c:v libx264
 -crf 20 -preset fast -c:a copy -sn -vsync 1 output.mkv

 -> No delay

 ffmpeg.exe -i source.ts -vf -s 1280x720 -sws_flags lanczos -c:v libx264
 -crf 20 -preset fast -c:a copy -sn output.mkv

 -> Negative delay

 ffmpeg version N-83280-gcba4f0e (latest zeranoe build)

 Thanks a lot!

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Ticket URL: <https://trac.ffmpeg.org/ticket/6114>
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