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• FFMPEG Seeking with concat demuxer causes video & audio to be out of sync

  20 février 2023, par Garuuk

  I have a very simple use case that's driving me bananas.

  


  My problem and question :

  


  I'm using ffmpeg version 5.1.2 on a MacOS and i'm using ffmpeg seeking and concat demuxer to cut many 1 minute videos into 15 seconds chopped up over 12 clips where every clip is just 2 seconds from the same video (kind of like a mini teasers for the video). I would really like to not have to re-encode to make the video processing as fast as possible.

  


  First, I take each 1 minute video and cut it up into 12 clips (I do all this programmatically in python fwiw)

  


  ffmpeg -ss 0 -i input.mp4 -t 2 -c copy -y cut_1.mp4
ffmpeg -ss 4 -i input.mp4 -t 2 -c copy -y cut_2.mp4
ffmpeg -ss 8 -i input.mp4 -t 2 -c copy -y cut_3.mp4
...
...


  


  I then write all the output file names to my concat_manifest.txt

  


  file cut_1.mp4
file cut_2.mp4
...
...


  


  Then I run my concat command :

  


  ffmpeg -f concat -i concat_manifest.txt -c copy -y concat_video.mp4


  


  This works really fast but the audio and video at the stitch point get out of sync and sometimes the video just chokes & lags. It's mostly not a smooth experience.

  


  What I have tried :

  


  

  1. using the concat protocol with intermediate profiles : ffmpeg.org/wiki/Concatenate#demuxer

  


  2. Putting the -ss when I seek after the -i. This makes everything worse

  


  3. Playing around with different -ss values. This has some noticeable affects but it's not obvious why yet.

  


  4. I've also read from the ffmpeg resource regarding seeking and copying :

  


  


  Which leads me to believe that maybe because ffmpeg is using timestamps instead of frames, seeking isn't accurate using -ss when using the concat demuxer

  


  Is there a way to get concat demuxer cutting and concatenating the video where the audio is somewhat in sync with the video ?

  


  Thanks

  


  EDIT : I found an answer and i'll be posting the solution in the coming few days.

  


• Two pass high quality theora/vorbis ffmpeg encoding

  25 septembre 2017, par Lea Chescotta

  I want to achieve the same video encoding that I had with ffmpeg2theora with standard ffmpeg, this is because i need the flexibility ffmpeg has to make the container mkv, with subtitles other than srt.

  In ffmpeg2theora i have the following command that output a very high quality and very small filesize file :

  $ ffmpeg2theora --videobitrate 2000 --two-pass --first-pass firstpass --speedlevel 0 --width 640 --height 360 --resize-method lanczos --noaudio input.mkv
  
  $ ffmpeg2theora --videobitrate 2000 --two-pass --second-pass firstpass --speedlevel 0 --width 640 --height 360 --resize-method lanczos --noaudio input.mkv --output output.ogv

  Being the most interesting options here i think (From ffmpeg2theora manual page) :

  --two-pass
  
  --first-pass <filename>
  
  --second-pass <filename> 
  
  --speedlevel
  
      encoding is faster with higher values the cost is quality and bandwidth (default 1)
  
  </filename></filename>

  But i can only found a simple way to encode theora/vorbis in standard ffmpeg (from : https://trac.ffmpeg.org/wiki/TheoraVorbisEncodingGuide) :

  ffmpeg -i input.mkv -codec:v libtheora -qscale:v 7 -codec:a libvorbis -qscale:a 5 output.ogv

  That produces a very bad quality output even in the best quality setting (10)

  How can I do a 2 pass ’high quality’/’not so big filesize’ theora/vorbis in plain ffmpeg ?

• What does Elementary Stream mean in Terms of H264

  5 août 2015, par user1767754

  I read what an Elementary Stream is on Wikipedia. A tool i am using "Live555" is demanding "H.264 Video Elementary Stream File". So when exporting a Video from a Video Application, do i have to choose specific preferences to generate a "Elementery Stream" ?