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• Slow audio-video sync drift when merging wav and mp4 with ffmpeg

  27 juin 2018, par charlie80

  I have an mp4 file with only a single video stream (no audio) and a wav audio file that I would like to add to the video using ffmpeg. The audio and the video have been recorded simultaneously during a conference, the former from a mixer output on a PC and the latter from a digital videocamera.

  I am using this ffmpeg command :

  ffmpeg -i incontro3.mp4 -itsoffset 18.39 -i audio_mix.wav -c:v copy -c:a aac final-video.mp4

  where I’m using the -itsoffset 18.39 option since I know that 18.39s is the video-audio delay.

  The problem I’m experiencing is that in the output file, while the audio is perfectly in sync with the video at the beginning, it slowly drifts out of sync during the movie.

  The output if ffprobe on the video file is :

  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'incontro3.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.25.100
  
    Duration: 00:47:22.56, start: 0.000000, bitrate: 888 kb/s
  
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 886 kb/s, 25 fps, 25 tbr, 12800 tbn (default)
  
      Metadata:
  
        handler_name    : VideoHandler

  and the ffprobe output for the audio file is :

  Input #0, wav, from 'audio_mix.wav':
  
    Metadata:
  
      track           : 5
  
      encoder         : Lavf57.25.100
  
    Duration: 00:46:32.20, bitrate: 1411 kb/s
  
      Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, 2 channels, s16, 1411 kb/s

  I’m using the latest ffmpeg Zeranoe windows build git-9591ca7 (2016-05-25).

  Thanks in anticipation for any help/ideas !

  ---

  UPDATE 1 : It looks like the problem is upstream the video-audio merging, and could be in the concatenation and conversion of the MTS files generated by the video camera into the mp4 video. I will follow up as I make any progress in understanding...

  ---

  UPDATE 2 : The problem is not in the initial merging of the MTS files generated by the camera. Or, at least, it occurs identically if I merge them with cat or with ffmpeg -f concat

  ---

  UPDATE 3 : Following @Mulvya’s suggestion, I observed that the drift rate is constant (at least as far as I can tell judging by eye). I also tried to superimpose the A/V tracks with another software, and the drift is exactly the same, thereby ruling out ffmpeg as culprit. My (bad) feeling is that the issue could be related to the internal clocks of the digital video camera and the laptop used for audio recording running at slightly different rates (see here the report of an identical issue I just found).

• Audio drift issue. Creating video frames programmatically and syncing with midi/audio in java

  6 septembre 2017, par user6688986

  I want to create a video that changes to the notes on a midi file. I am recording the midi then outputting it as wav.

  In java I read the midi file, create images based on the notes played. I am calculating the time between notes and using this to determine the number of frames with this code :

      for(int i = 0; i < noteImages.size(); i++) {
  
          for(int j = 0; j < frameRate * (noteLengths.get(i) / 1000); j++) {
  
              videoFrames.add(noteImages.get(i));
  
              frameCount++;
  
          }
  
      }

  I then use ffmpeg to create the video from the images with audio using this command :

  ffmpeg -r 60 -f image2 -i pic%05d.png -itsoffset 3 -i audio.wav -c:v libx264 -c:a aac -pix_fmt yuv420p -crf 23 -r 24 -shortest -y output.mp4 

  The audio starts okay then gradually goes out of sync. If I use higher fps when creating the images it is less pronounced, but I need to use at least 300fps to get it close and it is still not quite right. Any ideas on how to correct this ?

• avformat/mxfdec : use first valid sourceclip found if material track has multiple...

  18 septembre 2016, par Mark Reid

  avformat/mxfdec : use first valid sourceclip found if material track has multiple components
  This commit fixes a issue with mxf footage having multiple
  
  components on a material track.
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  

  • [DH] libavformat/mxfdec.c