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I am experiencing a very strange behavior and I just cant explain whats happening.

Setup: I have a video shot with a GOPRO at 59.95 fps. I want to use in a 29.97 fps environment inside blender, so I use ffmpeg to change the fps value.

Problem: When I import 29.97 video, the audio and video are off by 1 frame. Specifically, the audio is one frame longer than the video. I would like to have the audio and video to have the same==identical length.

What have I done so far: Now comes the funny part. So when I import the 29.97 fps video, the audio is one frame longer, as described above. However, the actual audio information is indeed a little shorter than the video. So I split up the audio and video, encoded the video separately and then joined the two parts together. That did not change a thing. However, when I just import the audio file, then the resulting strip is actually 1 frame shorter than the video! Both audio strips (from the video and the single one) line up perfectly in regards of their waveform, so there is only a difference in the padding at the end of the file.

BTW: The audio part from the original 59.94 video would line up perfectly with the encoded 29.97 video. But that would be too complicated to always import two videos and then combine audio and video...

Question: Has anyone ever experienced such a thing? I believe the way blender determines the audio length (in number of frames) maybe does something wrong.

Thanks for your help.

ffmpeg 59.94 to 29.94

ffmpeg -i GOPR7304-59.94fps.MP4 -y -probesize 5000000 -s 640x368 -c:v prores -profile:v 0 -qscale:v 13 -vendor ap10 -pix_fmt yuv422p10le -acodec copy -r 30000/1001 GOPR7304-29.97.mov

ffmpeg splitting

ffmpeg -i GOPR7304-59.94fps.MP4 -map 0:0 -vcodec copy 04.m4v -map 0:1 -acodec copy 04.m4a

Thanks!

VSE-User
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  • Related: http://blender.stackexchange.com/a/49219/2843 (using AviSynth before importing your footage into Blender will allow you for precise fps rates) – Samoth Apr 29 '16 at 12:01
  • Thanks for the comment. I will have a look at this. However, I still think the error lies within blender, specifically in how blender calculates the length of an audio strip. How is it possible, that the identical audio stream is represented in different lengths inside blender (one coming from a video file, the other from a single audio file) ? – VSE-User Apr 29 '16 at 23:53
  • Related: Bug report https://developer.blender.org/T21138 Still I think this is a real bug, I might open a new bug report. – VSE-User Apr 30 '16 at 04:00
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    Might it be an unwanted side effect of https://developer.blender.org/T47135? You are using 2.77? Is it the same problem in an older version? – Samoth May 01 '16 at 18:56
  • So i tried importing the same files into 2.76b and indee I observe a different bevavior as such, that all different strips are all 1 frame shorter than in 2.77, but still different in length. My 29.97 video file now comes out correct, but it still bothers me, that the identical audio stream leads to different strip lengths. When I use ffprobe, the audio stream will aways show the identical length, no matter if its embedded in a video or not. How can I retrace how blender determines the length of an audio stream? – VSE-User May 04 '16 at 04:31
  • The sound file in question is about 32s long. I used audacity to shorten the sound to about 20s and then joined the resulting audiofile with my video. When importing to blender, the sound strip shows the same length as the original one. So Blender actually uses the length of the video or some meta stream (in frames / fps) to predict the length of the audio stream and does not use the actual audio length information. This in conjunction with link leads to inconsistent audio strip lengths. Can someone confirm? – VSE-User May 04 '16 at 05:16
  • I'm voting to close this question as off-topic because it doesn't appear to be caused by Blender. – someonewithpc May 18 '16 at 09:57
  • The audio strip of an imported video does not represent the actual audio length. imho this is indeed blender. I'll file a bug report within the next days. – VSE-User May 21 '16 at 13:46

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