Loss of audio sync: results of research into problem

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Loss of audio sync: results of research into problem

Postby andrewfn » Fri Aug 01, 2014 3:22 am

Problems with losing sync between audio and video have led me to spend tens of hours testing out many combination of rendering options and versions of Melt etc.

Here are my findings.

Under certain circumstances, the audio will get ahead of the video at a rate of approx 1 second in 50mins (1:3000)

Test conditions:
  • OS: Ubuntu 13:10 & 14:04
  • Melt: 0.8.8 & 0.9.0 (also 0.9.2)
  • Openshot: 1.4.3

How to reproduce:

Using Ubuntu 14:04 (Openshot 1.4.3 Melt 0.9.0)
  • Take two video files, ideally each around 30 min long so that the problem is noticable
  • Place one file in the first track, and the other in the second.
  • Disable both sound and video for the second track (this is not necessary, but it demonstrates that no mixing is happening)
  • Render the video at 720p, 29.97fps, MP4 h.264, Med quality, AAC audio
The audio will be 0.5 seconds ahead of the video in the rendered file.

Results of experiments:
  • All the problems have occurred with Melt 0.9.0 and never with 0.8.8.
  • I attempted some tests with 0.9.2 and it seemed the problems were still there, (but I cannot be 100% sure--needs more testing)
  • The problem only occurred with a frame rate of 29.97. Rendering at 30fps was always in perfect sync.
  • The two videos simply need to be overlapping. It does not matter which track is displaying
  • Audio is advanced during the duration of the overlap. If the second video is only 15 mins long, then the audio will only be 0.25 seconds ahead
  • All audio is out of sync by the end, whichever track it is in.
  • Audio sample-rate made no difference. The problem was the same for 44.1 kHz and 48 kHz either for source files or rendered video
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Re: Loss of audio sync: results of research into problem

Postby XXLRay » Fri Aug 15, 2014 10:17 am

Interesting. Does the type of input video matter?
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Re: Loss of audio sync: results of research into problem

Postby andrewfn » Wed Aug 20, 2014 4:23 am

It doesn't seem to make a difference what format the input video is in. I have obviously not tried every possible combination but I have made a few variations and there has been no difference.
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