frustrating audio sync problem

[DaneClark]

ALL I am trying to do is slow down an avi file so that it plays a few minutes longer with the audio still in sync. After a very long step by step process that took several days that involved changing framerates and importing the audio into audacity, I STILL can't get the slowed down audio and video to be in perfect sync with eachother. The weird thing is, the audio and video are only out of sync for the first few minutes of the video, after that it's synced perfectly. What is going on? And I've been able to do this before without any problems.

[poisondeathray]

You probably need to split into 2 or more sections . Separate first few minutes that still has a problem and work on that separately

[manono]

Yes, and don't slow the video but speed up the audio (in my opinion). Or work on just the audio in an audio editor such as Audacity where you can adjust only the part needing adjusting, while leaving the rest alone.

One might ask how it came to be that way? It was like that when you downloaded it and you never had a chance to get the source? Or you did it and the source was fine but somehow you messed it up? Might there be VFR video involved (like a phone video, perhaps)?

[Selur]

I know that the input is an avi, but assuming the input file was an mp4 file one could also just change the time stamps by using something like:

ffmpeg -i "path to inpu file" -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" "path to output file"

this would slow down the video by a factor of 0.5.
Note: 0.5 is the inverse of 2.0 (so if you use other numbers keep in mind that atempo needs to be the inverse of the pts multiplicand)

Why I write this: This might also work for avi. (but I never tried it)