The elusive Audio/Video sync problem

[CloudEmeron]

I'm making a DVD from about six AVI files on my computer. Episodes of a show. The process I'm using is saving wavs with VDubMod, then video without audio. Then I convert using TMPGEnc Plus, ffmpeggui the audio, and add it all together with Adobe Encore.

The problem I'm having is that one of the episodes, even though I've separated the audio and video, doesn't sync up correctly. Even in the AVI file, you can tell there's a point where it jumps, but for some reason it fixes itself. When they're separated, it's like it goes away in one of the streams and they don't sync up correctly. It's not a gradual loss, it's sudden. It occurs at a point where there would have been a commercial cut out of the episode. Any ideas?

[Tommyknocker]

Check for bad frames. If that doesn't work, you may want to remove the frame(s) where the "jump" occurs.

[CloudEmeron]

Check for bad frames. If that doesn't work, you may want to remove the frame(s) where the "jump" occurs.

Strangely, when I try this, the jump now appars in the new AVI that has that area cut out of it. It's almost like when the error is present, and play detects it and fixes the problem. I'm really not sure how to fix this. I'm out of ideas.

[Gillies]

try divfix. works a treat for me

[CloudEmeron]

try divfix. works a treat for me

DivXFix doesn't see a problem.

Sometimes I really hate working with video/audio...

[mpack]

I was investigating A/V sync problems of my own a few days ago. I discovered that there are three main causes of lost A/V sync :-

1. Slight difference in clock values between audio and video capture cards. This one is easily fixed by stretching the audio length to match the video (especially easy when you separate the streams as you have). This is the way I prefer to do it too.

2. Video capture dropped frames, and also dropped the audio that went with them in an attempt to maintain audio sync. Unfortunately the capture software helpfully marked every point in the AVI where a frame was dropped, and some software which later reads the AVI creates substitutes for the dropped frames - but it does nothing with the audio - resulting in the audio being ahead of the video (you hear the words, then the lips move).

3. Video capture dropped frames, and this time did NOT drop the audio that went with them. If the AVI reading software skips dropped frames (insteading of substituting them as in scenario 2) then you get an A/V sync problem again, this time with the audio trailing the video.

In 2 and 3 you can't just stretch the audio track because the loss of sync isn't linear.

Notice that on 2 and 3 it is the combination of the exact behaviour of both capture software and AVI reading software (media players, VDub etc) that creates a particular kind of A/V sync problem. You can fix the A/V sync problem by changing either one to suit the other: and since you've already captured your video then in an ideal world you need to change how your AVI reader handles it.

Example: open the original captured AVI with VDub and then save it (using full processing mode - not just a direct stream copy!) to another AVI, then check the number of frames in the dest AVI. If VDub discards dropped frames then this should be shorter than the original, and that will tell you why you are losing sync. You would want to replace the dropped frames with good frames (somehow). You've already had a suggestion which deals with the other case.