Audio out of sync

[CrookitDough]

Hi, I'm more or less a complete newbie when it comes to video editing. But I do have Avidemux installed and have used it to shift audio on videos that were out of sync. Lately I've been trying to get a particular video to sync, but it just won't work.

It's a mostly silent video so I've tried using several different reference points in time but whenever it seems I've gotten one of the points synced I find that all the others are still out of place, so I'm starting to wonder if shifting the audio is enough or whether it was somehow collapsed/expanded or some other weird effect like that.

So I started googling and found these forums, so I'm hoping someone here can lend me a hand in at least diagnosing the problem. However, the video is a completely explicit pornographic video (and quite rough), so it's against the rules to post it. If anyone here has no problem looking at porn and is willing to take a shot at this beast, I could PM it. Besides being adult content, it's 100% percent legal to view and share.

No, this isn't a joke. And sorry if this community frowns upon this kind of request, I don't mean to offend anyone!
Thanks!

[hello_hello]

Once you have the audio synced at the beginning, you could try adjusting the frame rate a little. I don't use Avidemux much so I don't know if it makes it easy to do so, but for AVI you can adjust it with VirtualDub (select direct stream copy as the video compression method if you don't want to re-encode) or if you're not fussed about the output format MKVToolNixGUI will open lots of file types and remux as MKV, and it'll apply an audio delay and/or change the frame rate in the process.
It also has a "stretching" option, although it's best used as a last resort (it can cause audio glitches if the audio is stretched).

Chances are only a small frame rate adjustment will be required, depending on how badly the audio drifts out of sync, but if the video is currently 23.976fps (for example) try 23.974 or 23.977 etc before getting too carried away.

Even assuming the audio and video belong together it's generally a trial and error process, but if the video and audio are from different sources they may never match up, even though they appear to be the same. All it takes is an extra frame or two at an edit point, or a couple of frames less somewhere, and the sync will never be right.

[alfresco]

Three major kinds of audio/video sync errors

One is a offset error where the starting point of the video and audio are different. Easy to fix with a audio delay.

Second is a length error where the video and audio are different lengths. That results in a sync error that usually gets worse as the file plays.
Fix by stretching or shrinking the audio most times.

Third is a combination of both sync errors or just a chopped up and badly edited video or audio file.

If the video is complete, then I would suspect the audio file is not. These type of errors can be a lot of work to fix.

[CrookitDough]

I forgot to mention that I actually have two files: one is an MP4 which is the full length of the video 11m38s and has the audio sync problems. The other is a 35s Webm with apparently no syncing problems (but it might just be because it's so short). The whole thing is just one shot, it doesn't seem to be edited in any way.

Thanks for your replies! I'll mark down as many reference points as I can to try and figure out what the problem is.

[CrookitDough]

Okay, so I've watched the whole thing while writing down all the reference points I could and discovered that the delay increases during the first half and decreases during the second half. I'm having a hard time plotting these values in a chart so I can't tell if it's a linear increase or not. But I also noticed that in the second half of the video there's some kind of skipping, as you can tell the video seems to move more quickly for a split second.

Can you guys tell which of the cases alfresco presented we're seeing here? It's obviously not the first. Any suggestions are welcome, thanks!