what causes sync fluctuatons?

[spiritgumm]

I got a couple dvdrs from different traders (ntsc format, ac3 audio) - a dvd recorder was involved at some point on both, but only one was probably reauthored. The sync seems to come and go (it's not worsening, nor consistent by a certain delay time). The possibly-reauthored dvd might have been originally made with a PAL dvd recorder (has a converted look). Any ideas? I don't know more about their pedigree. It doesn't seem fixable, short of tweeking the audio manually while watching the movie.

[drewzor]

There are alot of factors that can play part in audio loss. Yours soulds like dropped frames. This is exactly what you think, the audio continues as the video frames drop and become shorter.

However!! it could be vbr audio and where it is quiet some players decode 0bits as nothing and collapse the audio, therefore making the audio track shorter then the frame rate...

A Solution?
I dont know, in my limited experience and knowlege i would convert it all to avi and segment it into smaller chapters and sync each using vegas or vdubmod's (interleaving)

good luck

[redwudz]

Just generally, if the sync error varies throughout the video, it's probably missing frames. Not much you can do about that, if that's the way you got it. The solution is complex. You would need to chop the video into parts and repair each separate section that has a sync problem with adjustment of the sync. Usually the audio is easier to adjust with an offset. If the sync error varies during the section, even more difficult. That would mean the audio and video lengths are different and you would have to stretch or compress the video or audio to match. After all of that, you would have to paste them all back together.

At this point, you will probably see that it is rarely worth this much work. Sorry.

What causes sync fluctuations?: Missing frames or audio, bad joining of multiple video files, bad conversion from PAL>NTSC are a few causes.

[spiritgumm]

If it's not the vbr audio, and it's dropped frames, what usually causes that? Could it be these were PAL dvds, and someone changed the frames per second but didn't change the audio - so both audio and video are same length but the video frames don't always match up with the audio at any given moment? Well, that probably wouldn't cause it, but you know what I'm saying - some kind of tinkering that someone did inexpertly?
I didn't think there was a solution - I was just wondering why it happened. Why these traders don't notice is an even more troubling question...

[drewzor]

Well really the only un-intentional thing you could do would be a frame rate change as you say but this would always have a linear effect, A consistant or gradual loss of audio sync. The fact that it is a variable loss of sync indicates an error more so then a applied effect/setting.

To offer another explination, when i began working with mpg files I would experience a varible loss of sync if I was cutting up different mpegs and then constructing one from the all the segments. It had to do with the fact i was cutting B.I.P frames at the wrong location and when put together would equate to a lost I frame...this is still lost frames but could give rise to another reason as to what may have happened.

To confirm the droped / lost frames theory you could check with g-spot the video file and see how long the duration of video vs audio is. Or de-multiplex your .vob or .mpg with mpeg-vcr, if the video runtime is shorter then the video then it is dropped frames

[redwudz]

If the sync error does vary quite a bit, with the audio ahead of the video in some parts and behind in others, dropped or missing frames would be my guess. These can come from bad splits/joins that weren't done on keyframes. Or the video may have been converted to multiple RAR files, then converted back to a single file with a poor conversion on either end. I have also seen a few cases where the audio was edited separately from the video then rejoined.

If the video was downloaded, and uploaded a few times, it could end up with transmission errors. Without some kind of error checking or correction like RAR and PAR files, this can happen.

There are lots of way to mess up a video. You may be able to drop it into VirtualDub Mod and check for missing frames, or if you can identify where there is a start of a sync error, go through it frame by frame and you may see where something is missing. Software players may skip right over a missing frame or two and you wouldn't notice.

If I get one like that, I usually just delete it as the effort to try to fix it isn't generally worth it.