New (?) audio sync problem

[oldblues]

I am trying to make a DVD out of a bunch of mpeg-1 files. There are a bunch that all seem to be the same, but one or two have mysterious problems that I can't seem to resolve. So, I found an old copy of one of them that doesn't actually crash Toast and/or the OS, and ran it through the DVD-lo preset, and everything seemed to go okay but the audio gets progressively more out of sync. Like the rate is just slightly off somehow. The original and re-encode both are 48000, but the original was 384 kbps while the target is 224 kbps. So, I tried resetting the encoding settings to 384. That was much worse.

The only other rate difference is in the video, which starts at 1150 and is upsampled to 2300. Doesn't seem like that should change it's duration, however.

[tomlee59]

One standard thing to try is to decode with QT (which seems to hold sync somewhat better, although it is not bulletproof).

Also check the source and target framerates. Is the sync drift so large that it corresponds to a pulldown-related issue? That is, are you accumulating 3-4seconds of error per hour? If so, then suspect a pulldown problem.

If the drift is steady, but by an amount inconsistent with a pulldown problem, you may have to perform an ad hoc correction of the audio track duration. Audacity is an excellent (and free) tool for doing this. It has a "tempo change" tool, which allows you to stretch/shorten the audio track while automagically keeping pitch unchanged.

Good luck; sync error problems are altogether too common, and not always easily fixed. But a linearly increasing error is correctable.

[oldblues]

Oy, trial and error time-stretching? I suppose if I have to, I could try it in something like Cubase that edits audio as part of a quicktime video, with video display.

I already had the QuickTime option checked, that must be part of the pre-set. I just tried it with pulldown checked, and that put the whole thing WAY off.

[tomlee59]

If you use, say, VLC, you can dial in a corrective offset (under the preferences menu) in playback. So, take note of the sync offsets at several points throughout the movie; take care to do this accurately, since 100ms is already a noticeable error. This exercise will allow you first to determine whether or not the sync error is in fact drifting at a constant rate. If so, then you can proceed to the correction. The data allows you to determine the two parameters you need to know: Offset and length correction. Use the latter value to adjust the length using Audacity, and use the former value to specify a muxing offset prior to authoring. Btw, the offset value you use in ffmpegx is the algebraic inverse of the number you used in VLC.