Very Strange Sync Problem with MPEG.

[DivXExpert]

I download a MPEG. Its MPEG-1 at VCD standard, and plays 44m 3.641s. The file plays perfectly in sync.

I am converting to DivX3 with Nandub.
So I start by demuxing the audio with TMPGEnc (Mpeg Tools).
The Audio is 43m 59.916s.
I did not change the streams at all. Just demuxed.

The original MPEG plays perfectly in sync, so when I convert I don't change a thing about frame rate, sample rate ect.

But when the conversion is done, it is out of sync. The audio is ahead of the video. Gradual Sync loss.

I think this is rather strange. So I take the demuxed streams and mux them back to a new MPEG file (TMPGEnc Mpeg Tools). The new MPEG is now out of sync. Nothing changed, just demuxed, them muxed back. Audio ahead of video as in the DivX conversion. Again, Gradual Sync loss.

So I think this has something to do with the way the MPEG was muxed, beacuse nothing was changed except demux them mux. This is very strange.

I really need some help.

[DivXExpert]

Oh yeah. The demuxed video plays at 44m 0s. But VirtualDub says its 44m 3.04s.

Audio demuxed is 43m 59.916s, and plays for this amount of time.

The orginal MPEG file, untouched, plays for 44m 3.641s, and virtualdub agrees that this is the true length.

I am using Windows Media Player 6.4 to play the original MPEG, and the demuxed .m1v. And VirtualDub to examine the true length of both.

Winamp to play the mp2, and SoundForge to examine its true length.

Strange that the video would play for 44m. But VirtualDub says its 44m 3.04s.

This has got to be some strange muxing thing.

[DivXExpert]

Also. If I encode (same framerate, samplerate, nothing touched) without demuxing. I get the same sync problems.

[DJRumpy]

Your original MPEG has a few bad frames. They get skipped during a re-encode, causing your audio to lose sync. You need to stretch/shrink your audio to match the new length of your video. You can try simple techniques, like delaying the audio start, but these rarely work for dropped frames. Use a decent audio editor to stretch it to the precise length of your video stream.

[DivXExpert]

V-Dub-MP3-Freeze, says no bad frames were found.

[DivXExpert]

Do these bad frames get skipped in a simple demux as well?

[DJRumpy]

Yes. When you demux, you essentially unlock the audio from the video. If a stream looked like this:

1 2 3 4 5 6 7 8 9 10 (video stream)

1 2 3 4 5 6 7 8 9 10 (audio stream)

And say for example, frames 5 6 and 7 were corrupt in some way, making the video unreadable in those frames. When you demux it, the video represented in those frames would simply be dropped, shortening your video length, and causing a skew, or loss of sync, so the output demuxed video stream would look like this:

1 2 3 4 8 9 10 (video stream)

1 2 3 4 5 6 7 8 9 10 (audio stream)

In the above example, the audio would be longer than the video.

Same situation for you, but reverse it, with the Audio being corrupt. For some reason, the Audio appears to be the most common piece to have problems. When the audio becomes corrupt, then of course your audio will be a bit shorter than your video in the demuxed stream, since those corrupt bits are dropped.