Audio Syncing Issues

[Alexstarfire]

Seems that audio problems plague me. I've found this one to be a bit unique though I suspect I know why. So, I have this MKV file which plays fine on my computer. Trying to convert this to AVI seemed simple enough since it's only stereo AC3. After converting it the sound was off. Not a simple offset type sync issue either. More like the audio was playing far slower than the video. Anyway, it has nothing to do with the actual conversion of the audio. I found this out when I simply demuxed the audio file from the MKV and just loaded the AC3 file as the sound source while playing the MKV file. So, the video came from the MKV file and the audio from the demuxed file. It has the EXACT same issue. It would seem to be that it depends upon the video file for playback, which is an h264 stream if it matters.

I am time stretching, or in this case shrinking, the audio in virtualdub so that it'll sync, but I'm noticing that when playing the resulting AVI file compared to the MKV file that the audio seems to be sped up, which is only really noticable because the ratio I'm using is about .8 which is an insane amount for a syncing issue. I'm hoping that someone can shed some light on audio of this sort. Using MKVInfo on the MKV file shows that the plaback rate of the audio and video are indeed different, but not by nearly as much as the ratio I'm using.

EDIT: Something else I just found out is that even with a good ratio the audio is still off in some parts and not in others. I would normally conclude this is either ABR or VBR audio, except that I know I'm using a CBR MP3 file to figure out this ratio.

[El Heggunte]

Probably your problematic MKV has variable frame rate.
The AVI container requires Constant-Frame-Rate video.

[Alexstarfire]

That was something that crossed my mind, but wouldn't it get converted to CBR when I re-encode the video?

[El Heggunte]

That was something that crossed my mind, but wouldn't it get converted to CBR when I re-encode the video?

Your question strongly suggests you haven't really understood what "VFR" means.
Please start by (re-)reading the pages below:

http://avisynth.org.ru/docs/english/advancedtopics/hybrid_video.htm

and

http://avisynth.org.ru/docs/english/corefilters/directshowsource.htm

[Alexstarfire]

You sir would be correct. I have not dealt with many VFR videos and the others that I have weren't ones I wanted to convert. Those two pages are a great deal of help. I'll try a couple of the things they suggest and see how that turns out.

Thanks for providing those links.

[Squash]

Alexstarfire,

Here's another good article/thread that helped me out a lot, a few years back:
http://forums.animesuki.com/showthread.php?t=34738

[Alexstarfire]

Interesting, but that makes me wonder why VFR even exists. I suppose I could understand if this was like 10 years ago. Obviously if you have less frames then you can get better quality in the same filesize from the remaining ones. But since we have formats like h264 I just don't see it being as useful anymore. In fact, it seems to be more of a pain unless you are simply watching it and don't want to do anything else to the file.