Audio sync problems

[jelockwood]

I am trying to convert an MKV file to an MPEG2 file (and ultimately DVD). The MKV file contains a H.264 video stream and Ogg Vorbis audio tracks. The MKV file plays the video and audio fine with both in sync all the way through.

However each time I use ffmpegX to convert the file, the resulting file has the audio start out in perfect sync but by the end is out of sync by over 3 seconds. From having used other utilities, it seems there are about 85 bad frames in the video which ffmpegX is discarding, at 23.976 fps this results in the 3+ seconds loss of sync by the end of the video.

Now it might be fair enough to say the fault is the bad video, but this other utility (ConvertX2DVD) is able to convert the same file with no loss of sync. My guess would be that it pads out the missing frames (perhaps by duplicating the previous frame) and thus keeps the video and audio in correct sync. Obviously one would expect ffmpegX to do the same.

Note: I tried both ffmpeg and mpeg2enc options

So my first point is can ffmpegX be improved to automatically cope with this not uncommon situation (like ConvertX2DVD does). Secondly, any suggestions on how to workaround this issue using ffmpegX. Thirdly in the meantime, are there any other MKV converters for the Mac (that hopefully don't have this problem), or a free converter for Windows (ConvertX2DVD is commercial, I was using the trial version).

[tomlee59]

Unfortunately, this is a very common problem. There doesn't seem to be a fix forthcoming soon, so your options are limited.

What I've often done (but not with Ogg Vorbis audio) is use Audacity to stretch/shorten the audio track to line up with the video as well as possible. If the troublesome video frames are more or less uniformly distributed throughout, then this hack works pretty well. If they are so tightly clustered as to produce large jumps in sync errors, I chop it up into several segments within which the error behaviors are constant, resync those, and then splice. Luckily, Audacity does support import and export of Ogg Vorbis files, so this option is available to you.

Btw, it's not just bad frames that cause this problem. Scene breaks with blank frames are typically mishandled, and some movies are authored with oscillating frame rates. The encoders aren't quite sophisticated enough to handle those situations, either.

Yes, a major PITA.