I have read a number of posts in which people have complained of problems with audio tracks that will not remain in sync with the video tracks. Various reasons have been suggested (i.e. variable frame rates, mixed film/video content, etc.) but none seem to explain what I have observed myself, and so I am going to try to describe it.
As an example I am going to use a UK PAL DVD of the movie 'ET' which has a reported frame rate in Avidemux of 25.000fps and 16:15 pixel ratio (although the correct pixel ratio is in fact 16:9).
Open VTS_01_1.VOB from the ET DVD using Avidemux, and it will automatically append all the constituent VOB files and show 3 audio tracks. There are two AC3 6 channel tracks at 384kbit/s (for which Avidemux requires a 350ms static time shift to play in sync), and a third DTS 6 channel 768kbit/s channel. (For completeness, there is also in fact a fourth channel containing subtitles that Avidemux doesn't report).
If I select either of the AC3 audio tracks with a 350ms time shift, on playback in Avidemux the soundtrack remains perfectly in sync throughout the movie. However, if I select the DTS channel with the same 350ms time shift, although the soundtrack starts out perfectly in sync, as the movie progresses it steadily departs, gradually becoming more and more out of sync.
If you study the durations of the audio and video track durations, sure enough, the DTS audio track is a couple of minutes shorter (i.e. Video is 91:55:01.520, AC3 Audio is 01:55:01.184, but DTS Audio is 01:52:59.840).
If I play these same VOB files using VLC Media Player, regardless of which audio track I select, they all play perfectly in sync throughout the movie.
So here is the big question. Please would someone explain why, if I use Avidemux to simply play back these files (i.e. no transcoding or anything - simply playback), the DTS audio track runs 'faster' than the movie, whilst if I use VLC it all runs in sync?
(I have tried adjusting video frame rates from normal to PAL to Film, and to Film to PAL with absolutely no success).
Is there a fundamental issue here somewhere, or is VLC just a bit better at finding correct defaults?