Author Topic: Audio Track Synchronisation  (Read 2050 times)

rickyh

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Audio Track Synchronisation
« on: July 16, 2012, 12:03:55 AM »
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?

Many thanks,
Richard


zakk

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Re: Audio Track Synchronisation
« Reply #1 on: July 16, 2012, 07:37:11 AM »
My advice is forget the playback synchronization. Often it will be fixed on the output anyway.

rickyh

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Re: Audio Track Synchronisation
« Reply #2 on: July 16, 2012, 03:47:43 PM »
Zakk,

Thank you for the reply, however the problem is not restricted to playback - it applies to all the output file tests I have performed as well.

The issue here is a genuine audio sync problem, it is specific to Avidemux, and I would really like to get a handle on it.

Regards
Richard

zakk

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Re: Audio Track Synchronisation
« Reply #3 on: July 16, 2012, 03:55:39 PM »
Ok, sorry. But keep in mind that DVD is a complex and always different structure (that's why there are .IFO files ind the folders). Don't expect too much of Avidemux ;)
You can also give a try to Avidemux 2.6, or read your .VOB's properties with Mediainfo (it sometimes gives audio delay information).

rickyh

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Re: Audio Track Synchronisation
« Reply #4 on: July 16, 2012, 04:03:26 PM »
Zakk,

Thank you again for the incredibly swift response! I think you're probably spot on when you say that the IFO files probably hold the key to this.  It may even be a deliberate strategy to obstruct pirating.

I just ran another test in which I calculated the video frame rate required to play all 172537 frames of the movie within the precise duration of the DTS audio file.  The answer is 25.448 fps.

This is close enough not to matter, and of course could be corrected later in any transcoded output file if that were important.

The result is perfect. That is to say, no initial timeshift is required and the DTS audio remains in sync throughout.

Perhaps someone with more experience that me might recognise that as some sort of relevant or magic frame rate, but I've not seen it before! :-)

Still, that is very good news since the audio quality of the DTS track is much higher than that of the AC3 6 channel track.

Regards
Richard

ABBrittain

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Re: Audio Track Synchronisation
« Reply #5 on: July 17, 2012, 11:05:28 AM »
I've seen that FPS before - Blue Ray - I think