I'm attaching the tail end of an exchange I just had with the lead
developer of dvdauthor. I was confused about DVD VOBs having
different PTSs. This is the one way I'm aware of that DVD VOB files
aren't legal MPEG Program Streams - the different VOBs with the file
can have independent PTS ranges. A "real" MPEG Program Stream (I
think) can't reset its PTSs halfway through - only a Transport Stream
can do that, and only at places flagged by a bit in the TS header. So
I was starting to think that if I take a bunch of different clips out
with gopchop, as long as they're put into a DVD as different VOBs,
then I don't have to worry about "trimming" the audio like I described
in my last post - the DVD player will re-sync the PTSs when it jumps
to the next VOB. But you see Scott's suggestion - transcode the audio
- and you see his reason why.
---------- Forwarded message ----------
Date: Thu, 26 May 2005 12:15:10 -0700
From: Scott T. Smith <scott@...>
To: Brent Baccala <baccala@...>
Subject: Re: do PTSs restart in each cell?
On Thu, 2005-05-26 at 11:44 -0700, Brent Baccala wrote:
> > You'll probably get a/v sync issues. By far the best way is to encode
> > them into a single MPEG, though rather than reencode the video, just
> > concatenate it together. Instead reencode the audio, which is less time
> > consuming, and in my opinion, less prone to requantization artifacts.
> > > I can see that individual PGCs have their own PTS timestamps, but do
> > > cells within a PGC have to have contiguous timestamps? Or does
> > > dvdauthor rewrite them?
> > The PTS of one vob have nothing to do with the PTS of another. So yeah,
> > they do reset.
> OK, I'm still confused. Your two answers seem (to me) inconsistent.
> I'd figure on getting a/v sync issues if the PTSs ran from one
> straight into the next. Then I'd think re-writing the PTSs (as you'd
> have to, since they're two different clips) would create the a/v sync
> problems (since there's more audio than video). But if the PTSs reset,
> then the decoder has to re-sync, the original timestamps remain
> unchanged, and there should be no sync issue. Am I missing something?
Yeah -- many DVD players don't properly use the PTS to re-sync. They
assume that the audio and video line up properly. It sucks.