[linux-dvb] Re: AC3 the fastest way ???

[Matthias Weingart <matthias@pentax.boerde.de>]

On Thu, 21 Jun 2001, Klaus Schmidinger wrote:

> > > VDR will get a command line option which defines the program into which
> > > to pipe the AC3 data while replaying or viewing. That way everybody can
> > > do this the way the actual system needs it.
> > 
> > Sounds fine, unfortunately i found those programs having a large latency, so
> > that the audio comes much to late - should vdr have an option for delaying
> > the video against the audio in [ms] then?
> 
> VDR can surely do this when replaying a recorded file.
> But what about live viewing? The video will be delivered directly
> to the Video-Out of the DVB card, and the AC3 audio will go through
> some program that ultimately sends it to a sound card or an external
> AC3 decoder. I would assume that the AC3 data broadcast from the
> tv station should by default arrive somewhat earlier than the
> video data in order to compensate for this delay. But we'll have to
> see what happens when we actually have the AC3 data steam...

I think the main problem is syncronisation; you need to know what
video-PTS is currently played by the card and output and decode the AC3
with the same (or similiar) PTS at the same time.
 Do it only with a fixed delay is very unreliable because of the many
buffers in the way. Is it possible to get the PTS (and only the PTS) of
the video frames that are currently decoded? (independent of the kind of
viewing - live or replaying). The audio decoder should be synced to that
PTS. 
 With this you have 2 links/pipes - one (buffered) for the audio data; and
one (unbuffered and fast) for transmitting the PTS (ideally the PTS-link
is a cable that connects the video and the audio decoders, sure this
is not possible, but a software link should fast enough (<20ms) )

        Matthias




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