[vdr] Re: Direct AC-3 yes or no ?

[Holger Waechtler <holger@convergence.de>]

Dr. Werner Fink wrote:
> On Wed, Jul 16, 2003 at 05:07:06PM +0200, peter.zeipelt@arcor.de wrote:
>
> > Hi
> >
> > ----- Original Nachricht ----
> > Von:     "Dr. Werner Fink" <werner@suse.de>
> > An:      vdr@linuxtv.org
> > Datum:   16.07.03 16:56
> > Betreff: [vdr] Re: Direct AC-3 yes or no ?
> >
> > Thank you for this very fast answer.
> > You show me the problems of this solution, but as I can see, it
> > seems to be interesting.
> > Unfortunately I have no experience in C and my last programming
> > activity in pascal
> > is about 10 years ago.
> > However, if it's an opportuny to fix some problems (e.g. your
> > mentioned synch problems)
>
> The synch problem is due to the different time reference of e.g. Pro7
> and the clock/quart of the sound card.  This leads to the problem that
> 48kHz from live view isn't 48kHz from the sound cards reference clock.
You could implement a (maybe interpolating, maybe simply 
sample-doubling) resampling algorithm, this would have the same effect 
as if you were able to calibrate the soundcard's clock as you can do in 
real STBs.

Sounds computationally expensive on the first look but remember that 
ESD, gstreamer & co are able to resample+mix multiple streams in 
realtime since a long time.

> In fact you get underuns (sound card clock to fast) or overruns (sound
> card clock to slow).  Only sound cards which could use the S/P-DIF intput
> as reference clock (some very expensive cards can do so) and if you've
> put the S/P-DIF (5V TTL!) out of the DVB card to the S/P-DIF in of the
> sound card (hopefully able to handle 5V TTL) do not have this problem.
> (Similar problem you get with two different sound cards! No quart/clock
> is exactly in synch with each other!)
>
> If you using the S/P-DIF out of DVB card the synch problem is gone
> even if a sound card is used as S/P-DIF router.
That's the hardware approach, due to jitter it may or may not be of 
worse quality than a smooth resampling algorithm...

Depends on how well they are smoothing the clock inside the av711x.


> > and brings some new benefit, don't you think that the programmers
> > might bring it in the future?
>
> It would be a pleasure but, ... as I've noticed ..., none of the
> firmware programmers do have an AC3 receiver or arn't interested
> on hearing AC3 over DVB card.
*g*
:) implement it using a software decoder and maybe a library like OpenAL 
to re-render the 5.1 channels (or whatever is transmitted) optimally for 
your own speaker installation.

Holger



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