[Gta04-owner] Low latency audio routing

Bob Ham rah at bash.sh
Tue Jan 17 22:43:24 CET 2012

On Wed, 2012-01-18 at 07:55 +1100, NeilBrown wrote:
> On Tue, 17 Jan 2012 20:37:42 +0000 Bob Ham <rah at bash.sh> wrote:
> > On Tue, 2012-01-17 at 21:27 +0000, Radek Polak wrote:
> > 
> > > I am 
> > > going to continue with experiments, but any hints in mean time are highly 
> > > appreciated.
> > 
> > Have you tried JACK?
> > 
> >   http://jackaudio.org/
> > 
> > I though I saw mention of it on the list before but I don't recall
> > whether it was actually tried?
> > 
> jackd only works with a single sound-card.
> We have 2 we want to route between.

Ah :-/

> There are two ways you can approach this in jackd.
> 1/ Tell ALSA to create a virtual sound card that slaves the two sound cards.
>  This apparently will lead to clicks if the two cards don't have the same
>  clock, and as the GSM module has its own clock, I suspect this will lead to
>  clicks.  I haven't tried it.

Yes, this is a no-no unless the cards' clocks are synced (eg, in a
studio you can sync cards using word clock).

> 2/ use the 'alsa_in' and 'alsa_out' jackd clients to feed data from one sound
>   card into jack, which then directly controls the other sound card.
>   This is supposed to handle differences in clocks correctly.
>   I tried this but didn't get it working in any useful way.

This would be the appropriate way.  Those clients should compensate for
jitter and smooth out any clicks from unsynchronised clocks.  They're
strongly discouraged for music production environments but should be
fine for a telephone conversation.

What was the problem getting it working?

Bob Ham <rah at bash.sh>

for (;;) { ++pancakes; }
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