[Gta04-owner] Low latency audio routing

Bob Ham rah at bash.sh
Tue Jan 17 22:33:20 CET 2012


On Tue, 2012-01-17 at 21:51 +0000, Radek Polak wrote:
> On Tuesday 17 January 2012 20:37:42 Bob Ham wrote:

> > Have you tried JACK?
> > 
> 
> Yes, i did. I tested it on my PC and it can do what we want, but for some 
> reason i couldnt make it working on GTA04 (qjackctl application behaves 
> strange and is unusable)...

The jackd server is quite easy to run from the command-line.  (QJackCtl
is just a front-end for it.)  There's good documentation for the
options:

  http://trac.jackaudio.org/wiki/jackd%281%29

Probably good options to try would be:

  jackd -R -d alsa -d hw:0 -n 2 -p 512 -r 44100

It's good to change the period size (-p) to be as low as the hardware
will do without giving xruns.  The values have to be powers of two, so
256, 128, 64, ... .

The best sample rate (-r) will be hardware-dependent; I'm not sure what
the OMAP sound devices support.


If you run as root, then you should be able to use -R (real-time
scheduling) without any problems.  I'm not sure how QtMoko is set up.
If you don't run as root, you need to put the following in
your /etc/security/limits.conf (which is in the Debian package
libpam-modules):

@audio - rtprio 90
@audio - nice -10
@audio - memlock 3040839

That should allow any user in the 'audio' group to use real-time
scheduling.  The jackd server will then use those privileges when called
with -R.  This is worth doing.


Hope that helps,

Bob

-- 
Bob Ham <rah at bash.sh>

for (;;) { ++pancakes; }
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