[LAD] ALSA multi plugin and JACK xrun callbacks

[torbenh]

On Sat, Dec 18, 2010 at 03:19:14PM +0100, Jörn Nettingsmeier wrote:
> On 12/16/2010 08:49 PM, Lee Azzarello wrote:
> > On Thu, Dec 16, 2010 at 1:36 PM, Lee Azzarello <lee at rockingtiger.com> wrote:
> >> On Thu, Dec 16, 2010 at 6:58 AM, torbenh <torbenh at gmx.de> wrote:
> >>> On Thu, Dec 16, 2010 at 08:02:29AM +0100, Luis Garrido wrote:
> >>>> On Thu, Dec 16, 2010 at 4:21 AM, Lee Azzarello <lee at rockingtiger.com> wrote:
> >>>>> Hello, I'm looking for some advice for an interesting symptom of
> >>>>> putting my M-Audio Fast Track Pro USB sound card into ALSA multi mode.
> >>>>
> >>>> I have no idea what might be happening, but an alternative to use
> >>>> several alsa devices in jack would be the alsa_in and alsa_out
> >>>> utilities shipped with jack, perhaps that will give you less trouble?
> >>>
> >>> hmm... the interesting thing is that he actually claims to not have
> >>> trouble. he only gets xrun callbacks. this is weird.
> >>
> >>
> >> I'll test the alsa_(in|out) utilities later today.
> > 
> > Great success! alsa_out on the second device exposes those two
> > additional channels to JACK and I can connect any application to them
> > without jackd printing messages about xrun callbacks.
> 
> torben, i wonder: will alsa_[in|out] be perfectly bit-transparent when
> the two interfaces it bridges are externally synced, or will its
> tracking algorithm keep oscillating around the correct samplerate?

it will oscilate, but the oscilation will get slower and slower,
and its about 0.00001% so this is going to be pretty good.

but its not perfect.
there are a bit of scheduler uncerteinties, which might perturbate the
timebase.

if your interfaces are synced just use the pcm multi approach.
if you dont care for bit-transparency, and dont want to edit a config
file, use alsa_io.

the main purpose of alsa_io is jackd running with a jittery timebase.
and i prefer optimizing it at that end.

> 
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-- 
torben Hohn