Re: Connecting two USB-Soundcard to JACK

> On Tue, Jun 12, 2007 at 03:42:03PM +0200, Carsten Beth wrote:
> > Hello!
> >
> > Since I have no responces at the alsa-devel list, I would to ask for some
> > help help here.
> >
> > We want to connect two USB soundcards to one multichannel device at the
> > alsa layer and to use that virtual device with JACK. But we don't find a
> > way to do so.
> >
> > The USB soundcard is self-made. It woks as a high speed USB device with
> >  sample rates of up to 96 KHz, has four capture and two playback cannels.
> > The clocks of two cards can be synchronized at hardware level. Using one
> > card by its own works perfectly both, at the alsa and at the JACK level,
> > also as a duplex device. All is done with alsalib 1.0.13, jackd 0.101.1
> > and qjackctl 0.2.21.
> >
> > Now I'm trying to build a virtual device with 8 capture and 4 playback
> > channels at the alsa level. But, as it can be read at several locations,
> > this only works for capture only or playback. It has been reported, that
> > duplex operation is possible after patching pcm_multi.c. I have applyed
> > the patch, but this has not changed anything. Now I want to ask you for
> > support, to get the device running.
> >
> > I made some experiments with different configurations of alsa/jackd and
> > had some strange results. You can read about the results and used the
> > asoundrc below. Questions, that I can't answer are
> > - Why do I see just the channels of one card when using the multi plugin
> > in try 2), 5) and 7)?
> > - Why is the processor load so high with playback only in try 2) and 3)
> > but not in duplex operation?
> > - Why are there lots non-adible xruns with playback only in try 2) and
> > 3)?
>
> Could be because pcm_multi causes jack_set_xrun_callback to be called
> when there aren't any audible xruns. This was mentioned in a recent
> thread on this list, but I'm not sure why it's happening. It appears to
> be harmless, as long as you can't hear anything wrong.
>
> > 1) playback only with p_multi
> > ALSA: mmap-based access is not possible for the playback stream of this
> > audio interface
>
> This is because you're using a version of jack without MMAP_COMPLEX
> support. Either apply MMAP_COMPLEX patch or use a later version.
> That will allow you to get rid of plug and route plugins, which
> increase system load by a lot.
>
> John