Problems with ALSA pcm_multi
The ALSA pcm_multi plugin provides a means of using multiple sound cards with
applications requiring a single audio interface (eg. JACK).
Here's an example .asoundrc file showing
how to use it. There are a couple problems with pcm_multi that need to be
Realtime (-rt) Kernel Doesn't Always Work with pcm_multi and JACK
Users may have problems using the -rt kernel with pcm_multi.
Many versions of the -rt kernel give lots of xruns or even lock up
completely when using pcm_multi with jackd,
even though they work fine with a single sound card. Users of pcm_multi may
get better results with a non-rt kernel with CONFIG_PREEMPT=y.
JACK Clients Report Lots of xruns
There is some kind of interaction between jackd and pcm_multi which
causes some JACK clients to report lots of xruns even though alsa_pcm doesn't
report any and there is nothing audibly wrong. JAMin, for example does this,
and some qjackctl users
have reported it too. I find it necessary to keep xrun markers turned off in
Ardour to prevent hundreds of them appearing whenever I use pcm_multi.
This is inconvenient but appears to be harmless (on my own system, anyway).
Users should still watch out for xrun messages from alsa_pcm,
which indicate real xruns.