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 worked around:

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.

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Last updated July 3 2012