[ILUG] HOWTO: software mixing with ALSA

[Justin Mason]

I ran into this problem a while back, and it's well worth noting the
solution.   Basically, a lot of cheapo soundcards and laptop sound
hardware doesn't support hardware mixing, which means that when
one app has the audio device open, all others will freeze attempting
to access it.

With some help from http://alsa.opensrc.org/index.php?page=AlsaSharing and
http://alsa.opensrc.org/index.php?page=DmixPlugin , here's the
/etc/asound.conf stanza that I use to do software mixing with ALSA:

  pcm.dmix0 {
      type dmix
      ipc_key 219345           # any unique number here
      slave {
              pcm "hw:0,0"
              period_time 0
              buffer_time 0
              period_size 2048    # jm: much smoother than 1024/8192!
              buffer_size 32768
              rate 44100
      }

      bindings {
          0 0   # from 0 => to 0
          1 1   # from 1 => to 1
      }
  }

  pcm.dsp0 {
    type plug
    slave.pcm "dmix0"
  }

  pcm.!default {
    type plug
    slave.pcm "dmix0"
  }

  ctl.dsp0 {
    type hw
    card 0
  }

# this makes native ALSA apps default to using dmix
  pcm.!default {
    type plug
    slave.pcm "dmix0"
  }

  ctl.!default {
    type hw
    card 0
  }



Note: I use a bigger period_size/buffer_size than the default DmixPlugin
settings -- it gives much smoother output at the cost of being slower to
respond to track/volume changes.  c'est la vie.

Then, configure your apps to use "alsa" as the output driver, and it'll
work nicely.

Application success reports:

  - xine and amaroK work nicely;
  - mplayer attempts to always open the "hw0:0" device, sidelining the
    DMix mixer for some reason, so it freezes;
  - JuK (by far the best KDE music player) uses KDE's gstreamer
    bindings, which hardcode use of "osssink" internally, so it freezes;
  - the Mozilla Flash player plugin also doesn't seem to support ALSA
    output, so there's no sound output from that if you have another app
    using ALSA at the same time.  But at least it doesn't freeze. ;)

I have a patch in for KDE (http://bugs.kde.org/show_bug.cgi?id=92786)
to support use of other gstreamer outputs instead of OSS, but no
word yet...

--j.