Inspired by a very good article about ALSA in the German magazine c't 26/2003, I finally got the ALSA dmix plugin working fine and thus rid of problems with apps blocking each other on sound playback, without needing any of the sound servers which all are unsatisfactory for me (not to mention that I couldn't get many of them working correctly at all on my TiBook). I thought I'd share my experience, hoping it will be useful for others.
First of all, you need to get ALSA working to the point where you basically hear correct sound. This shouldn't be hard with the Debian ALSA packages, in particular with a 2.6 kernel, so I won't go into details here. The next step is setting up ALSA to use the dmix plugin (official documentation at http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html#pcm_plugins_dmix). Put something like this in /etc/asound.conf or ~/.asoundrc: pcm.!default { type plug slave.pcm dmixer } pcm.dmixer { type dmix ipc_key 1024 # This must be a unique IPC key (see ipcs output) ipc_key_add_uid true # UID will be added to IPC key slave { pcm "hw:0" period_time 0 period_size 1024 # Must be power of 2 for alsa-oss buffer_size 8192 # ditto rate 44100 } } This sets up a PCM device called dmixer using the dmix plugin and overrides the default PCM device to use it via the conversion plugin, which will convert the sound format, rate and number of channels on the fly if necessary (the dmix plugin only supports a fixed constellation of these parameters due to the way it works). Now you should be able to use any number of ALSA apps concurrently. The problem is that still only a minority of apps use ALSA directly, at least by default. Luckily, most (can) use a standard library for sound playback, e.g. SDL; for those, you may have to set these environment variables: SDL_AUDIODRIVER=alsa AUDIODEV=default # This isn't necessary with the default device fix from http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=225252 For OpenAL, I use (define devices '(alsa sdl native)) in ~/.openalrc, which tells it to try ALSA first (the current OpenAL packages in Debian don't support ALSA directly though), then SDL and only then direct access to /dev/dsp. So far, so good, but there's still the odd app which can only access /dev/dsp directly for sound. That's what alsa-oss is for, which unfortunately isn't available in Debian yet (though I'm told that will change after the next DeMuDi release, planned for mid January), but almost as conveniently from deb http://apt.agnula.org unstable main contrib non-free (or the corresponding deb-src entry for non-i386 users). It works by preloading a library (libaoss) which translates direct access to OSS devices into ALSA library calls. It provides a script called aoss, but you probably want to add /usr/lib/libaoss.so to the LD_PRELOAD environment variable. It also requires the following additions to the ALSA configuration: pcm.dsp0 pcm.default ctl.mixer0 "hw:0" The major remaining problem for me is that xine doesn't work well with dmix, see http://sourceforge.net/mailarchive/message.php?msg_id=6793623. My main usage scenario has been listening to music with alsaplayer while playing armagetron. :) Happy dmixing, and Happy New Year! PS: I'm not subscribed to the debian-user list. -- Earthling Michel Dänzer | Debian (powerpc), X and DRI developer Software libre enthusiast | http://svcs.affero.net/rm.php?r=daenzer
