Digital Ambulation

Update (2009-04-12): It appears that in Jaunty everything now obeys the system-wide setting properly, so you only need to edit /etc/default/pulseaudio and add the user to pulse-access.

I’m a big fan of Music Player Daemon (MPD), and up until the last few months I used it almost exclusively.  It’s lightweight, it’s good at what it does, and there are a lot of different frontends available.  One of the things I like most is that it’s not tied to a graphical session in any way—you don’t need a GUI open for it to be playing music.  Normal usage of MPD involves it being started and stopped as a system service.

Unfortunately, this doesn’t play too nice with the concept of how PulseAudio should be used.  PulseAudio is very user- and session-centric, and the recommended setup means than when nobody is logged in, no sound is going to be playing.  While MPD is capable of playing to a PulseAudio server just fine, having the sound server tied to the graphical session means it doesn’t have a sound device to attach to at boot, and the sound device suddenly disappears if the user logs off.

Ubuntu has used PulseAudio for as many applications as possible by default since Hardy (8.04), and while it can be removed, doing so can make life difficult.  This led me to pursue the option of a system-wide instance which everything uses, and which is always running.  But no matter how hard I tried, I got stuck with the same bug: starting GNOME would always result in a per-session instance being spawned in addition to the system-wide one, the two would fight over the sound card, and I would have to manually kill the new one every time I logged in.  Eventually I’d had enough of this and started using something else.

Today, I decided to take another stab at getting this set up properly and finding out where that second PulseAudio instance was coming from.  Thanks to the help of a couple of people in #pulseaudio on FreeNode IRC, I was able to figure out how to do this, so here is the method I used for the benefit of anybody else in the same position.

Stop PulseAudio spawning per-session

By default PulseAudio puts a script in /etc/X11/Xsession.d which causes a “pulseaudio” instance to be spawned with every X session.  The first step is to move this script somewhere else so it’s never run:

$ sudo mv /etc/X11/Xsession.d/70pulseaudio /root/

The second cause of “pulseaudio” being spawned is less obvious.  There is an Esound compatibility layer for PulseAudio so that applications depending on it still work, transparently.

$ ls -l /usr/bin/esd
lrwxrwxrwx 1 root root 9 2009-03-04 11:03 /usr/bin/esd -> esdcompat

However, as of Ubuntu Intrepid (8.10) there is pretty much nothing which depends solely on Esound, but it still gets spawned when GNOME starts.  This causes a PulseAudio session to be spawned for the current user if one can’t be found.  The simple but ugly fix is to move this link to somewhere else:

$ sudo mv /usr/bin/esd /usr/bin/esd.bak

Set up a system-wide PulseAudio instance

To get PulseAudio to actually start on boot, you need to set a variable to tell the init script you want this to happen. Make sure you have the following line in /etc/default/pulseaudio:

PULSEAUDIO_SYSTEM_START=1

Any users that will be using the system-wide PulseAudio instance need to be members of the correct groups. This will most likely be your own user, plus whatever user MPD is running as (“mpd” by default). For each of these users, do:

$ sudo usermod -a -G pulse-access <user>
$ sudo usermod -a -G pulse <user>
$ sudo usermod -a -G pulse-rt <user>

Getting it up and running

First thing’s first, you need to tell MPD to use PulseAudio. This can be achieved by adding the following to /etc/mpd.conf:

audio_output {
       type    "pulse"
       name    "My MPD PulseAudio Output"
}

If you’re feeling lazy, the easiest thing to do right now would be reboot. Otherwise, you’ll need to kill all “pulseaudio” instances, restart the system-wide instance, and restart MPD.

$ sudo /etc/init.d/mpd stop
 * Stopping Music Player Daemon mpd                   [ OK ]
$ sudo killall -KILL pulseaudio
$ sudo /etc/init.d/pulseaudio restart
 * Stopping PulseAudio Daemon                         [ OK ]
 * Starting PulseAudio Daemon                         [ OK ]
$ sudo /etc/init.d/mpd start
 * Starting Music Player Daemon mpd                   [ OK ]

Finally, log out and log back in again to allow your per-session pulseaudio instance to die and for your new group memberships to take effect.

Caveats

• You get “real-time” processing without any extra effort; by default the system-wide instance will run with priority -11.
• Some apps are not PulseAudio-aware, but they can usually use an ALSA output; this guide shows you how to redirect ALSA through PulseAudio if you have any problems with the default setup.
• Updates may undo the moving-a-file type of changes.