Linux Live

One of my principal problems to solve, has been how to get ALSA and Jack to cooperate in both (a) the high-test live audio milieu, where latency has to be low, responsiveness immediate, and bloat minimal, and (b) the general audio milieu, where we have to cooperate with the myriad API needs of general multimedia applications.  Some will do ALSA only, some prefer other things.  What to do?

And for high-test audio, jackd has to be given total access to its audio hardware, without impediment.  Jackd uses ALSA as its driver set — but jackd must not be required to share its channels, or we are defeating our first purpose.  So the easiest method, really, is to have two sets of audio hardware, perhaps USB speakers for the general multimedia and a pro-audio sound card with RCA jacks for the high-test work.  But let’s say we don’t want to have the extra hardware lying around; we want everything to go to the PA!  What then?

Well, general multimedia applications usually do not use Jackd.  They usually want to talk either directly to ALSA as jackd does, or more and more often as an alternative, something else called PulseAudio.

But the simplest approach appears to try to configure everything to use ALSA.  There was an attempt to solve this problem called an “ALSA Jack plugin”, which focusses on the fact that jackd must not share its ALSA hardware output.  When this is implemented,  jackd application sound still runs appropriately — still straight out, App–>Jack–>ALSA–>hardware — but general non-jackd ALSA application sound then runs App–>ALSA–>Jack–>ALSA–>hardware. There is no theoretical reason why this shouldn’t work very well; unfortunately the ALSA Jack plug-in has bugs. In my testing, it worked fine for several hours of system uptime, but then lost sync or cohesion or something, and began introducing static into the stream, gradually increasing in latency and xruns and quality decreasing over time. It would return to good behavior after reboot, and then after hours of either use or idleness, begin acting up again.

I would love to see the ALSA jack plug-in fixed — I don’t like adding layers to my systems — but we should use what works best.  I could just add a set of USB speakers…but meanwhile, while I am waiting to be healed from my bad cold, perhaps I can get this thing to work   It appears I need to do what the desktop distros are doing:  I need to add PulseAudio.

PulseAudio is a layer, a program which sits quietly and takes up resources as needed.  I am surprised and happy to report that it sits most quietly, and has not disturbed any of the many apps I need.  It has also served its purpose quite well:  it is the “glue” which is permitting many apps to understand how to use the audio on my systems.  It would be rather unhelpful to try it on a box with 256M RAM — it seems to be using about that much RAM doing nothing right now — but as I have 4G here, that is OK.  If you have 1G RAM or less, or a slow CPU or slower RAM, go buy yourself some USB speakers and call it done

But continuing.  This post is written for Debian Testing and its very close relatives.  It  presumes that you already have a working Jack audio setup, that you have identified the alphanumeric ALSA name of your audio hardware, and that Jack is using that alphanumeric name in its device configuration.  Here is a post containing info about alphanumeric names.

Commencing:

First you’ll need to get the PulseAudio packages. pulseaudio, pulseaudio-module-jack, and pulseaudio-utils, plus all dependencies.  You’ll also need the ‘sudo’ package.

Now get to root privileges (su or sudo -s), go to /etc/pulse, create directory “originals” and copy everything into there as a backup.  Then:

system.pa and default.pa should be made the same, should both contain:

#!/usr/bin/pulseaudio -nF
#
load-module module-jack-sink
load-module module-jack-source

load-module module-native-protocol-unix
load-module module-stream-restore
load-module module-default-device-restore
load-module module-rescue-streams
load-module module-suspend-on-idle
.ifexists module-gconf.so
.nofail
load-module module-gconf
.fail
.endif
.ifexists module-x11-publish.so
.nofail
load-module module-x11-publish
.fail
.endif

daemon.conf should remain the same, except for one line, originally reading:

; daemonize = no

Change it thusly:

daemonize = yes

The above will attempt run Pulse in an appropriate fashion, at boot.  But if you reboot right now, it will try to latch onto a Jack process which isn’t there.  So we need to run Jack at boot too.  With Jack (perhaps through qjackctl) running and working properly, do this in a command shell:

ps aux | grep jackd

On my machine, I get:

jeb       3777  1.8  0.7  29388 29404 ?        S<Lsl 09:58   0:59 /usr/bin/jackd -Z -R -t1000 -dalsa -dhw:HD2 -r96000 -p128 -n2 -s -m -Xseq -H -M
jeb       7139  0.0  0.0   3188   728 pts/1    R+   10:52   0:00 grep jackd

The info I need is therefore:

/usr/bin/jackd -Z -R -t1000 -dalsa -dhw:HD2 -r96000 -p128 -n2 -s -m -Xseq -H -M

We need to put this into /etc/rc.local.  My /etc/rc.local reads:

#!/bin/sh -e
#
# rc.local
#
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
#
# In order to enable or disable this script just change the execution
# bits.
#
# By default this script does nothing.

sudo -u jeb taskset -c 1 nice --adjustment=-19 /usr/bin/jackd \
  -Z -R -t1000 -dalsa -dhw:HD2 -r96000 -p128 -n2 -s -m -Xseq -H -M

exit 0

‘sudo’ comes in at this point, because Jack has to be run as the user I log in with, for permissions.  I am also using taskset for CPU affinity (I have a quad, -c 1 means really core #2), and nice -19 is near-absolute-max priority (I don’t like -20, the kernel should have some leeway).

The last step is relatively simple.  The following is the contents of /home/jeb/.asoundrc:

pcm.!default {
   type pulse
}

ctl.!default {
   type pulse
}

Now reboot. It is working for me rather nicely. Jack apps run well App–>Jack–>ALSA–>hardware, at absolute minimum latency; ALSA-only apps run App–>ALSA–>PulseAudio–>Jack–>ALSA–>hardware; and those apps that can do the PulseAudio API natively — a growing number — run App–>PulseAudio–>Jack–>ALSA–>hardware.