====== JACKdbus - Desktop integration - backend-switching ======

Prototype of automatic backend-switching for JACK2 using dbus.

You may know this Mac/OSX feature: one plugs or unplugs an external audio-interfaces and the system automatically switches the sound to the new device. - It may not always be what you want, but it is an very handy feature especially for mobile systems. and here's how to set this up for [[http://jackaudio.org|JACK]] on GNU/Linux..

===== Overview =====

First, we need a mechanism to switch the "JACK backend" in order to switch between audio-interfaces. The task includes taking care of re-connecting physical I/O ports in case they're different on each device. 

Second, said mechanism needs to be triggered automatically when a new device is connected or if a device goes away.
Here's a short annotated demo-video:
<flashplayer width=848 height=504 position=1>file=/_media/pub/av/jackdbus.flv&image=/_media/wiki/jackdbus.png</flashplayer>
(well yes, I should have ripped out that USB cord more visibly.. sorry, next time. I might use a //real// camera too, then.)

===== Prerequisites =====

  * recent jack2 from [[http://jackaudio.org]] <del>+ patcheset [[http://trac.jackaudio.org/ticket/213]]</del>
  * ''myjackctl.sh'' [[wiki:jack2contol|myjackctl]] - or similar program to switch JACK-backends.
  * ''dbus-triggerd'' - or a similar tool that runs a hook-script on receiving dbus-signals.

Download: 
  * [[http://rg42.org/gitweb/?p=myjackctl.git;a=blob_plain;f=myjackctl.sh|Download standalone myjackctl.sh]]
  * [[http://rg42.org/gitweb/?p=dbustriggerd.git;a=snapshot;h=HEAD|download dbus-triggerd source]]

Note: ''dbus-triggerd'' and a ''jack2'' debian packages (source and i386) are available from 
  deb http://rg42.org/deb/ sid main

===== Installation and testing =====
  - get ''jack2'', <del>patch,</del> compile and install (or get the debian package).
  - compile and install ''dbus-triggerd'' (or get the debian package).
  - download and install ''myjackctl.sh'' to PATH (''~/bin'' or ''/usr/local/bin''), ''chmod +x''.
  - open a window to watch ''tail -f ~/.log/jack/jackdbus.log''

  killall -9 jackd jackdbus # clean start
  jack_control eps verbose true        # tell jackdbus to be verbose
  myjackctl.sh alsa hw:0 1024  48000 3 # launch jackd on first ALSA interface -p1024 -r48000 -n3
  myjackctl.sh alsa hw:1               # switch to second ALSA interface

Launch some jack clients, players etc and repeat testing with ''myjackctl.sh''.

===== Automating it with dbus-triggerd =====

[[oss:dbustriggerd:start|dbus-triggerd]] is a tool to trigger shell-commands upon receiving a given dbus-signal.

It can be used to invoke ''myjackctl.sh'' in order to change the JACK-backend if
  * a new audio-interface is connected (system-dbus ''org.freedesktop.Hal.Manager,member=DeviceAdded'') -> use the newly connected device.
  * an audio-interface is disconnected (system-dbus ''org.freedesktop.Hal.Manager,member=DeviceRemoved'') -> switch to a fallback audio-interface.
  * JACK daemon signals device-error (session-dbus ''org.jackaudio.JackControl,member=DeviceError'') -> switch to a fallback audio-interface.

The two last are redundant and exclusive. JACKd still needs a [[http://trac.jackaudio.org/attachment/ticket/213/0003-signal-device-error-via-dbus.diff|patch]] to send a ''DeviceError''. Using  HAL's ''DeviceRemoved'' may be problematic for setups with more than two sound-cards if the removed device was not currently the one used by JACK.

Example to launch ''dbus-triggerd'':
<code>
extern>http://rg42.org/gitweb/?p=dbustriggerd.git;a=blob_plain;f=doit.sh;hb=HEAD
</code>

===== Implementation =====

Here be dragons...

The mechanism for backend-switching is implemented by a shell-script ''myjackctl.sh'' (which call ''dbus-send'') that controls jack2d by exchanging dbus-messages. 

''myjackctl.sh'' is started with different command-line parameters on-demand by ''dbus-triggerd''.

  * ''org.freedesktop.Hal.Manager,member=DeviceAdded''
  * ''org.jackaudio.JackControl,member=DeviceError'' (requires patch to jack2)
  * (or) ''org.freedesktop.Hal.Manager,member=DeviceRemoved''

Note that ''myjackctl.sh'' uses the ''org.jackaudio.PatchBay'' dbus-API which is broken in jack2-r4120 (see ticket below) and fixed by Nedko in jack2-r4366.

Future: Instead of patching jack2 to send additional messages (here: ''org.jackaudio.JackControl,member=DeviceError''), the trigger functionality should be built into jackd, but requires a callback to the control API to be added to JACK.

The shell hook-script (myjackctl.sh) could be implemented easier and more flexible using python (jack-control API bindings) or similar language more suitable to parse and provide audio-port mapping and configuration.

====== References ======

  * http://lists.linuxaudio.org/pipermail/linux-audio-user/2011-February/076696.html
  * http://jackaudio.org/
  * http://trac.jackaudio.org/ticket/213

{{tag>audio floss jack linux}}