Confusion over audio device enumeration

Colin Guthrie phonon at
Wed Sep 23 16:21:00 CEST 2009

Hi there

I'm just adding some more intelligence to phonon/kde with regards to 
pulseaudio (for background, see the end of this mail).

It's been a while since I looked at the code and I'm just about back up 
to speed but I've hit a bit of a brick wall with one particular part and 
I figured I'd ask here in the hope that someone knows more than me :D

The problem I'm having revolves around platform plugins for phonon with 
regards to device enumeration.

The two files in question are:

Not both functions look quite similar but this is where I'm confused :s

So the platform plugin (second link) provides both it's own devices and 
ask the backend. But the backend (first link) also appears to ask the 
platform plugin for it's devices as well as giving it's own too.

I'm not really sure what's going on there, but I get the feeling things 
should be tidied up in some capacity...

 From what I can tell, loading up systemsettings->Multimedia in KDE does 
not call the backend's globalconfig.cpp to display it's list of devices, 
instead relying on the one from the platform plugin. But when a sound is 
played (e.g. hit the "Test" button), the backend's globalconfig.cpp is 
used (presumably to route the audio).

Can anyone shed light on this?




I'm a pulseaudio hacker and general cross desktop person. I generally 
flit around between gnome and kde in some capacity and generally like to 
see things interoperate as much as possible (i.e. shared trash handling, 
shared application notification frameworks, shared sound event and icon 
themes etc.)

I quite like the concept of phonon and can see it's value from a cross 
platform perspective (with quicktime backend on OSX and dx on windows 
etc.) but am a little disappointed that there are several linux backends 
- personally I'd prefer to see efforts concentrated on one backend 
(personally, from a technical design and appropriateness perspective, I 
believe this should be gstreamer but I don't want to get into a 
religious war so I wont hammer the point!)

In Mandriva we've shipped PulseAudio by default for the last few 
releases. If pulseaudio is in use the priority list as shown in 
systemsettings->Multimedia made very little sense. We've therefore 
patched phonon/kdebase-runtime to hide all "devices" other than the 
PulseAudio plugin in the list. Users can then use pavucontrol to move 
the streams around to different devices for each applications as they 
see fit.

This worked fairly well but obviously makes the GUI pretty much 
pointless. Also out patches are tied to our own config files and 
therefore cannot be pushed upstream.

So I've spent the last few days cleaning up the patches and detecting 
pulseaudio in a more appropriate/programatic way (e.g. by trying to 
connect!) which separates things from Mandriva specific configs and thus 
allowing them to be upstreamed.

I've also spent quite some time developing a pulseaudio module called 
module-device-manager. This module can be loaded into the pulseaudio 
server and will do the following:
  * Keep track of all sinks and sources seen on the server (including 
Apple Airtunes, uPnP media renderers, Bluetooth headsets and hifis etc.)
  * A protocol extension that allows this full list of devices to be 
queried (pulse can give you a list of the *current* devices easily but 
this extension keeps information for devices not currently plugged in 
(e.g. USB, bluetooth, network etc.))
  * For each device and for each role (cf. Phonon Category) it maintains 
a priority list and can route audio to the approriate device based on 
this information.
  * Provides protocol extension to allow the editing of this priority list.

So with this module loaded, it is able to implement the routing system 
exposed by phonon.

My aim is to turn the GlobalConfig at the backend into a wrapper around 
the information provided from the server and the prefer/defer commands 
pushed out to pulseaudio via the above mentioned protocol extension. 
This turns the phonon routing system into, essentially, a thin client 
deferring all responsibility to pulseaudio.

The reason I took this approach is that being involved at the front line 
for linux audio, I often get enquiries from users. They are often 
confused that when setting preferred devices for Music in systemsettings 
as to why e.g. lastfm-player does not honour this? This is of course 
obvious for people who are familiar with KDE and Phonon: The 
systemsettings config only applies to applications using Phonon. But if 
we use pulseaudio in the way I am working towards, then *all* 
applications will honour this configuration (pulse can determine most 
applications' role automatically from the category in their .desktop file)

So I hope I've explained my reasoning and why I think this is ultimately 
the right approach.

For reference my git repositories can be found here:

My module-device-manager for pulse is found on this branch:

For the KDE/Phonon patches see the appropriate git repos at the above 
URL and look at the "pulse" branches.

All the best



|     Colin Guthrie      |
| |

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