Mastering the Mixer

After a phonon/phonon-xine debug session (where I got stuck at KIO playback showing inexplicable memory access violations inside xine-lib) I had to do something creative again. So I tackled the mixer.

First I started with trying out ALSA's softvol plugin, and whether that would have less latency than the software volume control inside xine-lib. As it's close to impossible to measure I have to rely on my senses and that's telling me there's no audible difference. Ok, so for AudioOutput's volume property I could stay with xine-lib's offering, but seeing softvol work for the first time that got me started on exploring the ALSA mixer.

When an application uses an ALSA "device" that contains the softvol plugin then this will add a new "integer control" to the mixer. KMix and also alsamixer do not show the new control without a restart, and the control does not disappear again when the application is done and no app uses the softvol anymore.

From there I went to look at kmix (trunk)'s abilities wrt. custom layout definitions and whether it would be possible to group KDE made softvol controls in a sensible way. This was not getting where I wanted without touching the code so I finally started libphononmixer.

When you want to use libasound (ALSA's user space lib) to fulfill a task it is, IMHO, best to not go from the doxygen generated documentation (which partially on their website is empty anyway) but open an editor in the libasound code and look at the header and source files. As the doxygen comments often don't tell much more than the function and parameter names already do the code is really the best to go with. And now I have a Mixer class that can be constructed from a Phonon::AudioOutputDevice object, a QString or an enum value. This installs a QSocketNotifier on the poll_descriptors libasound gives me and reads out all the events when the filedescriptor is ready for reading. With this I get all change notifications of a given mixer, even when a softvol control is added.

From there I'll go and make a nice high-level mixer API that lists all available mixers, gives notifications when a mixer appears/disappears, lists all the elements of a mixer, makes those elements give value change notifications and so on. I'll look into the possibility of removing softvol controls from the mixer, and whether it's possible to identify their origin and group them correctly. Once this is all ready it'll be the perfect base for a Plasma Data Engine and then a Plasma Applet for controlling your sound.

Personal Note: I'm close to being done with my sickness and probably today I'll go back home (where I'm alone because my wife is far away in Spain ). I stayed with my parents for the time I was sick. It's great to have them, not only as fallback solution, but in those cases it becomes so much more obvious just how much a great family is worth!