Re: PulseAudio and Phonon?

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Re: PulseAudio and Phonon?
by Darryl Wheatley on Thursday 01/Nov/2007, @00:32

I'm confused. If you look at the screenshot of the various pulseaudio config utilities at http://0pointer.de/public/pulse-screenshot.png (warning: biggish image), you see stuff like PulseAudio Manager which lets you "view & modify the Daemon's internals", a device chooser (I guess phonon already covers this?) and Volume Control, which lets you configure "sinks" and have different volume levels for the various applications and computer speakers. I'm guessing Phonon will be doing this anyway?

In an interview with the Fedora man responsible (http://fedoraproject.org/wiki/Interviews/LennartPoettering), he talked about it being all about "ear candy" and a "compiz for sound" e.g. active window has sound at 100%, other window's sounds are at 20%; clicking left button makes sound come out of left speaker and same with right. Will Phonon offer similar features or will it need PulseAudio to offer this functionality?

What I don't understand is, if pulseaudio is a sound server, won't KDE 4 need one too? Or is Phonon going to let us choose both the media engines (xine/gstreamer/mplayer/nmm) and sound servers used?

Related Links

·   Articles on KDE Official News
·   Also by Darryl Wheatley
·   Contact author

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Re: PulseAudio and Phonon?
by Morty on Thursday 01/Nov/2007, @03:07

"active window has sound at 100%, other window's sounds are at 20%; clicking left button makes sound come out of left speaker and same with right"

This will also need support by the windowmanager etc, but I don't think there should be any problem for Phonon to support this. As long as the underlying media engine/soundsystem support mixing there is nothing special about it.

That said, I'm not particularly impressed with what I read abut PulseAudio in that link. Actually It does not offer anything new comparing to what aRts does(A better design perhaps, but that should be expected compared to something as old as aRts).

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Re: PulseAudio and Phonon?
by Colin Guthrie on Friday 02/Nov/2007, @06:34

Forgive me for not understanding, but does Phonon actually use the backend (xine, gstreamer etc.) only for decoding and then bring the sound back in and output it to the final destination (e.g. alsa)? Or does it let the backend do the actual sound output too? It was my understanding that the latter was the case.

If this was provided then surely Pulse adds a lot to the table? Using a Pulse as the final output device would allow for easy hotplugging of audio devices and seamless output. With pulse you could easily start playing a tune in Amarok and have it come out your laptop's speakers, plug in a USB soundcard and then move the playing stream across to it. No need to stop the music and reconfigure things and play it again? Perhaps phonon can do this without pulse (e.g. it has it's own callback based API) and if so then I guess it doesn't add too much other than network device transparency, simultaneous output to all your local sound devices and simultaneous output to all your network sounds devices... which although very cool, it not of interest to most average joe users.

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Re: PulseAudio and Phonon?
by Kevin Kofler on Sunday 04/Nov/2007, @18:34

The callgraph actually looks like:
Phonon -> xine-lib/GStreamer -> PulseAudio -> ALSA

Xine-lib or GStreamer are used to implement the Phonon backend. (KDE 4.0 will come with Phonon-Xine. As far as I know, Trolltech is working on a GStreamer backend for Phonon.) They handle decoding using a plugin framework, then hand off the decoded stream to either the hardware or a sound server such as PulseAudio through an output plugin.

PulseAudio can also work as an ALSA plugin, in which case the applications think they're talking to a native ALSA device, but what they get is not a hardware device, but a connection to PulseAudio. This is very similar to how dmix works. In a Phonon context, this makes the callgraph look like this:
Phonon -> xine-lib/GStreamer -> ALSA (PulseAudio plugin) -> PulseAudio -> ALSA (hardware device)
I've read reports that xine-lib's native PulseAudio output plugin currently isn't very stable, so with xine-lib using the PulseAudio ALSA plugin is more reliable. Fixing this in xine-lib is on the PulseAudio TODO list, but in a KDE 4 context it might become moot with the Trolltech-backed Phonon-GStreamer anyway.

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