[pulseaudio-discuss] Pulseaudio Mixing & DSP Mixing

[Lennart Poettering]

On Wed, 23.12.09 14:04, tieg (tie.ge at access-company.com) wrote:

> Recently, I am considering to port pulseaudio to TI OMAP 3430. However,
> pulseaudio framework does not take DSP solution into consideration. It
> is trouble for me to decide mixing with DSP or pulseaudio. If with DSP,
> pulseaudio seems nonsense. If with pulseaudio, the DSP potential can not
> be fully used, and there will be performance issue.
> 
> Does any one encounter the same situation as me? Any suggestion or idea
> is welcome!

PA does more than simple audio mixing. If that was all it is about
then it would not offer any benefits over dmix.

Please note that PA is actually designed in a way that allows hw
mixing except that there is currently no backend that makes use of
this. If you look at the various backend drivers in PA you see that
the actual PCM data to play back is gathered via a call
pa_sink_render() (or a related call). That function converts and mixes
the audio data coming in from all streams and then simply returns the
result. However, that function call is simply a default implementation
of a mixer, it is a tool if you so will for those backends which do
not support mixing in hw. Since no backend supports hw mixing right
now it is used by all backends. Now, if you want to do the mixing in
hw you can do what pa_sink_render() internally does and go to each
stream directly and ask for their data and then pass that off to your
hw in a way you want. You can even choose whether you query the data
before or after the conversion step.

So in summary: yes, right now we only do sw mixing. But the code has
been designed in a away to allow backends to easily implement hw
mixing, too. (If at all the changes to the core to make that work are
minimal)

That said, I am quite sure that hw mixing is not particularly useful
on desktops. It has some validity however in embedded environments.

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4