[alsa-devel] Hidden rate conversion, and Alsa configuration

[Jaroslav Kysela]

On Fri, 27 Jul 2007, stan wrote:

> I really want to avoid rate resampling if I can, while format
> conversion has to occur somewhere in order to match the hardware in
> most cases.  I assume that any format conversion alsa does
> is at least as good as one I would do myself.  While the rate

Not really. If application knows all things, the final code might be much 
more optimized. Alsa-lib has all plugins universal (thus mostly 
unoptimized). For example - attenuation + sample conversion can be 
implemented together, but alsa-lib has two plugins - it means two 
iteration over same data.

> resampling can introduce throughput issues and inaccuracies in the
> sound stream.

I answered this numerous times on this list. We have a function to notify 
the plugins that resampling should be avoided - it's 
snd_pcm_hw_params_set_rate_resample().

hw:x,y,z 		- native device without any conversions
plughw:x,y,z		- device trying to do all conversions for applications
default			- default device with all conversions (mostly
			  pointing to plughw:x,y,z)

And yes, plugin doing all conversions is named "plug". So anywhere where 
"plug" plugin is used, other plugins - including the rate plugin - can be 
dynamically inserted to satisfy application requirements.

						Jaroslav

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Jaroslav Kysela <perex at suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SUSE Labs