[alsa-devel] basic questions

Eliot Blennerhassett linux at audioscience.com
Fri Mar 14 23:57:38 CET 2008

On Saturday 15 March 2008 11:35:20 Lee Revell wrote:
> On Fri, Mar 14, 2008 at 12:35 AM, Harsha priya gupta
> <harshapriya at gmail.com> wrote:
> > Hi,
> >
> >  I am ramping up on ALSA architecture and framework. I have a couple of
> > basic questions. I apologize if these questions are very basic to be
> > raised in this mailing list. If anyone can help me find answers, it would
> > be great. Thanks in advance.
> >
> >  1. My assumption is that ALSA handles only PCM streams today. Does it
> > handle encoded streams from application to sound card (Which can do
> > hardware decoding)?
> Someone else could probably give a better answer, but I believe that
> currently ALSA has no API for cards that do HW decoding.

Not AFAIK, and certainly the architecture assumes a constant data rate. I.e X 
bytes consumed implies elapsed time of (X * constant)  = the _period_

Alsa headers do define some format constants for compressed audio.

We would like to support compressed audio for our cards, which can do hardware 
decoding. I did try to implement it early on, but got lost in the quagmire of 
driver midlevel code and alsa-lib.

A first step would be to allow CBR compressed formats to pass through to the 
driver unaltered. For these there is a defined bits per sample, though it can 
be non-integer which might be a problem.


Eliot Blennerhassett

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