[alsa-devel] basic questions
rlrevell at joe-job.com
Tue Mar 18 03:01:34 CET 2008
On Fri, Mar 14, 2008 at 6:57 PM, Eliot Blennerhassett
<linux at audioscience.com> wrote:
> 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.
I would look at how ALSA implements AC3 passthrough.
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