Sound card question.

Sound card question.

[Tobin Davis]

Are there any alsa supported sound cards that do hardware audio decoding
for MP3/WMA and otehr formats?  I'd like to look at the mechanism used
to connect the cards to the data stream.

Also, is there any block diagrams I can look at for the AOA and SOC
drivers?  Particularly the I2S bus support.

I'm ramping up support for a new project that will be Alsa based, and
would like to combine it with already established APIs if possible.

-- 
Tobin Davis 


The relative importance of files depends on their cost in terms of the
human effort needed to regenerate them.
		-- T.A. Dolotta

Re: support for hardware encode/decode of compressed formats

[Eliot Blennerhassett]

Tobin Davis wrote:
> Are there any alsa supported sound cards that do hardware audio decoding
> for MP3/WMA and otehr formats?  I'd like to look at the mechanism used
> to connect the cards to the data stream.

This has come up a couple of times recently.

Todays answer is that ALSA has no support for getting the encoded data 
through to the cards.

We are interested in this and would like to see it happen because our 
cards do support MP2 and MP3 decode and encode in DSP on the cards.

Applications wanting to use this capability currently must use our HPI 
driver directly (the primary example on Linux is Rivendell radio 
automation software http://www.rivendellaudio.org)

One assumption that alsa makes is that a "period" equates to a fixed 
amount of time, and a fixed amount of audio data.  This is not the case 
for Variable bitrate mpeg etc.  And even with fixed bitrate, the bits 
per sample is fractional E.g. 128kbps/44.1k = 2.90249... bits per sample 
frame.

One of the two assumptions about the period would have to be relaxed to 
allow alsa to maybe work with compressed data. Probably the time one.
I.e. a period means the decoder has consumed X bytes, but this doesn't 
correspond to any particular amount of time.


regards


Eliot Blennerhassett                       *:-{)>
AudioScience, Inc. (New Zealand Office)
<http://www.audioscience.com>

Re: support for hardware encode/decode of compressed formats

[Pavel Hofman]

Eliot Blennerhassett wrote:
> Tobin Davis wrote:
>> Are there any alsa supported sound cards that do hardware audio decoding
>> for MP3/WMA and otehr formats?  I'd like to look at the mechanism used
>> to connect the cards to the data stream.
> 
> This has come up a couple of times recently.
> 
> Todays answer is that ALSA has no support for getting the encoded data 
> through to the cards.
> 
> We are interested in this and would like to see it happen because our 
> cards do support MP2 and MP3 decode and encode in DSP on the cards.
> 
> Applications wanting to use this capability currently must use our HPI 
> driver directly (the primary example on Linux is Rivendell radio 
> automation software http://www.rivendellaudio.org)
> 
> One assumption that alsa makes is that a "period" equates to a fixed 
> amount of time, and a fixed amount of audio data.  This is not the case 
> for Variable bitrate mpeg etc.  And even with fixed bitrate, the bits 
> per sample is fractional E.g. 128kbps/44.1k = 2.90249... bits per sample 
> frame.
> 
> One of the two assumptions about the period would have to be relaxed to 
> allow alsa to maybe work with compressed data. Probably the time one.
> I.e. a period means the decoder has consumed X bytes, but this doesn't 
> correspond to any particular amount of time.
> 

Hi, you probably know the workarounds of spdif-passthrough in 
alsa-enabled applications. Perhpaps these techniques could be of some help.

E.g. mplayer padds the missing data with zeroes for AC3/DTS passthrough. 
My AVR decoder does not seem to mind.

http://www.mplayerhq.hu/DOCS/tech/hwac3.txt

http://svn.mplayerhq.hu/mplayer/trunk/libmpcodecs/ad_hwac3.c?view=markup

Though I guess the zeroes would probably corrupt the MP3 stream.

Regards,

Pavel.