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.