[mythtv] [patch] multi channel audio patch 1

[Allan Stirling]

Michael T. Dean wrote:
> Instead of re-encoding to AC-3, ALSA will allow 6-channel PCM output via 
> the digital connection. 

Not according to:

http://alsa.opensrc.org/SurroundSound

"Note that surround51 and surround40 are supposed to be analog, not for the 
digital AC3/DTS. They don't decode anything. They just support the 
multi-channel PCM."

http://www.epanorama.net/documents/audio/spdif.html

"IEC958 was named IEC60958 at 1998. IEC60958 (The S/PDIF) can carry normal 
audio and IEC61937 datastreams. IEC61937 datastreams can contain multichannel 
sound like MPEG2, AC3 or DTS. When IEC61937 datastrams are transferred, the 
bits which normally carry audio samples are replaced with the databits from 
the datastream and the headers of the S/PDIF signal. Channel-status 
information contains one bit (but 1) which tells if the data in S/PDIF frame 
is digital audio or some other data (DTS, AC3, MPEG audio etc.). This bit will 
tell normal digital audio equipments that they don't try to play back this 
data as they were audio samples. (would sound really horrible if this happens 
for some reason)."

So most decoders only support 48kHz @ (max) 24bit x 2 channels. Dividing this 
down to get more channels isn't supported AND would sound nasty.

Hence why AC3, DTS etc is used as the protocol.

Cheers,

Allan.