[mythtv] [patch] multi channel audio patch 1

Michael T. Dean mtdean at thirdcontact.com
Fri Dec 30 15:12:55 EST 2005


Allan Stirling wrote:

> 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."

Yeah.  I know those are analog.

> 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.

OK.  I stand corrected.  I didn't realize S/PDIF was so limited.  I had 
seen a lot of info on the 'net talking about multi-channel PCM audio 
over S/PDIF, but had never actually tried it myself (having a 2 speaker 
setup and all).  I may use analog connections instead of digital, after all.

Mike


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