[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:
> "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.
> "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.
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