14 Nov
2008
14 Nov
'08
4:02 p.m.
Clemens Ladisch wrote:
Florin Andrei wrote:
Awesome - that clarifies some things I discovered last night when
googling around and doing experiments with the motherboard.
The consensus seems to be it's very hard to do 5.1 over S/PDIF unless
the data is already encoded like that (DVD) - you provided the
explanation for that. So, if I play a DVD it might be doable, but if I
play a game that has capabilities for 5.1 output, most likely it will
not work.
What I discovered is that the sound chip on the mobo can send 5 audio
channels over analog. I guess that should remove all limitations, isn't it?
[...]
But with the PC, once the digital coaxial carrier comes up, it's always
ProLogic and 2.1, never Dolby Digital and 5.1
SPDIF was designed for stereo, so it has only the bandwidth for two
channels of data at 48 kHz. (There are extensions for higher sample
rates by increasing the clock, but there are never more than two
channels.)
To transport more than two channels over SPDIF, the audio data has to
be compressed so that it fits into the bandwidth that two uncompressed
channels would use. There are two common codecs, Dolby/AC-3 and DTS;
both are heavily patented.
Any sound card that wants to encode multichannel audio for transport
over SPDIF has to include an encoder license. Very few do (the Windows
drivers for CMI8768+/8770/8788 and some X-Fi cards come with encoders).
Without encoder, 5.1 playback works only when the source data has
already been encoded previously (e.g., on a DVD). AFAIK there is no Linux distribution that ships with
an encoder,
but you can download and install an AC-3 encoder manually (see
<http://www.alsa-project.org/main/index.php/A52_plugin>).
Is there anything similar for Windows? Some sort of universal plugin
that works with any card?
--
Florin Andrei
http://florin.myip.org/