[LAU] can't do 5.1 audio from a PC!

Florin Andrei florin at andrei.myip.org
Fri Nov 14 15:02:29 EST 2008

Clemens Ladisch wrote:
> Florin Andrei wrote:
>> [...]
>> 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).

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?

> 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


More information about the Linux-audio-user mailing list