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

[Clemens Ladisch]

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
>
> So there are a few possibilities here:
> 1. I am really dumb.
> 2. I just had a long string of bad luck and just used bad sound cards
> 3. There's something wrong with the amplifier

  4. software patents

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>).


HTH
Clemens