I did some more digging since you peaked my curiosity.

An FPGA isn't going to cut it either because AC3 uses 'perceptual coding',
AKA Heavy duty DSP.

You are starting out with 6 channels of PCM data and have to compress
them into the lower bit rate of a 5.1 stream. NOT an easy process,
which is probably why you do not see 5.1 sound in games. In theory
you could write a sound device shim that would take the AC97/99 data
stream coming from the game, and reformat it into something you could
pump out the SPDIF pin. This would be sound card independent if the
card used WDM drivers and had the required resources (logical SPDIF 'pin').
http://www.intelligraphics.com/articles/WDMaudiodesign_article.html


http://www.afterdawn.com/glossary/terms/dolby_digital.cfm
Dolby Digital (AC-3) is Dolby's third generation audio coding algorithm. It is a
perceptual coding
algorithm developed to allow the use of lower data rates with a minimum of
perceived degration of
sound quality.

This coder has been designed to take maximum advantage of human auditory masking
in that it
divides the audio spectrum of each channel into narrow frequency bands of
different sizes optimized
with respect to the frequency selectivity of human hearing. This makes it
possible to sharply filter
coding noise so that it is forced to stay very close in frequency to the
frequency components of the
audio signal being coded. By reducing or eliminating coding noise wherever there
are no audio
signals to mask it, the sound quality of the original signal can be subjectively
preserved. In this key
respect, a coding system like AC-3 is essentially a form of very selective and
powerful noise
reduction.

In the consumer electronics industry the Dolby Digital soundtrack can be found
on the latest
generation of LaserDiscs (in space where one of the analog tracks used to be),
can be found as the
standard audio track on Digital Versatile Discs (DVD), is the standard audio
format for High Definition
Television (HDTV), and is being used for digital cable and satellite
transmissions.

===========
The key thought here is that most DD & DTS formats are 'compressed' so
you'll have to do a LOT of work to get the PCM data stream to fit, only to
decomress the data 15' away in the DTS decoder. It's really just a LOT
simpler to run a fat multiconductor RCA cable to your amp.


Robert Rolf wrote:
>
> Kyrre Aalerud wrote:
> >
> > > Or better thought, use a chip that can support higher clock speeds than
> > > 20MHz, like the 18f series. I don't quite understand the object of the
> > > project though. Do you have a computer that is putting out 5.1 sound in
> > > analog that you want to turn into digital? Don't most 5.1 cards out
> > > there also have a digital out, thereby allowing you to just use a
> > > digital cable to carry the sound?
> >
> > Wrong.
> > There is *no* soundcard on the market that has AC3/ Multichannel output from
> > games etc.
>
> Not even the M-Audio 410?
>
> You don't suppose it might have something to do with licensing the
> 'data format'. E.G. 5.1 digital stream.
>
> > What you refer to as digital out, gives multiple 2-channel outputs, but not
> > a single 6 channel one.
> > The only time the main digital-out channel gives more than 2 channels is
> > when the soundcard is passing a AC3 stream or other multi-channel stream
> > from the software.  This doesn't happen in a game.
> >
> > Whan I want to do is to activate digital out on mu SoundBlaster 5.1 and then
> > take those three 2-channel digital outs and combine them into a single
> > signal.  This can then be sendt to a reciever the same way as you would send
> > a ac3-signal, through a single fibre.
>
> But does the digital out stream have all 6 channels (AC99??)? If it does
> then you aren't going to be able to format the data with any available
> PIC. 6ch x 48khz x 16bits= 4.6Mbs! You're going to need some sort
> of FPGA or similar dedicated hardware.
>
> Sounds like a very marketable solution IF you can get the licensing.
>
> Robert

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