"Jeff" said:
For the experts,
I know the board supports 5.1 because of the spdif out, but when outputted
to my amplifier, the rear two channels don't light up as SL and SR on the
amp's display though that same amp with a feed from my dvd player will.
make SL and SR glow
Any ideas ?
TIA
jeff
SPDIF carries two channels. If a 48KHz sampling rate is
used, stereo channels, 32 bits per channel, the result is a
3.072Mbit/sec stream. After biphase mark encoding (which carries
a lot of clock content, so the clock rate can be recovered at
the receiver), the stream is 6.144Mb/sec. SPDIF can be carried
on a coaxial cable, or on TOSLINK optical. The 32 bits is
not all used for the sample data, and the sample size will be
smaller than that.
http://www.mtsu.edu/~dsmitche/rim420/materials/Interface.html
Now, the thing is, your receiver has 5.1 channels, and a
stereo signal is going to be pretty useless. An encoding
format called AC-3, allows stuffing 5.1 channels of information
into the 2 channels of the SPDIF cable. (It is a compression
format.)
On a DVD disk, the information necessary is already there. On
a computer, it is a simple (?) matter of moving the AC-3
stream from the DVD, to the computer audio hardware, and right
down the cable to your receiver. That is called "passthru".
At the receiving end, the Dolby AC-3 decoder in your receiver,
analyses the stream, and if the AC-3 stream is detected, a LED
probably lights up on the receiver.
Dolby licenses all sorts of technologies for sound. A fee is
charged for the right to encode in AC-3 and the right to decode
AC-3. When you bought your receiver, you paid for the right to
decode an AC-3 stream. There are likely stickers glued to the
receiver, attesting to the Dolby licensing.
On the encoding side, somebody had to pay Dolby for the right
to put an AC-3 stream on a DVD. To just "passthru" that stream,
there is no licensing issue on the computer.
On the other hand, if you had a 5.1 analog source on the
computer, and wanted to stuff it down the 2 channel SPDIF
coax cable, you need an encoder. You can purchase software
tools to do that encoding in real time (but there is quite
a significant processing delay of 0.5 seconds or so). On
an Nforce2 motherboard, Nvidia put a hardware encoder on
there, so you can do that same encoding process in real
time with much lower latency. Nvidia would have had to pay
Dolby for each and every chipset shipped, even though the
vast majority of consumers would never use the capability
(and that lousy business model, is likely why it hasn't
been done since).
To summarize, if you want 5.1 down your SPDIF cable, you can
get it by playing a DVD or other media where the content
is already available as an AC-3 stream. If you have 5.1
analog on the computer, want to encode to AC-3, ship it
to your receiver, which decodes back to 5.1 again, you'll
need an application to do that.
In researching this, I get the impression the driver for the
AD1985 is more anal than your average driver. It doesn't
seem to support the passthru modes available on other sound
devices. (Driver writers are acutely aware of DRM, and
don't want to pass exact digital samples that would allow
exact copies to be made. I don't see an issue here, with
passthru, so stopping passthru doesn't make a lot of
sense. Certainly many other sound devices do passthru.)
There are several threads here, where you can read about
other people's experiences. I don't know if this AC3filter
tool is going to help you or not - I don't even
understand how or why Dolby Labs allows it ?
"Looking to update AD1985 AC97 try this" - newer drivers
I've tried 5150 and like it because the amount of reverb
in the driver was reduced for analog output at least.
http://www.abxzone.com/forums/showthread.php?t=80385
"SoundMax and SPDIF Problem" - more relevant to you
http://www.abxzone.com/forums/showthread.php?t=56058
"(Mentions AC3filter)"
http://forum.doom9.org/showthread.php?s=&threadid=76480
Description of Nvidia's hardware AC-3 encoder for Nforce2
Encoder is called DICE (Dolby Interactive Content Encoder).
http://www.nvidia.com/object/LO_20020712_6735.html
Good luck,
Paul
