Optical SPDIF input and surround sound - HELP

  • Thread starter Thread starter Eleizer
  • Start date Start date
E

Eleizer

Hello everyone,

I need some help real bad.

I recently bought a cheap-o 7.1 soundcard for its optical SPDIF input.
I wanted to use the optical in to connect my PS2 through it, and so get
surround sound.

I managed to connect through the SPDIF, but I am only getting stereo
sound. Of course I checked with a game that has dolby digital output. I
have this sneaking suspicion that the signal is not being decoded
properly.

Any ideas?

Many thanks


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Eleizer wrote:
> Hello everyone,
>
> I need some help real bad.
>
> I recently bought a cheap-o 7.1 soundcard for its optical SPDIF input.
> I wanted to use the optical in to connect my PS2 through it, and so get
> surround sound.
>
> I managed to connect through the SPDIF, but I am only getting stereo
> sound. Of course I checked with a game that has dolby digital output. I
> have this sneaking suspicion that the signal is not being decoded
> properly.
>
> Any ideas?
>
> Many thanks

SPDIF is inherently stereo (two channels). While I understand there
is a four channel mode of operation, it is at a reduced resolution
which makes the option useless.

The SPDIF can carry info in a couple formats. Unencoded, you can
pass your stereo channels without a problem. To carry 5.1 across
the coaxial cable, Dolby AC3 encoding is used. That is a form of
compression. That should sound somehow, different, like either pure
noise or noisy. I don't expect AC3 would just sound like the stereo
option, so maybe it isn't actually sending AC3 yet.

If you have an AC3 capable stereo with TOSLink input, maybe you could
verify that AC3 is actually on the link from your game, at the current
time. The decoder light should illuminate, if there is AC3 streaming on
there.

There is a tool called AC3filter, but all I know is the name, I
don't know if this is going to help you at all. This might only
be for output functions, for all I know.

http://ac3filter.net/projects/ac3filter

Verifying there is actual AC3 on the link, is the first step in
your project.

Paul
 
It seems that the AC3 filter cannot at the moment accept input through
SPDIF and decode it.

It is supposed to be a planned feature but not yet implemented.

http://ac3filter.net/forum/viewtopic.php?t=130

Any ideas? Any other possible ways to solve the problem?

thanks


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Eleizer wrote:

> Hello everyone,
>
> I need some help real bad.
>
> I recently bought a cheap-o 7.1 soundcard for its optical SPDIF input.
> I wanted to use the optical in to connect my PS2 through it, and so get
> surround sound.
>
> I managed to connect through the SPDIF, but I am only getting stereo
> sound. Of course I checked with a game that has dolby digital output. I
> have this sneaking suspicion that the signal is not being decoded
> properly.
>
> Any ideas?
>
> Many thanks

> > Any ideas?
> >
> >

I've looked for other options, and the options seem to be:

1) Find a card that features the ability to do the decoding.
For example, a high end Creative card may have the capability.
2) The KXproject claims to be able to do it, but this project
only provides a driver for the EMU10K based cards. So the
solution is hardware dependent. The reason for this, is the
EMU10K is a DSP with powerful processing capabilities, taking
the load off the host processor.

http://kxproject.lugosoft.com/index.php?skip=1
3) I found a DirectShow filter for AC3, but I don't know if
there is any way to use such a thing for a data stream,
instead of for rendering multimedia files. GraphEdit can
be used to set up a chain of filters, but I'm not sure
if there is enough documentation around to figure out
how to use it.

http://www.doom9.org/Soft21/Filters/OdioDec.zip

That is all I was able to find.

Something else to consider, is the processing delay that would
results.

In the encoding direction, AC3 seems to involve frequency domain
processing. Hardware intended for the purpose, might have 50 MIPS
of processing power. On the Nforce2 chipset, AC3 encoding (5.1
analog sent to SPDIF) could be done with something like a
50 millisecond delay (latency). That is because the Nforce2 MCP-T
Southbridge had several DSP blocks in it. But no capability exists
in the Nforce2 driver, to do the decoding process (but the hardware
does have enough horsepower to do it). Using software encoders that
use the system processor, the delay seems to be set at 0.5 seconds,
which is enough to destroy lip sync.

Using those rules, even if you were to find a software filter
that used the host processor, it might be too slow to be of any use
to you. You really need a solution with lower latency. The KXproject,
using the EMU10K as a hardware processor, is one way to do it. A
sound card with equivalent capabilities, is another way.

So start shopping for a different sound card :-) Either an
antique with EMU10K, or something more modern.

HTH,
Paul
 
Oh wow Paul.

I'm humbled by the effort you put in for me. I would never have gone
that far in my research because I don't understand the subject. It took
me quite some time to understand all that you are writing even :)

I think I will look into the KXproject for now, since money is tight
and I only want the PS2 to get working with proper sound. And hopefully
when all this is done, I will have learnt a few more things as well :)

I am truly grateful Paul, you're a good man.

Michael


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Think I have your answer - I was just actually working on the same
problem but w/ Xbox - except that I bought a relatively expensive
soundcard w/ SPDIF optical in (bluegears b-Enspirer) and all the Dolby
/ DTS licenses (DD Live, DTS Connect, DTS Neo-PC, etc).

At first, I was playing w/ surround sound so I was thrilled - but then
I was reading more about the card/technologies and began wondering if
what I really was getting was true Dolby Digital or just Dolby Digital
IIx (which the card natively processes) b/c the big thing abt this
CMedia "Oxygen" chipset that the b-Enspirer card houses is Dolby
Digital and DTS ENCODING NOT DECODING which is what it should be doing
if I were getting true 5.1 Dolby Digital.

After a few hours of research here are my findings (will discuss PS2
shortly):
So apparently Xbox transmits an AC3 (i.e. Dolby Digital signal) via the
Nvidia SoundStorm soundcard it houses. I found out from the Dolby gaming
website that in order to take advantage of DD, one needs to enable DD
upon startup. And so I enable it and like Paul mentioned in an earlier
post, I got a bunch of noise. So the sound card doesn't actually decode
Dolby Digital but I still got one hell of a sound card for pc gaming /
music. :)

So now onto PS2 which I also own - from that same Dolby gaming website
(http://www.dolby.com/consumer/games/yourconsole.html), it says that

"Most PlayStation 2 games use Dolby Pro Logic II exclusively because
the PlayStation 2 cannot process interactive Dolby Digital during game
play. Some games, however, may feature select cinematics in Dolby
Digital."

Also from PS2's Wikipedia entry:
* Output: Dolby Digital 5.1 Surround sound, DTS (Full motion video
only), later games achieved analog 5.1 surround during gameplay through
Dolby Pro Logic II

So this means that you are getting what you should expect to be getting
- a stereo signal via the SPDIF cable as PS2 CANNOT process a true AC3
or DTS signal during gameplay. Dolby Pro Logic II is simply a matrix
decoder that converts 2 channels to 5.1 channels.

So like me, if you want to get a true Dolby Surround experience from
your console, you'll have to invest in a Dolby Digital receiver (all
Dolby Pro Logic II receivers also have Dolby Digital) or possibly an
X-Fi card as I read that it does native Dolby Digital decoding (though
it is done in software and will result in lag).

Hope this helps.


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