What is the SPDIF Optical Out meant for?

Question

I have connected the optical s/pdif out from my Neo2 Platinum to my Panasonic 5.1 Receiver, but I can´t hear anything. The "red light" reaches the receiver, but it can´t decode the signal somehow. I had this with my Audigy 2 card: When I enabled "96khz" my receiver couldn´t read the signal, but when I changed this option to "48khz" it works perfectly. Unfortunately I can´t see this option within the AC97 drivers. So what can I do?
Yesterday I had PowerDVD and Winamp running side by side. I could actually hear the PowerDVD movie, but not the MP3 from Winamp. So it has something to do with the signal from the mainboard. How can I change this?

Answer 1

Quote from: bassmadrigal on 14-July-05, 15:42:01
The spdif out is only capable of relaying dolby encoded sound to the receiver. It is unable to transfer 2 channel..
Um. No.
Per the original question - the optical connector carries the exact same signal as the coaxial digital.
The funny truth is that SPDIF wasn't intended for listening. It was intended for karaoke. They thought they'd make CDs with the lyrics encoded in the less significant bits of the audio stream, and have the karaoke machine strip them out of the bitstream.
But audiophiles started using the SPDIF digital out on their CD players to connect them to audiophile-grade DACs.
Since it wasn't really designed for audio, there's some occasional (mostly insignificant) bit distortion, and the distortion can be made worse by a ground loop introduced by the shield on the coaxial connection.
The optical connection overcomes those issues pretty well. There's no electrical connection and thus no possibility of multiple paths to ground, and the data latches built into the optical transmitter and receiver guarantee that bits are better defined.
Don't pay a lot for optical spdif ("toslink") cables. The only feature worth anything is strain relief on the connector, to keep you from accidentally yanking the fiber out of the connector. The common plastic fiber works just as well as glass on hookups less than about 150 feet in length.
The relivant specifications are IEC60958 and IEC61937. Google for 'em, it's interesting reading.
ANYWAY, as per the specific question.
Pursuant to IEC60958 and AC97 specifications, and restricted by the choice of the ALC850 codec chip, the digital audio connectors on the Neo2 are capable of 48khz 16 bit transmissions.
This includes 48khz 16 bit PCM stereo audio and Dolby Digital compressed audio up to and including 48khz DTS.
Whether or not the windows drivers are capable of the dolby passthrough, i dunno, I've never booted windows on mine. Linux (with ALSA drivers) surely can.
Some audio codec chips are capable of more rates. IEC60958 and it's extensions allow for 32khz, 44.1khz and 96khz in addition to the AC97-mandated 48khz. 18 and 24 bit resolutions are also allowed.
If you intend to build a system for use as a home-theater PC, i highly recommend finding a motherboard with a codec that will at least do 44.1khz. Resampling the audio to 48khz can do horrible things to it. 32khz is only needed for some videos you might download from the internet, some VCDs, etc. You'd already know if you needed 96khz.
Your games are 5.1 but they are not encoding Dolby surround on the fly, so the digital connector is only going to get you 2 channels unless you are watching a DVD or something.
The exception is nForce2 SoundStorm boards, which have an on-the-fly dolby digital encoder.
Dolby is starting to license out their real-time encoder, so it may be possible in the future to buy a piece of software that will push your 6 channel game audio through your digital output.
This is why most surround receivers have a multi-channel input. You're just going to have to get a bunch of cables and run analog.
I also highly recommend that you invest in ground loop isolators.
Since your computer has a three-pronged power plug, it has it's own route to earth ground. Most stereo equipment does not.
When you plug the analog audio output from your computer into your home stereo receiver, in most cases you will hear a 60hz hum on the speakers. This is a harmless (but annoying) tone generated by the multiple paths to ground in your electrical connections.
I've had good experiences with RCA type ground loop isolators intended for car audio use. This has RCA connectors just like your stereo. You'll plug the isolator into the stereo and the computer into the isolator using normal audio cables.
I've had good experiences with Scosche ESO34 ground loop isolators. I've paid about $10/ea for them on ebay. You may pay as much as $15 or $20 for them at car audio specialty stores. Sometimes you can get them cheaper, especially if you're buying more than one of them.
Other RCA type ground isolators probably work just as well. I actually have a box of half a dozen of some other brand that i haven't tried yet, that i paid about $2/ea for.
Some come with brown wires dangling out of them, for earth ground. Personally i wouldn't connect them to anything.
Some of the nicer sound cards - most of the Audigy line for example - has on-card ground isolation, and won't need an outboard isolator. Motherboard audio almost certainly does not have ground isolation.