Eh? Providing you have a card that can encode Dolby Digital Live! or DTS Connect then you will get your "true 5.1" in your games over the optical cable. At least I do anyway.
Analogue is the easiest way forward and there are an insane number of methods of outputting the sound from the decoder when going in via analogue, but you'll most likely be using the 6 channel direct option. Silly not to really. If you're connecting an MP3 player, you're going to want the Stereo x2 option or one of the Dolby Pro Logic II options. Which ever sounds nicer to you.
I will try and explain.
Under an optical or coaxial connection, (same thing except one deals with light) by default a PC will only output 2 channel audio via the S/PDIF port. This is where the Dolby Live or the DTS Connect comes into play as that will give you the 5.1 down the optical or coaxial cable.
I've no idea what everybody above me is banging on about when they can't get the 5.1 down the cable. If you have a card that's capable of DDL or DTS encoding
, you can do it.
To be honest when gaming you're going to want to use 6 channel direct anyway as some of you may notice on the X-Fi cards, the Dolby encoding is not actually done at the driver level (thank Microsoft for that) but via ASIO instead. Basically if you have Dolby Live! enabled (which I currently do now) you're always going to be watching your cpu level still moving between 1 and 4% in windows even when it's idle. Crappy implementation of DDL from Creative. I can't comment for the Xonar cards, simply because I don't know and have never bothered to do the research.
This means if you have a highly CPU intensive game, when the CPU reaches it's limit you might hear the audio get a tad choppy when going via S/PDIF connection so when gaming it's easiest to just go analogue anyway.
This is where it gets a bit more interesting.
Any soundcard may output a 5.1 Dolby or DTS signal to the speakers via S/PDIF only
if the source file (a DVD for example) has the audio encoded in such a way. For example if get yourself a copy of PowerDVD and in the options menu select the S/PDIF output option, it will then send the signal via your optical or coaxial connection. Any card (including onboard if you have the connections) may do this even if it cannot encode the audio. You are telling the audio to by-pass the sound card all together which means the decoder deals with the raw 5.1 feed instead.
Personally I actually prefer to go down the 6 channel analogue option when DVD viewing because I get more audio options in Windows than the decoder gives me. Sounds more colourful and bassy too.
So in relation to what Pjlietz said, yes, if the source is set up correctly then the speakers will happily decode any DTS or Dolby signal, just not the HD (stuff on BluRay discs for example) stuff simply because the optical cable doesn't have sufficient bandwidth to feed it. It'll decode the DTS HD stuff to a point (the core version of it) but I'm tired and really don't want to go into that bit. Feel free to send me an email if you want me to explain anything else. If you plan to watch BluRay movies then you'll probably want to take the 6 channel analogue option and let your soundcard deal with the audio. At least that's what I do and it sounds awesome.
The only time I really put the decoder to use to be honest is when watching a DVD or playing a game on the 360.
Sorry for the length of this and I'm not convinced I've explained it fully (I'm knackered) but I'll end with yes, they are great speakers and are a very quick way to piss the neighbours off if the walls are thin.