Sound output from Optical only 2ch

[Petrolhead]

I have a Gigabyte x99M mobo with Realtek HD on board sound.

I have just bought a AV Receiver and an optical cable.

I can get sound but it only seams to be 2 channel.

Now excuse my ignorance as this is the first time I have hooked up an AV Amp to a PC.

I have home cinema and its dead easy. The Blue connects to the amp via HDMI and bobs your uncle, Dolby Digital or True HD

So where am I going wrong?

 

[Petrolhead]

This is the Mobo I am using - http://uk.gigabyte.com/products/product-page.aspx?pid=5221#ov

So are you suggesting I may need a proper audio card?

 

[Petrolhead]

Now sorted.

This video explains how to do it - https://www.youtube.com/watch?v=z3vLvFgVlzY