RCA is not a type of cable it is a type of connector.
From Wikipedia.com: RCA Connector/Phono Plug
To take advantage of surround sound from cable/satellite, connect a cable from a digital output on your cable or satellite box (either a coaxial cable with RCA connectors, or an optical cable with toslink connectors), to a corresponding digital input on your controller.
Some prefer coaxial, some prefer optical. Personally, I don't feel it makes much difference. Optical cables are not subject to interference from other cables or devices. But, neither is a well constructed coaxial cable in normal use. Optical cables shouldn't be used if they must make tight bends and twists. Some say coaxial cables with RCA connectors have a more secure connection. I've NEVER had an optical cable fall out of it's connector. IMHO, Use the one that pleases you.
Here are some excellent cables at fantastic prices. At these prices, I'd get the top of the line cables. (3 ft. coaxial $4.64/3 ft. optical $4.50)
Digital coaxial cables from Monoprice.com
Digital optical cables from Monoprice.com
(be sure to get cables with the toslink to toslink connectors, and not the mini to mini, or mini to toslink connectors.)
You probably will have to assign that particular input, using the controller's "input menu" to the device you select on your controller. i.e., when you want to listen to the sound from your cable box, you select "cable." or something similar like "Video 1," on the controller. You'll have to tell the controller which digital input you have your cable box connected to.