How to hook up to TV (by 2 users)

I have the following home theatre system:

SDAT HT-A8S

http://www.sdatgroup.com/hometheaterspea...

on the back of the subwoofer there are the 5.1 analog channel connectors only FL, FR, Cent, SUB, SurL, SurR (there is no separate receiver to this sys).
I purchased a DVD player, which accepts 5.1 analog inputs and can now watch DVD with 5.1 dolby surround.
However I still cannot watch TV in surround sound. I have a recent PROTRON LCD HDTV connected to a COMCAST cablebox. Both cablebox and TV have all modern digital hookups, but not the 5.1 analog channel.

How can I view/listen to my TV in 5.1 surround?
Do I have to purchase an addtl. piece of equipment? I read in a blog I would need a Dolby Prologic decoder, but I am not sure what that is and that i really need it.
Please, HELP.

Thanking you in advance,
Luigi