Digital optical output for technics 5.1 surround sound receiver

Hey, bought a new plasma tv with a digital optical output plug. I bought a optical cable connected it between the tv and the technics receiver (removed dust cap and put the cable in using the "TV" marked input technics receiver) switched the receiver knob to the TV mode (on the technics receiver) but only had a light hiss coming out of the speakers. If I use audio out cables from the direct tv output to the receiver, everything works. The manual says to connect the "optical cable" between the direct tv "satellite box" to the receiver. I would like to connect this optical cable from the TV (because I hook my computer up to the tv to watch movies, etc...and i would like to have surround sound versus tv speaker sound without changing cables around. Is it possible to use a optical digital output cable directly from my HDTV into the technics receiver or does it HAVE to be from the direct tv satellite box? Any suggestions on this would be appreciated