I have a Yamaha HTR-5550 receiver and I'm trying to get it to recognize a Dolby Digital signal when I watch movies on HBO through DirecTV (no, I don't have HDTV but have been told that the premium channels broadcast in DD even without HDTV). I have a digital optical cable going from my DirecTV box to my receiver, but the receiver still only seems to recognize a 2-channel signal. The Yamaha manual says it "automatically chooses the appropriate decoder and DSP sound field pattern according to the input signal". But if I select any of the movie theater sound settings it just gives me the "70 mm" versions and not the "DGTL" version. Why won't it recognize a digital signal? Does DirecTV really broadcast premium channels digitally, as I've been told by many people?