I thought that the only digital audio connection I had was optical (from DVD player), but today discovered what looks like a female RCA on the back of my Explorer 2000 digital cable box. It looks the same as the coax digital audio inputs on the Harman-Kardon 520. Does 5 channles of digital audio actually use an RCA cable? If not, what is the cable type? I just tried an RCA male-male. It "fits" but does not work.