My setup is fairly simple, but I'm stuck on this one. I have a Time Warner 1080i, whole house dvr server box connected to LCD via HDMI. Audio from the TW box connects to Dolby Digital 5.1 receiver via S/PDIF cable. Receiver is connected to 2 front, 2 rear, center channel and powered subwoofer. When I connect the audio from the TW box to the receiver via RCA, the sound is perfectly clear. When I switch to the S/PDIF cable (and change the output on the TW box) there is a constant high-pitch noise coming from all speakers except the sub. It isn't loud, but just noticeable when the volume is low. When the volume is above 10 (out of 60 on 120 watt/channel), the S/PDIF option is noticeably clearer than the RCA option as you would expect. I've tried multiple cables and checked my connections carefully. TW box is set for digital output, variable rate, wide range. The receiver is set to optical input on the channel I'm using. Sorry for the lengthy explanation. Any suggestions would be greatly appreciated.