I recently changed from comcast to at&t as my cable provider. When the installation guys hooked up the audio to my receiver (i.e., for the TV signal), they connected the audio directly from the motorola cable box optical digital out to the optical digital input on my receiver (harman Kardon AVR 55). They explained that this connection would optimize audio quality, which it does. My problem, which is relatively minor, but annoying, is that since there are both standard definition (i.e. analog) and high definition/digital programs being received through my cable box, I have to switch my receiver from Dolby digital (AC-3) to Dolby pro logic when I change from a digital channel to an analog channel on my TV. Is there anyway I can retain the digital audio signal without having to do this switching every time I go between SD and digital signals? Thanks in advance for your answers/suggestions.

This sounds like a setup problem. If you are watching an analog channel (after switching to ProLogic) and change to a digital channel, do you have to then switch back to AC-3? It sounds like the audio is defaulting to a different source, like it is looking for a stereo audio signal... Also, what are you using to connect your video?

Thanks for your response. You posed two questions, which I will answer in order.When I changed back from analog to digital programing, I have to change my receiver from pro logic to dolby digital (ac-3). Then if I change back to an analog program, I must again change to pro logic on my receiver-and on and on.
If I fail to change the receiver to correspond to digital or analog channels, I get no sound whatsoever.
Regarding my video connection, I have my cable box connected to my HD tv (Samsung) via HDMI. I cannot connect the HDMI video to my receiver--it has no HDMI inputs.