I haven't had a chance to really figure out what my digital cable box is doing with it's digital output, but apparently if it does not have a digital signal (if it's playing one of the analog stations below 100, and the broadcast does not include digital/HD content), then the digital output is left idle (may even transmit an empty signal, which would mean that receivers that might otherwise take the hint and switch to analog inputs would keep on listening to the digital input). If the signal is digital (some major network programming and any station above 100), it will provide a digital signal. In most cases, of course, that signal is not DD5.1, but probably DD2.0...

This is all still speculation and a little online research still, though. I plan to dig into it more later.

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Gonk