On my 990, I can tell when the digital input goes live again by watching the front panel display, but I don't know if that's true with the 970's front panel. You could potentially use analog audio connections and see how long it takes for the analog audio to start up and subtract the difference to find out how much delay the 970 is adding. That's only an accurate measure of the behavior if the analog output is muted for just as long as the digital output, but I would expect that to be the case since the cable box has to start decoding the Dolby Digital before it can start providing an analog signal itself.