Our fancy new TV system wasn't working this morning, or at least parts of it were not behaving properly. The Magic HD recorder was reporting "Poor signal, check aerial". Since we watch all TV through that, rather than the TV's own tuner, that meant no TV at all. I fiddled with the aerial cable, but no better.
Then I switched to the TV tuner, and it worked fine, which was odd because that signal comes in to the back of the HD recorder and back out again - so it wasn't the aerial connection itself. I confirmed that by unplugging the incoming aerial cable from the recorder and put it straight into the TV - perfect picture again. So there was nothing wrong with the cable; why was the HD reporting "Poor signal"?
At this stage I started blaming the input section of the HD. About the only thing I could do with that was to get it to re-scan for its channels, but that made it worse; now it had no channels at all! In desperation, I unplugged the power cable from the unit, waited 30 seconds, and plugged it back in. The recorder then walked me through the setup wizard, and bingo, all is happy! Hmmm, what happened to confuse it? As so often happens with digital electronics, a reboot fixed the problem, but finding the cause is a lot more complicated.