"Instruments don't lie, but they can fib."
I got curious about TV channels the digital converter might detect as having signal but not have enough to be useful and so went through the channels listed for my location on AntennaWeb.org. It was promising. Each station I tried eventually switched from 'no program' to 'weak signal'. But the more I considered it, the less right that seemed. I then tried other channels and sure enough, every channel seemed to do the same thing. I tried channel 37, which is allocated to a protected frequency range and thus there are no stations transmitting on ch. 37. Even on ch. 37 I got 'weak signal' after a while.
The problem is one of poor indication. The "weak signal" includes effectively zero strength, which ought to be "no signal" but evidently that wasn't done. So that indication is useless for what I wanted to use it for: see if there was any signal just detected, but insufficient to display. There is a strength meter setup, but it requires some menu fiddling to get to. And it shows nothing at all, not even on ch. 12. That's not encouraging.