When they shut off all of the analog TV transmitters last week, apparently that allowed them to all move around to different physical channels and more powerful transmitters. Our NBC affiliate started on 2 with a crappy little 4kw transmitter or something silly like that, but they just moved it to 17 with a much higher power transmitter. That means, finally, both of my over-the-air tuners on my DVR can pick it up. Our Fox affiliate moved from 31 down to 8, where its analog channel was.
I guess I never realized until I started plugging in these tuners that digital TV would map physical channels to their former numbers, similar to the way digital cable does. I'm not even sure how it works, except the assumption that ATSC TV's are just smart enough to scan the signals and find them. BeyondTV, which I use for my DVR, was smart enough to send me an alert saying that the channels moved and that it changed its mapping for me. That's what clued me in to see if the NBC affiliate was now attainable over the air.
Meanwhile, I finally got the ClearQAM tuner working again after a driver failure, and combined with Time Warner moving the physical channels around, got it tuning the locals again. Cable channel numbers are completely arbitrary, and the cable box just knows to map them to physical channels. The only thing they send you in the clear are the locals and home shopping, but they don't tell anyone if they move them around. So with this computer based tuner, if they change stuff, I need to have it scan everything, and then manually map the physical channels to the cable channel numbers so it matches up with the program guide it downloads. Fortunately, now that everything is available over the air, I won't need to rely on it unless there are three shows I want to record simultaneously in the fall.
EDIT: Here's how TV knows which channel to use...