What's this "rescan" for TV?

You may have seen references to an upcoming "rescan" for over-the-air television receivers. I'll explain it.

Radio frequencies for over-the-air or "broadcast" TV — the only kind of TV there was, before cable TV and later satellite TV were started in the 1970s —  are assigned in terms of channels. In the U.S., over-the-air TV channels initially went from 2-13 ("VHF") and 14-80 ("UHF"). Most of the radio spectrum from 54 to 890 MHz was tied up, essentially, for over-the-air TV. Of course there are many other users of radio spectrum in that same range: FM radio stations, aircraft traffic control, police and fire, industrial radio, and on and on. God isn't making any more radio spectrum; what we have is all that we'll ever have, so naturally there is contention for spectrum among the various users of it.

When the technology to provide cellular telephone service became feasible in the late 1970s, some amount of radio spectrum had to be made available for it. The solution was to take away channels 70-83 from over-the-air TV and reallocate those frequencies to the cellular system operators. A small number of broadcast TV stations using channels 70-83 had to relocate to other channels. This explains why TV sets quit having channels 70-83 on their dials after 1983.

Cellular turned out to be a bigger success than most people anticipated, and it didn't take them long to use up all the frequencies that had been TV channels 70-83. The solution was to give the over-the-air TV stations another haircut. Channels 52-69 were taken and reallocated to the cellular system operators. This time, those operators (AT&T, Verizon, Sprint, etc) had to pay billions of dollars for the spectrum they obtained. Some of this money was used to offset the costs that broadcast TV stations in channels 52-69 incurred when they had to move below channel 52.

The next big change was digital TV. To prevent confusion, the TV industry developed the notion of a "virtual" channel number. In Raleigh, for example, the dominant broadcast TV station has been WRAL, using channel 5 since it went on the air in the 1950s. Because there was going to be a transition period when both the new digital transmitters and the old analog transmitters were on the air, the digital transmitters were assigned to different "physical" channel numbers so that the digital and analog signals did not interfere. But the viewing public would still use virtual channel 5 to receive WRAL on digital TV; there was intelligence in the TV receiver that mapped virtual channel 5 to whatever physical channel the digital transmitter for WRAL happened to have been assigned…in this example, physical channel 48. When you turned your digital TV to WRAL, you might have thought you were still watching channel 5, and in a sense you were; but the signal was actually being sent to you on channel 48.

Time passed, and we began to have more cellular telephones in operation than people. (Some people have more than one cellular phone.) Once again, the cellular system operators began to run out of spectrum — complicated by the fact that data had displaced voice as the major use of cellular phones. It's time again to reallocate spectrum from broadcast TV stations to the cellular system operators, who again have spent billions of dollars for their new assignments.

In September, all broadcast TV stations must use physical channel numbers 36 and below. Channel 37 has always been reserved for radio astronomy, and channels 38-51 will be reused for cellular. This time, nearly all TV stations are moving their physical channels in what the industry calls a "repack".

With one exception, you the viewer can be oblivious to all this because you use only the virtual channel numbers that you're familiar with. In Raleigh, WRAL will still be virtual channel 5. However, in reality WRAL is moving from physical channel 48 to physical channel 17. Your TV set doesn't know that this is happening unless you press a RESCAN button (or something similar) on your TV set so that it remaps all the virtual channels to physical channels. That's why all the broadcast TV stations are telling you about rescan day.

For Raleigh, this is how the repack plays out:

• WRAL, virtual 5, current physical 48, new physical 17
• WRAZ, virtual 50, current physical 49, new physical 15
• WLFL, virtual 22, current physical 27, new physical 18
• WUNC main transmitter in Chapel Hill, virtual 4, current physical 25, new physical 20
• WUNC aux transmitter in Garner (the WRAL antenna farm), virtual 4, current physical 30, new physical 19
• WTVD, virtual 11, current physical 11, new physical 9
• WNCN, virtual 17, current physical 17, new physical 8
• WRDC, virtual 28, current physical 28, new physical 14

No one in the Triangle area should have to get a new antennas for over-the-air TV. WTVD had remained on VHF when it went digital, so everyone in the Triangle should already be using an antenna that does both VHF and UHF. However, if you live in another area of the country, it's possible that you had all UHF before the repack and you will have a mix of VHF and UHF after the repack. If that's the case and your antenna was UHF-only, you may have to get a new antenna that does both VHF and UHF.

Seem complicated? Well, this is the 35,000-foot view. The details are a lot more complicated than I've written here, but I hope I've given you the basic information to understand what's happening.

If you get TV from cable, fiber, or satellite, none of this matters to you. Channel numbers on cable, fiber, and satellite are a different world.