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.