North American Context
In North America, amateur radio bands that are suitable for a television signal (wide enough to fit such a signal) are higher in frequency than VHF broadcast TV. The lowest frequency ham band suitable for television transmission is 70 centimeters, which is between broadcast channels 13 and 14. While outside of broadcast television channels, this frequency falls into CATV frequencies, on channels 57 to 61 (420-450 MHz). As such, ATV transmissions can be viewed by setting a television to cable input and attaching a terrestrial antenna. For more sensitive reception, some users may use a purposely-built ATV down-converter, which is a kind of set-top-box. Other bands are also used for ATV, most of them in the UHF region on frequencies higher than UHF broadcast TV. 33 centimeters and 23 centimeters are two other commonly used bands for ATV, but reception of these higher bands requires the use of a down-converter.
Most ATV signals are transmitted in either Amplitude modulation (AM) or vestigial sideband (VSB) NTSC (North American analog TV broadcast modulation standard). AM and VSB signals are inherently compatible with each other, and most televisions can receive either. AM signals are wider than VSB signals, but VSB transmitters are more difficult and expensive to construct. For practical reasons, most individual ATV users transmit in AM, and VSB is transmitted by repeater stations. On the 33 cm and higher bands, Frequency modulation (FM) ATV may be used, and on the SHF and EHF ham bands, FM is more commonly used than VSB or AM. FM ATV is incompatible with AM/VSB ATV, and a separate demodulator is necessary to receive signals.
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