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Amateur television
Amateur television (ATV) is the transmission of broadcast quality video and audio over the wide range of frequencies of radio waves allocated for radio amateur (Ham) use. ATV is used for non-commercial experimentation, pleasure and public service events. Ham TV stations were on the air in many cities before commercial television stations came on the air.〔(Kowalewski, Anthony, "An Amateur's Television Transmitter" ), ''Radio News'', April 1938. Early Television Museum and Foundation Website. Retrieved 19 July 2009.〕 Various transmission standards are used, these include the broadcast transmission standards of NTSC in North America and Japan, and PAL or SECAM elsewhere, utilizing the full refresh rates of those standards. ATV includes the study of building of such transmitters and receivers, and the study of radio propagation of signals travelling between transmitting and receiving stations.
ATV is an extension of amateur radio. It is also called HAM TV or fast-scan TV (FSTV), as opposed to slow-scan television (SSTV). SSTV is a method of transmitting still images over radio, when it is not possible to send video.
== 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 (IRC) (420–450 MHz). As such, ATV transmissions can be viewed by setting a television or analog cable-box to cable input and attaching an outdoor 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). DSB AM and VSB AM signals are inherently compatible with each other, and most televisions can receive either. DSB-AM signals consists of the carrier and both upper and lower sidebands. VSB-AM is where DSB-AM is filtered and the lower sideband is highly attenuated at frequencies more than 1.25 MHz from the carrier signal. A VSB filter can be added to a DSB=AM transmitter to make it a VSB signal. The filters, depending on power usage, will cost anywhere from $100 to $1,000. For practical reasons, most individual ATV users transmit in DSB-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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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