Amateur television is the transmission of broadcast quality video and audio over the wide range of frequencies of radio waves allocated for radio amateur use. ATV is used for experimentation, pleasure and public service events. Ham TV stations were on the air in cities before commercial television stations came on the air. ATV includes the study of building of such transmitters and receivers, ATV is an extension of amateur radio. It is also called HAM TV or fast-scan TV, in North America, amateur radio bands that are suitable for a television signal are higher in frequency than VHF broadcast TV. The lowest frequency ham band suitable for transmission is 70 centimeters. While outside of broadcast television channels, this falls into CATV frequencies. As such, ATV transmissions can be viewed by setting a television or analog cable-box to cable input, 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 used for ATV, most of them in the UHF region on frequencies higher than UHF broadcast TV. 33 centimeters and 23 centimeters are two 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 or vestigial sideband NTSC, 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 US$100–1,000. For practical reasons, most individual ATV users transmit in DSB-AM, on the 33 cm and higher bands, frequency modulation 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 demodulator is necessary to receive signals. The 2-meter band, while too narrow in bandwidth for a signal, is often used by ATV operators for coordination with each other via FM voice transmissions. Operators seeking an ATV contact might first attempt calling on a regionally recognized ATV liaison-frequency, commonly 144.34 MHz, the 2 meter frequency may be used throughout the contact to talk back to the current station transmitting video. The receiving station may suggest adjustments the sending station can make, such as antenna direction, the 70-centimeter band is the most commonly used ham band for ATV
Signal circuit performance checks made when using a typical test card.
Typical fast scan test card showing "Hanover bars" (colour banding) effect in Pal S (simple) signal mode of transmission.
Test transmission signal for chrominance and luminance signals check using Pal D (delay line) encoding - colour "hanover Bars" effect no longer visible.