Television transmitter

(electronics) An electronic device that converts the audio and video signals of a television program into modulated radio-frequency energy that can be radiated from an antenna and received on a television receiver.

An electronic device that converts audio and video signals into modulated radio-frequency (rf) energy which can be radiated from an antenna and received by a television receiver. The term can also refer to the entire television transmitting plant, consisting of the transmitter proper, associated visual and aural input and monitoring equipment, transmission line, the antenna with its tower or other support structure, and the building in which the equipment is housed. In the United States, both analog NTSC (National Television Systems Committee) and digital 8-VSB transmitters are in service. The digital transmitters are used for what is termed high-definition television (HDTV).

An analog television transmitter can be thought of as two separate transmitters integrated into a common cabinet. Video information is transmitted via a visual transmitter, while audio information is transmitted via an aural transmitter. Because video and audio have different characteristics, the two transmitters differ in terms of bandwidth, modulation technique, and output power level. Nevertheless, a common transmitting antenna is generally used, and the two transmitters feed this antenna via an rf diplexer or combiner.

A digital transmitter accepts a single encoded digital bit stream that may contain video, audio, and data. In the United States, the digital terrestrial transmission standard is known as 8-VSB, which is an eight-level, vestigal sideband format. The FCC has mandated that all U.S. television stations convert to difital and terminate analog transmissions.

Television stations are licensed to operate on a particular channel, but since it takes a very wide bandwidth to transmit a television picture, these channels are allocated over a broad range of frequencies. Channels 2 through 6 are low-band very-high-frequency (VHF) channels, while channels 7 through 13 are high-band VHF channels. Channels 14 through 69 are ultrahigh-frequency (UHF) channels. Each channel is 6 MHz wide. Because of the wide range of frequencies, television transmitters are designed to work in only one of the foregoing groups, and employ specific circuits which are most efficient for the channels involved. See also Radio spectrum allocations.

The horizontal radiation pattern of most television transmitting antennas is circular, providing equal radiated signal strength to all points of the compass. Higher-gain antennas achieve greater power in the direction of the horizon by reducing the power radiated at vertical angles above and below the horizon. Since this could result in weaker signals at some receivers close to the transmitter, beam tilt and null fill are often used to lower the angle of maximum radiated power. Because television signals travel in a “line of sight,” transmitting antennas are usually placed as high as possible above ground with respect to the surrounding service area. This allows viewers to orient their receiving antennas in one direction for the best reception from all of the stations. See also Antenna (electromagnetism).

There are two broad classes of VHF analog visual television transmitter design philosophy. The classical approach modulates the visual carrier at a moderate power level, amplifies the carrier to rated output power by means of high-power linear amplifiers, and then filters this high-power carrier to obtain the required vestigial-sideband signal. The more contemporary approach, used by nearly all transmitter manufacturers, employs modulation at a very low power level of an intermediate-frequency (i-f) signal. The required vestigial-sideband filtering is imposed on this low-level signal, generally by means of a highly stable surface-acoustic-wave filter, whereupon the signal is upconverted to the carrier frequency and amplified by linear amplifiers to rated output power. See also Amplifier; Surface-acoustic-wave devices; Television.