television: Broadcast, Cable, and Satellite Television Transmission
Television programs may be transmitted either
live or from a recording. The principle means of recording television programs for future use for many years was videotape recording, although programs were first recorded (when recorded) by kinescope, a method that uses motion-picture film. Appropriate changes in the signal-carrying circuitry allow kinescopes to be played back from a developed negative as well as from a positive. Videotape recording is similar to conventional tape recording (see tape recorder ; videocassette recorder ) except that, because of the wide frequency range—4.2 megahertz (MHz)—occupied by a video signal, the effective speed at which the tape passes the head is kept very high. The sound is recorded along with the video signal on the same tape. Television programs may also be recorded on a computer drive that uses hard disks or solid-state flash memory and on optical disks such as DVDs in a variety of formats.
When a television program is broadcast, the varying electrical signals are then amplified and used to modulate a carrier wave (see modulation ); the modulated carrier is fed to an antenna, where it is converted to electromagnetic waves and broadcast over a large region. The waves are sensed by antennas connected to television receivers. The range of waves suitable for radio and television transmission is divided into channels, which are assigned to broadcast companies or services. In the United States the Federal Communications Commission (FCC) currently has assigned 12 television channels between 54 and 216 MHz in the very-high-frequency (VHF) range and 47 channels between 470 and 698 MHz in the ultra-high-frequency (UHF) range; 32 additional channels at the upper end of the UHF range (698–890 MHz) originally assigned to television broadcasting were reassigned to other uses between 1983 and 2009 (see radio frequency ). Since the transition to digital broadcasting was completed in 2009, the UHF range has increased in importance for television broadcasting even as the number of viewers receiving broadcast television programs has declined. Originally a television station's channel number was identical to the channel number of the radio frequency channel it used for its broadcasts, but as a result of the digital transition that often is no longer true.
Most television viewers in the United States no longer receive signals by using antennas; instead, they receive programming via cable television . Cable delivery of television started as a way to improve reception. A single, well-placed community antenna received the broadcast signals and distributed them over coaxial cables to areas that otherwise would not be able to receive them. Today, cable television is popular because of the wide variety of programming it can deliver. Many systems now provide hundreds of channels of programming. A cable television company now typically receives signals relayed from a communications satellite and sends those signals to its subscribers over coaxial or fiber-optic cable. Some television viewers use small satellite dishes to receive signals directly from satellites. Most satellite-delivered signals are scrambled and require a special decoder to receive them clearly.
See also broadcasting .
- Evolution of the Scanning Process
- Development of Color Television
- Broadcast, Cable, and Satellite Television Transmission
- Development of the Television Camera and Receiver
- Television Technology Innovations
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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