Television

B

Cable Transmission

Cable television was first developed in the late 1940s to serve shadow areas—that is, areas that are blocked from receiving signals from a station's transmitting antenna. In these areas, a community antenna receives the signal, and the signal is then redistributed to the shadow areas by coaxial cable (a large cable with a wire core that can transmit the wide band of frequencies required for television) or, more recently, by fiber-optic cable. Viewers in most areas can now subscribe to a cable television service, which provides a wide variety of television programs and films adapted for television that are transmitted by cable directly to the viewer's television set. Digital data-compression techniques, which convert television signals to digital code in an efficient way, have increased cable's capacity to 500 or more channels.

C

Microwave Relay Transmission

Microwave relay stations are tall towers that receive television signals, amplify them, and retransmit them as a microwave signal to the next relay station. Microwaves are electromagnetic waves that are much shorter than normal television carrier waves and can travel farther. The stations are placed about 50 km (30 mi) apart. Television networks once relied on relay stations to broadcast to affiliate stations located in cities far from the original source of the broadcast. The affiliate stations received the microwave transmission and rebroadcast it as a normal television signal to the local area. This system has now been replaced almost entirely by satellite transmission in which networks send or uplink their program signals to a satellite that in turn downlinks the signals to affiliate stations.

D

Satellite Transmission

Communications satellites receive television signals from a ground station, amplify them, and relay them back to the earth over an antenna that covers a specified terrestrial area. The satellites circle the earth in a geosynchronous orbit, which means they stay above the same place on the earth at all times. Instead of a normal aerial antenna, receiving dishes are used to receive the signal and deliver it to the television set or station. The dishes can be fairly small for home use, or large and powerful, such as those used by cable and network television stations.

Satellite transmissions are used to efficiently distribute television and radio programs from one geographic location to another by networks; cable companies; individual broadcasters; program providers; and industrial, educational, and other organizations. Programs intended for specific subscribers are scrambled so that only the intended recipients, with appropriate decoders, can receive the program.

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Direct-broadcast satellites (DBS) are used worldwide to deliver TV programming directly to TV receivers through small home dishes. The Federal Communications Commission (FCC) licensed several firms in the 1980s to begin DBS service in the United States. The actual launch of DBS satellites, however, was delayed due to the economic factors involved in developing a digital video compression system. The arrival in the early 1990s of digital compression made it possible for a single DBS satellite to carry more than 200 TV channels. DBS systems in North America are operating in the K_u band (12.0-19.0 GHz). DBS home systems consist of the receiving dish antenna and a low-noise amplifier that boosts the antenna signal level and feeds it to a coaxial cable. A receiving box converts the superhigh frequency (SHF) signals to lower frequencies and puts them on channels that the home TV set can display.

VI

Television Receiver

The television receiver translates the pulses of electric current from the antenna or cable back into images and sound. A traditional television set integrates the receiver, audio system, and picture tube into one device. However, some cable TV systems use a separate component such as a set-top box as a receiver. A high-definition television (HDTV) set integrates the receiver directly into the set like a traditional TV. However, some televisions receive high-definition signals and display them on a monitor. In these instances, an external receiver is required.

A

Tuner

The analog tuner blocks all signals other than that of the desired channel. Blocking is done by the radio frequency (RF) amplifier. The RF amplifier is set to amplify a frequency band, 6 MHz wide, transmitted by a television station; all other frequencies are blocked. A channel selector connected to the amplifier determines the particular frequency band that is amplified. When a new channel is selected, the amplifier is reset accordingly. In this way, the band, or channel, picked out by the home receiver is changed. Once the viewer selects a channel, the incoming signal is amplified, and the video, audio, and scanning signals are separated from the higher-frequency carrier waves by a process called demodulation. The tuner amplifies the weak signal intercepted by the antenna and partially demodulates (decodes) it by converting the carrier frequency to a lower frequency—the intermediate frequency. Intermediate-frequency amplifiers further increase the strength of the signals received from the antenna. After the incoming signals have been amplified, audio, scanning, and video signals are separated.

Over-the-air digital television requires a special tuner to receive and decode the digital broadcast signals. These digital tuners must be compliant with standards set by an international body called the Advanced Television Systems Committee (ATSC). When all television broadcasting becomes digital, viewers who watch over-the-air broadcasts on analog television sets that do not have ATSC equipment will need a special converter box to turn the digital signal into an analog signal. Customers of cable or satellite television may also require new equipment to view digital television, though some customers may not be affected. Special additional equipment is needed to decode high-definition digital television sent through cable or satellite, or broadcast over the air.