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Television, system of sending and receiving pictures and sound by means of electronic signals transmitted through wires and optical fibers or by Electromagnetic radiation. These signals are usually broadcast from a central source, a television station, to reception devices such as television sets in homes or relay stations such as those used by cable television service providers. Television is the most widespread form of communication in the world. Though most people will never meet the leader of a country, travel to the moon, or participate in a war, they can observe these experiences through the images on their television.
Television has a variety of applications in society, business, and science. The most common use of television is as a source of information and entertainment for viewers in their homes. Security personnel also use televisions to monitor buildings, manufacturing plants, and numerous public facilities. Public utility employees use television to monitor the condition of an underground sewer line, using a camera attached to a robot arm or remote-control vehicle. Doctors can probe the interior of a human body with a microscopic television camera without having to conduct major surgery on the patient. Educators use television to reach students throughout the world.
People in the United States have the most television sets per person of any country, with 814 sets per 1000 people in 1994. More than 98 percent of households in the United States have television sets. Canada, Japan, Germany, Denmark, and Finland follow the United States in the number of sets per person.
How Television Works
A television program is created by focusing a television camera on a scene. The camera changes light from the scene into an electric signal, called the video signal, which varies depending on the strength, or brightness, of light received from each part of the scene. In color television, the camera produces an electric signal that varies depending on the strength of each color of light.
Three or four cameras are typically used to produce a television program (see Television Production). The video signals from the cameras are processed in a control room, then combined with video signals from other cameras and sources, such as videotape recorders, to provide the variety of images and special effects seen during a television program.
Audio signals from microphones placed in or near the scene also flow to the control room, where they are amplified and combined. Except in the case of live broadcasts (such as news and sports programs) the video and audio signals are recorded on tape and edited, assembled with the use of computers into the final program, and broadcast later. In a typical television station, the signals from live and recorded features, including commercials, are put together in a master control room to provide the station’s continuous broadcast schedule. Throughout the broadcast day, computers start and stop videotape machines and other program sources, and switch the various audio and visual signals. The signals are then sent to the transmitter.
The transmitter amplifies the video and audio signals, and uses the electronic signals to modulate, or vary, carrier waves (oscillating electric currents that carry information). The carrier waves are combined (diplexed), then sent to the transmitting antenna, usually placed on the tallest available structure in a given broadcast area. In the antenna, the oscillations of the carrier waves generate electromagnetic waves of energy that radiate horizontally throughout the atmosphere. The waves excite weak electric currents in all television-receiving antennas within range. These currents have the characteristics of the original picture and sound currents. The currents flow from the antenna attached to the television into the television receiver, where they are electronically separated into audio and video signals. These signals are amplified and sent to the picture tube and the speakers, where they produce the picture and sound portions of the program.
The Television Camera
The television camera is the first tool used to produce a television program. Most cameras have three basic elements: an optical system for capturing an image, a pickup device for translating the image into electronic signals, and an encoder for encoding signals so they may be transmitted.
The optical system of a television camera includes a fixed lens that is used to focus the scene onto the front of the pickup device. Color cameras also have a system of prisms and mirrors that separate incoming light from a scene into the three primary colors: red, green, and blue. Each beam of light is then directed to its own pickup device. Almost any color can be reproduced by combining these colors in the appropriate proportions. Most inexpensive consumer video cameras use a filter that breaks light from an image into the three primary colors.
The pickup device takes light from a scene and translates it into electronic signals. The first pickup devices used in cameras were camera tubes. The first camera tube used in television was the iconoscope. Invented in the 1920s, it needed a great deal of light to produce a signal, so it was impractical to use in a low-light setting, such as an outdoor evening scene. The image-orthicon tube and the vidicon tube were invented in the 1940s and were a vast improvement on the iconoscope. They needed only about as much light to record a scene as human eyes need to see. Instead of camera tubes, most modern cameras now use light-sensitive integrated circuits (tiny, electronic devices) called charge-coupled devices (CCDs).
When recording television images, the pickup device replaces the function of film used in making movies. In a camera tube pickup device, the front of the tube contains a layer of photosensitive material called a target. In the image-orthicon tube, the target material is photoemissive-that is, it emits electrons when it is struck by light. In the vidicon camera tube, the target material is photoconductive-that is, it conducts electricity when it is struck by light. In both cases, the lens of a camera focuses light from a scene onto the front of the camera tube, and this light causes changes in the target material. The light image is transformed into an electronic image, which can then be read from the back of the target by a beam of electrons (tiny, negatively charged particles).
The beam of electrons is produced by an electron gun at the back of the camera tube. The beam is controlled by a system of electromagnets that make the beam systematically scan the target material. Whenever the electron beam hits the bright parts of the electronic image on the target material, the tube emits a high voltage, and when the beam hits a dark part of the image, the tube emits a low voltage. This varying voltage is the electronic television signal.
A charge-coupled device (CCD) can be much smaller than a camera tube and is much more durable. As a result, cameras with CCDs are more compact and portable than those using a camera tube. The image they create is less vulnerable to distortion and is therefore clearer. In a CCD, the light from a scene strikes an array of photodiodes arranged on a silicon chip. Photodiodes are devices that conduct electricity when they are struck by light; they send this electricity to tiny capacitors. The capacitors store the electrical charge, with the amount of charge stored depending on the strength of the light that struck the photodiode. The CCD converts the incoming light from the scene into an electrical signal by releasing the charges from the photodiodes in an order that follows the scanning pattern that the receiver will follow in re-creating the image.
In color television, the signals from the three camera tubes or charge-coupled devices are first amplified, then sent to the encoder before leaving the camera. The encoder combines the three signals into a single electronic signal that contains the brightness information of the colors (luminance). It then adds another signal that contains the code used to combine the colors (color burst), and the synchronization information used to direct the television receiver to follow the same scanning pattern as the camera. The color television receiver uses the color burst part of the signal to separate the three colors again.
Television cameras and television receivers use a procedure called scanning to record visual images and re-create them on a television screen. The television camera records an image, such as a scene in a television show, by breaking it up into a series of lines and scanning over each line with the beam or beams of electrons contained in the camera tube. The pattern is created in a CCD camera by the array of photodiodes. One scan of an image produces one static picture, like a single frame in a film. The camera must scan a scene many times per second to record a continuous image. In the television receiver, another electron beam-or set of electron beams, in the case of color television-uses the signals recorded by the camera to reproduce the original image on the receiver’s screen. Just like the beam or beams in the camera, the electron beam in the receiver must scan the screen many times per second to reproduce a continuous image.
In order for television to work, television images must be scanned and recorded in the same manner as television receivers reproduce them. In the United States, broadcasters and television manufacturers have agreed on a standard of breaking images down into 525 horizontal lines, and scanning images 30 times per second. In Europe, most of Asia, and Australia, images are broken down into 625 lines, and they are scanned 25 times per second. Special equipment can be used to make television images that have been recorded in one standard fit a television system that uses a different standard. Telecine equipment (from the words television and cinema) is used to convert film and slide images to television signals. The images from film projectors or slides are directed by a system of mirrors toward the telecine camera, which records the images as video signals.
The scanning method that is most commonly used today is called interlaced scanning. It produces a clear picture that does not fade or flicker. When an image is scanned line by line from top to bottom, the top of the image on the screen will begin to fade by the time the electron beam reaches the bottom of the screen. With interlaced scanning, odd-numbered lines are scanned first, and the remaining even-numbered lines are scanned next. A full image is still produced 30 times a second, but the electron beam travels from the top of the screen to the bottom of the screen twice for every time a full image is produced.
Transmission of Television Signals
The audio and video signals of a television program are broadcast through the air by a transmitter. The transmitter superimposes the information in the camera’s electronic signals onto carrier waves. The transmitter amplifies the carrier waves, making them much stronger, and sends them to a transmitting antenna. This transmitting antenna radiates the carrier waves in all directions, and the waves travel through the air to antennas connected to television sets or relay stations.
The transmitter superimposes the information from the electronic television signal onto carrier waves by modulating (varying) either the wave’s amplitude, which corresponds to the wave’s strength, or the wave’s frequency, which corresponds to the number of times the wave oscillates each second (see Radio: Modulation). The amplitude of one carrier wave is modulated to carry the video signal (amplitude modulation, or AM) and the frequency of another wave is modulated to carry the audio signal (frequency modulation, or FM). These waves are combined to produce a carrier wave that contains both the video and audio information. The transmitter first generates and modulates the wave at a low power of several watts. After modulation, the transmitter amplifies the carrier signal to the desired power level, sometimes many kilowatts (1000 watts), depending on how far the signal needs to travel, and then sends the carrier wave to the transmitting antenna.
The frequency of carrier waves is measured in hertz (Hz), which is equal to the number of wave peaks that pass by a point every second. The frequency of the modulated carrier wave varies, covering a range, or band, of about 4 million hertz, or 4 megahertz (4 MHz). This band is much wider than the band needed for radio broadcasting, which is about 10,000 Hz, or 10 kilohertz (10 kHz). Television stations that broadcast in the same area send out carrier waves on different bands of frequencies, each called a channel, so that the signals from different stations do not mix. To accommodate all the channels, which are spaced at least 6 MHz apart, television carrier frequencies are very high. Six MHz does not represent a significant chunk of bandwidth if the television stations broadcast between 50 and 800 MHz.
In the United States and Canada, there are two ranges of frequency bands that cover 67 different channels. The first range is called very high frequency (VHF), and it includes frequencies from 54 to 72 MHz, from 76 to 88 MHz, and from 174 to 216 MHz. These frequencies correspond to channels 2 through 13 on a television set. The second range, ultrahigh frequency (UHF), includes frequencies from 407 MHz to 806 MHz, and it corresponds to channels 14 through 69 (see Radio and Television Broadcasting).
The high-frequency waves radiated by transmitting antennas can travel only in a straight line, and may be blocked by obstacles in between the transmitting and receiving antennas. For this reason, transmitting antennas must be placed on tall buildings or towers. In practice, these transmitters have a range of about 120 km (75 mi). In addition to being blocked, some television signals may reflect off buildings or hills and reach a receiving antenna a little later than the signals that travel directly to the antenna. The result is a ghost, or second image, that appears on the television screen. Television signals may, however, be sent clearly from almost any point on earth to any other-and from spacecraft to earth-by means of cables, microwave relay stations, and communications satellites.
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, will eventually increase cable’s capacity to 500 or more channels.
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. Network television stations use relay stations to broadcast to affiliate stations that are located in cities far from the original source of the broadcast. The affiliate stations receive the microwave transmission and rebroadcast it as a normal television signal to the local area.
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.
Direct-broadcast satellites are used in Europe and Japan to deliver TV programming directly to TV receivers through small home dishes. The Federal Communications Commission (FCC) has licensed several firms to begin DBS service in the United States; in the early 1990s actual launch of DBS satellites was delayed due to the economic factors involved in developing a digital video compression system. The arrival of digital compression, however, made it possible for a single DBS satellite to carry up to 200 TV channels. DBS systems in North America are operating in the Ku 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.
The television receiver translates the pulses of electric current from the antenna or cable back into images and sound. It consists of a tuner, an audio system, and a picture tube.
The 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. The particular frequency band that is amplified is determined by a channel selector connected to the amplifier. 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.
The audio system consists of a discriminator, which translates the audio portion of the carrier wave back into an electronic audio signal; an amplifier; and a speaker. The amplifier strengthens the audio signal from the discriminator and sends it to the speaker, which converts the electrical waves into sound waves that travel through the air to the listener.
The television picture tube receives video signals from the tuner and translates the signals back into images. The images are created by an electron gun in the back of the picture tube, which shoots a beam of electrons toward the back of the television screen. A black-and-white picture tube contains just one electron gun, while a color picture tube contains three electron guns, one for each of the primary colors of light (red, green, and blue). Part of the video signal goes to a magnetic coil that directs the beam and makes it scan the screen in the same manner as the camera originally scanned the scene. The rest of the signal directs the strength of the electron beam as it strikes the screen. The screen is coated with phosphor, a substance that glows when it is struck by electrons (see Luminescence). The stronger the electron beam, the stronger the glow and the brighter that section of the scene appears.
In color television, a portion of the video signal is used to separate out the three color signals, which are then sent to their corresponding electron beams. The screen is coated by tiny phosphor strips or dots that are arranged in groups of three: one strip or dot that emits blue, one that emits green, and one that emits red. Before light from each beam hits the screen, it passes through a shadow mask located just behind the screen. The shadow mask is a layer of opaque material that is covered with slots or holes. It partially blocks the beam corresponding to one color and prevents it from hitting dots of another color. As a result, the electron beam directed by signals for the color blue can strike and light up only blue dots. The result is similar for the beams corresponding to red and green. Images in the three different colors are produced on the television screen. The eye automatically combines these images to produce a single image having the entire spectrum of colors formed by mixing the primary colors in various proportions.
The scientific principles on which television is based were discovered in the course of basic research. Only much later were these concepts applied to television as it is known today. The first practical television system began operating in the 1940s.
In 1873 the Scottish scientist James Clerk Maxwell predicted the existence of the electromagnetic waves that make it possible to transmit ordinary television broadcasts. Also in 1873 the English scientist Willoughby Smith and his assistant Joseph May noticed that the electrical conductivity of the element selenium changes when light falls on it. This property, known as photoconductivity, is used in the vidicon television camera tube. In 1888 the German physicist Wilhelm Hallwachs noticed that certain substances emit electrons when exposed to light. This effect, called photoemission, was applied to the image-orthicon television camera tube.
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