Thursday, January 12, 2012

Television


Television

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 835 sets per 1,000 people as of 2000. Canadians possessed 710 sets per 1,000 people during the same year. Japan, Germany, Denmark, and Finland follow North America in the number of sets per person.
II
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.
In digital television broadcasting, the video and audio signals are digitally compressed as sets of numbers. These numbers are carried by the broadcast signal but must be decoded by a digital receiver to be translated back into video and audio signals. Digital information takes up less bandwidth than an analog signal and greatly reduces interference and other problems. Picture and sound quality can be much clearer and more detailed than with analog signals. Multiple digital signals can be sent at the same time.
III
THE TELEVISION CAMERA
Early Television Camera
A television camera uses a sensitive electronic tube to change light into electrical impulses that can be transmitted to television receivers. Early television cameras used tubes called iconoscopes that required bright, controlled lighting conditions, but tubes developed later overcame this limitation. Pictured is a television camera from 1956.

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.
A
Optical System
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.
B
Pickup Device
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.
C
Encoder
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.
IV
SCANNING
Television Screen
This close-up photo of part of a television screen shows the green, red, and blue phosphor strips that make up a color image. Three electron beams, one for each color, strike the strips and make them glow. Different combinations of red, green, and blue create different colors; green and magenta are shown here.

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 for analog television is called interlaced scanning. It produces a clear picture that does not fade. 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.
Digital television uses progressive scan, the same as most computer monitors do. The image is produced 60 times a second, with odd and even fields scanned every 0.6 seconds. The higher scan rate can produce a better picture than analog.
V
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.
A
The Transmitter
Television Tower
Television antennas are built on tall towers so that high-frequency signals (which only travel in a straight line) can reach viewers without being blocked by nearby hills or buildings. Small dishes on this tower send and receive microwave signals from other stations or from reporters broadcasting live from nearby locations.

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 (1 kilowatt equals 1,000 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. However, channel 37 is used for radio astronomy and medical telemetry equipment, not for television broadcasting (see Radio and Television Broadcasting). When the transition to all-digital television broadcasting is complete, channels 52 through 69 will no longer be used for television signals. These frequencies may become available for other uses such as wireless communication.
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. Digital transmission, however, eliminates ghosts and snow since the picture that results is recreated from a digital code, not from analog waves. Television signals may also 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.
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.
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 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.
VI
TELEVISION RECEIVER
Television Sets from 1950s
Television pictures are formed by the transmission of a succession of tiny tonal elements on a screen, which appear as moving images to the human eye. The electronics giant Radio Corporation of America financed the development of early television, and by 1955, 67 percent of American households had television sets.

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.
B
Audio System
The analog 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. Digital audio decodes the digital signal and converts it into an electronic audio signal.
C
Picture Tube
Television Picture Tube
A color television picture tube contains three electron guns, one corresponding to each of the three primary colors of light—red, green, and blue. Electromagnets direct the beams of electrons emerging from these guns to continuously scan the screen. As the electrons strike red, green, and blue phosphor dots on the screen, they make the dots glow. A screen with holes in it, called a shadowmask, ensures that each electron beam only strikes phosphor dots of its corresponding color. The glow of all the dots together forms the television picture.

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.
VII
TELEVISION'S HISTORY
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.
Although several methods of changing light into electric current were discovered, it was some time before the methods were applied to the construction of a television system. The main problem was that the currents produced were weak and no effective method of amplifying them was known. Then, in 1906, the American engineer Lee De Forest patented the triode vacuum tube. By 1920 the tube had been improved to the point where it could be used to amplify electric currents for television.
A
Nipkow Disk
Some of the earliest work on television began in 1884, when the German engineer Paul Nipkow designed the first true television mechanism. In front of a brightly lit picture, he placed a scanning disk (called a Nipkow disk) with a spiral pattern of holes punched in it. As the disk revolved, the first hole would cross the picture at the top. The second hole passed across the picture a little lower down, the third hole lower still, and so on. In effect, he designed a disk with its own form of scanning. With each complete revolution of the disk, all parts of the picture would be briefly exposed in turn. The disk revolved quickly, accomplishing the scanning within one-fifteenth of a second. Similar disks rotated in the camera and receiver. Light passing through these disks created crude television images.
Nipkow's mechanical scanner was used from 1923 to 1925 in experimental television systems developed in the United States by the inventor Charles F. Jenkins, and in England by the inventor John L. Baird. The pictures were crude but recognizable. The receiver also used a Nipkow disk placed in front of a lamp whose brightness was controlled by the signal from the light-sensitive tube behind the disk in the transmitter. In 1926 Baird demonstrated a system that used a 30-hole Nipkow disk.
B
Electronic Television
Crookes Tube
Sir William Crookes constructed this forerunner of the modern television picture tube in the 1870s to investigate the properties of cathode rays. When the tube is evacuated and a high voltage applied, one end of the tube glows, caused by cathode rays (now known to be electrons) striking the glass. The modern television picture tube, also known as a CRT (Cathode Ray Tube) is a direct descendant of the Crookes tube. The major differences are that a CRT uses a heated cathode to increase the number of electrons, while the Crookes tube does not, and the CRT has extra electrodes to focus and deflect the beam as it travels toward the screen.

Simultaneous to the development of a mechanical scanning method, an electronic method of scanning was conceived in 1908 by the English inventor A. A. Campbell-Swinton. He proposed using a screen to collect a charge whose pattern would correspond to the scene, and an electron gun to neutralize this charge and create a varying electric current. This concept was used by the Russian-born American physicist Vladimir Kosma Zworykin in his iconoscope camera tube of the 1920s. A similar arrangement was later used in the image-orthicon tube.
The American inventor and engineer Philo Taylor Farnsworth also devised an electronic television system in the 1920s. He called his television camera, which converted each element of an image into an electrical signal, an image dissector. Farnsworth continued to improve his system in the 1930s, but his project lost its financial backing at the beginning of World War II (1939-1945). Many aspects of Farnsworth's image dissector were also used in Zworykin's more successful iconoscope camera.
Trinitron Cathode Ray Tube
Many televisions still use cathode ray tubes (CRTs) for receivers. Until the Sony Corporation patented the simplified Trinitron system in the late 1960s, RCA’s original and more complex color tube dominated the market. Today, flat-screen TVs, based on a different technology, are becoming increasingly popular.

Cathode rays, or beams of electrons in evacuated glass tubes, were first noted by the British chemist and physicist Sir William Crookes in 1878. By 1908 Campbell-Swinton and a Russian, Boris Rosing, had independently suggested that a cathode-ray tube (CRT) be used to reproduce the television picture on a phosphor-coated screen. The CRT was developed for use in television during the 1930s by the American electrical engineer Allen B. DuMont. DuMont's method of picture reproduction is essentially the same as the one used today.
The first home television receiver was demonstrated in Schenectady, New York, on January 13, 1928, by the American inventor Ernst F. W. Alexanderson. The images on the 76-mm (3-in) screen were poor and unsteady, but the set could be used in the home. A number of these receivers were built by the General Electric Company (GE) and distributed in Schenectady. On May 10, 1928, station WGY began regular broadcasting to this area.
C
Public Broadcasting
The first public broadcasting of television programs took place in London in 1936. Broadcasts from two competing firms were shown. Marconi-EMI produced a 405-line frame at 25 frames per second, and Baird Television produced a 240-line picture at 25 frames per second. In early 1937 the Marconi system, clearly superior, was chosen as the standard. In 1941 the United States adopted a 525-line, 30-image-per-second standard.
The first regular television broadcasts began in the United States in 1939, but after two years they were suspended until shortly after the end of World War II in 1945. A television broadcasting boom began just after the war in 1946, and the industry grew rapidly. The development of color television had always lagged a few steps behind that of black-and-white (monochrome) television. At first, this was because color television was technically more complex. Later, however, the growth of color television was delayed because it had to be compatible with monochrome—that is, color television would have to use the same channels as monochrome television and be receivable in black and white on monochrome sets.
D
Color Television
It was realized as early as 1904 that color television was possible using the three primary colors of light: red, green, and blue. In 1928 Baird demonstrated color television using a Nipkow disk in which three sets of openings scanned the scene. A fairly refined color television system was introduced in New York City in 1940 by the Hungarian-born American inventor Peter Goldmark. In 1951 public broadcasting of color television was begun using Goldmark's system. However, the system was incompatible with monochrome television, and the experiment was dropped at the end of the year. Compatible color television was perfected in 1953, and public broadcasting in color was revived a year later.
Other developments that improved the quality of television were larger screens and better technology for broadcasting and transmitting television signals. Early television screens were either 18 or 25 cm (7 or 10 in) diagonally across. Television screens now come in a range of sizes. Those that use built-in cathode-ray tubes (CRTs) measure as large as 89 or 100 cm (35 or 40 in) diagonally. Projection televisions (PTVs), first introduced in the 1970s, now come with screens as large as 2 m (7 ft) diagonally. The most common are rear-projection sets in which three CRTs beam their combined light indirectly to a screen via an assembly of lenses and mirrors. Another type of PTV is the front-projection set, which is set up like a motion picture projector to project light across a room to a separate screen that can be as large as a wall in a home allows. Newer types of PTVs use liquid-crystal display (LCD) technology or an array of micro mirrors, also known as a digital light processor (DLP), instead of cathode-ray tubes. Manufacturers have also developed very small, portable television sets with screens that are 7.6 cm (3 in) diagonally across.
E
Television in Space
Television evolved from an entertainment medium to a scientific medium during the exploration of outer space. Knowing that broadcast signals could be sent from transmitters in space, the National Aeronautics and Space Administration (NASA) began developing satellites with television cameras. Unmanned spacecraft of the Ranger and Surveyor series relayed thousands of close-up pictures of the moon's surface back to earth for scientific analysis and preparation for lunar landings. The successful U.S. manned landing on the moon in July 1969 was documented with live black-and-white broadcasts made from the surface of the moon. NASA's use of television helped in the development of photosensitive camera lenses and more-sophisticated transmitters that could send images from a quarter-million miles away.
Since 1960 television cameras have also been used extensively on orbiting weather satellites. Video cameras trained on Earth record pictures of cloud cover and weather patterns during the day, and infrared cameras (cameras that record light waves radiated at infrared wavelengths) detect surface temperatures. The ten Television Infrared Observation Satellites (TIROS) launched by NASA paved the way for the operational satellites of the Environmental Science Services Administration (ESSA), which in 1970 became a part of the National Oceanic and Atmospheric Administration (NOAA). The pictures returned from these satellites aid not only weather prediction but also understanding of global weather systems. High-resolution cameras mounted in Landsat satellites have been successfully used to provide surveys of crop, mineral, and marine resources.
F
Home Recording
In time, the process of watching images on a television screen made people interested in either producing their own images or watching programming at their leisure, rather than during standard broadcasting times. It became apparent that programming on videotape—which had been in use since the 1950s—could be adapted for use by the same people who were buying televisions. Affordable videocassette recorders (VCRs) were introduced in the 1970s and in the 1980s became almost as common as television sets.
During the late 1990s and early 2000s the digital video disc (DVD) player had the most successful product launch in consumer electronics history. According to the Consumer Electronics Association (CEA), which represents manufacturers and retailers of audio and video products, 30 million DVD players were sold in the United States in a record five-year period from 1997 to 2001. It took compact disc (CD) players 8 years and VCRs 13 years to achieve that 30-million milestone. The same size as a CD, a DVD can store enough data to hold a full-length motion picture with a resolution twice that of a videocassette. The DVD player also offered the digital surround-sound quality experienced in a state-of-the-art movie theater. Beginning in 2001 some DVD players also offered home recording capability.
G
Digital Television
High-Definition Television
Proponents of high-definition television claim that it offers viewers a sharper picture. An image of a butterfly, top, with a resolution like that of high-definition TV's digital transmission shows greater clarity and detail than the butterfly, bottom, with a resolution typical of ordinary TV's analog transmission.

Digital television uses technology that records, transmits, and decodes a signal in digital form—that is, as a series of ones and zeros. This process produces much clearer picture and sound quality than analog systems, similar to the difference between a compact disc recording (using digital technology) and an audiotape or long-playing record. It also permits additional features to be embedded in signals including program and consumer information as well as interactivities. Early digital equipment included digital television receivers that converted analog signals into digital code. The analog signal was first sampled and stored as a digital code, then processed, and finally retrieved. ATSC digital tuners designed to decode purely digital signals are now standard on new televisions.
There are three types of broadcast digital television (DTV), each with progressively better picture and sound quality: standard-definition TV (SDTV), enhanced-definition TV (EDTV), and high-definition TV (HDTV).
The high-definition television (HDTV) system was developed in the 1980s. It uses 1,080 lines and a wide-screen format, providing a significantly clearer picture than the traditional 525- and 625-line television screens. Each line in HDTV also contains more information than normal formats. HDTV is transmitted using digital technology. Because it takes a huge amount of coded information to represent a visual image—engineers believe HDTV will need about 30 million bits (ones and zeros of the digital code) each second—data-compression techniques have been developed to reduce the number of bits that need to be transmitted. With these techniques, digital systems need to continuously transmit codes only for a scene in which images are changing; the systems can compress the recurring codes for images that remain the same (such as the background) into a single code. Digital technology is being developed that will offer sharper pictures on wider screens, and HDTV with cinema-quality images.
A fully digital system was demonstrated in the United States in the 1990s. A common world standard for digital television, the MPEG-2, was agreed on in April 1993 at a meeting of engineers representing manufacturers and broadcasters from 18 countries. Because HDTV receivers initially cost much more than regular television sets, and broadcasts of HDTV and regular television are incompatible, the transition from one format to the next could take many years. The method endorsed by the U.S. Congress and the FCC to ease this transition is to give existing television networks a second band of frequencies on which to broadcast, allowing networks to broadcast in both formats at the same time. Engineers are also working on making HDTV compatible with computers and telecommunications equipment so that HDTV technology may be applied to other systems besides home television, such as medical devices, security systems, and computer-aided manufacturing (CAM).
The Congress of the United States has mandated that all over-the-air television broadcasting become digital, although the date for the end of all analog broadcasting has been changed a number of times. After the conversion date (now set as February 2009), viewers with analog televisions will need special converter boxes to watch over-the-air broadcasts.
H
Flat Panel Display
In addition to getting clearer, televisions are also getting thinner. Flat panel displays, some just a few centimeters thick, offer an alternative to bulky cathode ray tube televisions. Even the largest flat panel display televisions are thin enough to be hung on the wall like a painting. Many flat panel TVs use liquid-crystal display (LCD) screens that make use of a special substance that changes properties when a small electric current is applied to it. LCD technology has already been used extensively in laptop computers. LCD television screens are flat, use very little electricity, and work well for small, portable television sets. LCD has not been as successful, however, for larger television screens.
Flat panel TVs made from gas-plasma displays can be much larger. In gas-plasma displays, a small electric current stimulates an inert gas sandwiched between glass panels, including one coated with phosphors that emit light in various colors. While just 8 cm (3 in) thick, plasma screens can be more than 150 cm (60 in) diagonally.
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Computer and Internet Integration
As online computer systems become more popular, televisions and computers are increasingly integrated. Such technologies combine the capabilities of personal computers, television, DVD players, and in some cases telephones, and greatly expand the kinds of services that can be provided. For example, computer-like hard drives in set-top recorders automatically store a TV program as it is being received so that the consumer can pause live TV, replay a scene, or skip ahead. For programs that consumers want to record for future viewing, a hard drive makes it possible to store a number of shows. Some set-top devices offer Internet access through a dial-up modem or broadband connection. Others allow the consumer to browse the World Wide Web on their TV screen. When a device has both a hard drive and a broadband connection, consumers may be able to download a specific program, opening the way for true video on demand.
Personal computers have also taken on television-like functions. Webcasting includes the broadcasting of video content over the World Wide Web. Television programs and other types of video media can be viewed from Web sites. Streaming allows a live video signal to be played as it is sent over the Internet in small packets of data. Archived programs can be viewed on-demand or downloaded to a computer. Small, handheld portable media devices with video capability can also play television programs or other video as downloaded podcasts. Some devices can also receive television broadcasts and wireless Internet.
Consumers may eventually need only one main system or device, known as an information appliance, which they could use for entertainment, communication, shopping, and banking in the convenience of their home.



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