Television



The term television refers to any system for transmitting visual images at a distance. Research on such systems dates back to the 1880s, when German scientist Paul Nipkow (1860–1940) invented a device known as the Nipkow disk. This device was made of a metal or cardboard disk perforated with a series of square holes in a spiral pattern. As the disk was spun, a light was shined through the holes and onto a target. By looking through the holes, one could see the target revealed as a series of horizontal lines.

Nipkow's invention had no practical applications, but it established a model on which later television systems were based. The modern television system was invented in the 1920s at about the same time by two inventors working independently: American Philo Farnsworth (1906–1971) and Russian-born American Vladimir Zworykin (1889–1982). Of the two, Zworykin experienced the greater success in patenting and marketing his ideas.

How television works

A television system consists primarily of two parts: picture transmission and picture reception. A television camera used to photograph a television program is similar in some ways to a still camera. Light bounces off the subject being photographed and enters the lens at the front of the television camera. The lens forms a clear image of the subject being photographed on a screen, which is located behind the lens.

Transmission. The surface of the screen contains millions of tiny particles of selenium or some other photosensitive (sensitive to light) material. These particles act like tiny photocells. That is, when struck by light, they emit a small electrical pulse. An electron gun at the back of the television camera scans back and forth, up and down across the screen at the front of the camera. As it scans, it detects electrical pulses being given off by various parts of the screen. A bright region in the scene being photographed will give off a lot of light. That light will be converted by the selenium into a relatively large electrical pulse. The electron gun will detect that electrical pulse as being greater than other pulses around it.

Words to Know

Coaxial cable: A cable made of a conducting outer metal tube insulated from an inner conducting core that is used to carry telegraph, telephone, and television signals.

Electron gun: A device that gives off a stream of electrons.

Persistence: In terms of human vision, the tendency of an image to remain on the retina of the eye for a fraction of a second after the image has disappeared.

Photocell: A device that emits an electrical pulse when struck by light.

Photosensitive: Affected by light.

The electrical pulses detected by the electron gun are then amplified and sent to the broadcasting tower. In the broadcasting tower, the electrical current from the television camera is converted into radio waves and sent out through the air. The process is similar to the way in which a radio program is transmitted except the frequency is different.

Television reception. At the receiving station, the above process is repeated in reverse order. Radio signals are received, amplified, and then fed into an electron gun in a television picture tube. The electron gun is pointed at the back of a picture tube. It travels back and forth across the picture tube tracing 525 lines on the tube 30 times every second. The back of the tube is covered with a photosensitive material that gives off light whenever it is struck by an electrical pulse. An intense beam from the electron gun (corresponding to an intense beam originally seen by the television camera) produces a strong burst of light. A weaker beam from the electron gun produces a weaker burst of light.

What the electron gun in the picture tube is producing, then, is a series of individual dots, one at a time, spread out across the screen at a very rapid pace. This mass of dots appears as a coherent picture to the human eye because of a phenomenon known as persistence. The term persistence refers to the fact that a visual image projected onto the retina of the human eye tends to remain there for a fraction of a second. Thus, what our eye sees as the electron gun scans the picture tube is a collection of millions of individual spots of light that, taken together, makes up a complete picture. That picture is identical to the one photographed originally by the television camera.

Color television

Color television did not become commercially available until the late 1950s, about 30 years after black-and-white television had been invented. The principles of color television are largely the same as those of black-and-white television. The most important difference is that three different electron guns are required for both color television cameras and color television picture tubes. The three different guns detect and project one color each: red, green, and blue. As the three guns in a television camera scan a scene simultaneously, they detect all possible combinations of the three basic colors that produce all the hues in that scene. When the three guns in a television picture tube project the electrical counterpart of that scene, they produce the same combination of hues in the original scene.

Cable and satellite television

First known as CATV (community antenna television) or simple cable, cable television was developed to deliver a clear signal to rural communities. At the time, a CATV system generally consisted of a single large antenna mounted in a high, clear area to receive signals from distant television broadcasters. Cables were fed to the houses in the community and they usually delivered two or three channels. In the mid-1960s, new technology allowed for up to twelve channels to be carried through a single cable. In order to fill these new channels, cable operators began to bring in television signals from more distant sources. This allowed viewers to watch stations from large cities and neighboring states. With access to a wider variety of stations, the demand for cable increased.

In the early 1970s, several small companies in California and on the East Coast began offering pay-per-view broadcasting: first-run films and major sporting events delivered by cable to a viewer's home for a monthly fee. The popularity of these programs caused demand to skyrocket. By 1975, the first nationwide pay-per-view cable station—Home Box Office (HBO)—was in service.

What makes cable transmission practical is its use of coaxial cable. This thick, layered cable allows transmission of a wide band of frequencies and rejects interference from automobiles and electrical appliances. As coaxial technology improved, the number of stations available to cable operators rose from twelve to more than fifty. Now, that number can be increased to almost 150.

The antennas once used to deliver a signal to a cable system are long since gone, replaced by microwave dishes often fed by communications satellites. Once a signal is delivered to a cable company in this manner, it is distributed over cable lines to customers. Broadcasts are often scrambled to prevent nonsubscribers from splicing into a cable line without paying for the service. Cable television's clear image is unaffected by poor weather conditions and most types of interference.

Beginning in the late 1970s, satellite television systems were introduced. The television signals transmitted by a satellite are quite different from the television or radio signals that are broadcast over the air. Satellite television is transmitted by microwaves. Microwaves do not behave like lower frequency radio waves that can bounce off obstructions, clouds, and the ground. Microwaves are strictly line-of-sight. In order for a satellite dish to receive a signal, there can be no obstruction between the transmitting satellite and the receiving satellite dish. Because microwaves are highly directional, the satellite dish and associated components must be properly aligned.

Currently in the United States, there are two major types of satellite television. The first is TVRO (Tele Vision Receive Only). TVRO satellite systems have a large dish—6 to 12 feet (1.8 to 3.6 meters) across—that is movable. The movable dish enables a TVRO system to view programs on the many satellites that are positioned in orbit above Earth. The second type of satellite television is DBS (Direct Broadcast Satellite). DBS is broadcast by high-powered, high-frequency satellites, which make it possible for the signals to be picked up on a small dish ranging from 18 to 36 inches (46 to 91 centimeters) across. One of the big advantages of DBS systems is that the small dish does not have to move.

HDTV

Digital television refers to the transmission of pure digital television signals, along with the reception and display of those signals on a digital television set. The digital signals might be broadcast over the air or transmitted through a cable or satellite system. A decoder receives the signal and uses it, in digital form, to directly drive a digital television. A class of digital television is called high-definition television or HDTV. HDTV is high-resolution digital television (DTV) combined with dolby digital surround sound. HDTV is the highest DTV resolution in the new set of standards. Whereas traditional televisions have 525 lines of resolution, HDTV has 720 or 1080 lines of resolution.

HDTV requires new production and transmission equipment at the HDTV stations as well as new equipment for reception by the consumer. Optical fibers have proven to be an ideal method of transmitting HDTV signals. Because its transmission contains twice as much information as those of conventional television, HDTV features much greater clarity and definition in its picture. However, standard television technology cannot transmit so much information at once. Using optical fibers, the HDTV signal can be transmitted as a digital-light pulse, providing a near-flawless image. HDTV reproduction is far superior to broadcast transmission, just as music from a digital compact disc is superior to that broadcast over FM radio.



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ibrahim isaac
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May 7, 2008 @ 11:11 am
I like the way this website is set, but i want to know the use of micro-waves or the way it works in television signal transmission.

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