| Advances In Technology | ||||||||||||||||||||||||
| Television | ||||||||||||||||||||||||
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Television, the electrical transmission of moving images with accompanying sound, is designed to extend the senses of sight and hearing. The principles employed in television are based on human vision. | |||||||||||||||||||||||
| History & The Black and White Television | ||||||||||||||||||||||||
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The first proposals for television were made long before the electronic techniques of the present day were developed. In the 1880s the first proposal to transmit images by scanning was made by W. E. Sawyer, an American, and by Maurice LeBlanc of France. Black-and-White Television BLACK-AND-WHITE TELEVISION The principal elements of a typical black-and-white television camera are the lens, the camera tube, and the scanning and focusing coils. The lens (which is often of the zoom type, particularly in sports telecasting) focuses the scene on the front end of the camera tube. The tube that was most widely used in the late 1970s was the vidicon, which is an evacuated glass cylinder. At the front end of the tube is a flat glass plate, the inside of which is coated with a photosensitive material, a sulfur compound of antimony. (Another widely used tube, the plumbicon, is similar to the vidicon in operation but uses a compound of lead.) Underneath the antimony coating is a thin, transparent coating of metal. The electrical resistance of the antimony compound is lowered when light falls on it. The optical image from the lens falling on the antimony coating causes its resistance to change in proportion to the amount of light reaching it at each point on its surface; that is, a pattern of electrical resistance is formed that matches the pattern of light in the image. The metallic coating beneath the antimony coating is maintained at a positive voltage, causing each point on the antimony coating to assume a positive charge, with the amount of charge depending on the amount of light falling on that point. Thus a pattern of positive electric charge is built up, and the charge elements corresponding to the picture elements pass through the antimony to its rear surface, where they are stored. At the opposite end of the camera tube is a structure known as an electron gun. This forms a narrow ELECTRON BEAM that travels down the tube and encounters the charge pattern on the rear of the antimony coating. The focusing coils are arranged to keep the electron beam narrow (that is, sharply focused) so that the beam that strikes the coating has the size and shape of the picture element. | |||||||||||||||||||||||
| General | ||||||||||||||||||||||||
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Television must deal with an important difference between the way in which radio communications are transmitted through space and the usual way in which humans receive visual images. The retina of the eye receives an image of the whole scene before it, and hundreds of thousands of fibers in the optic nerve transmit to the brain, individually and simultaneously, signals that together represent the whole scene. Human vision thus uses hundreds of thousands of "channels" at once. | In television transmission by microwave or cable, by contrast, the entire content of the scene must be sent through a single channel . To accomplish this, the scene is broken down into many small pieces (called picture elements) that look like the half-tone dots used in printed pictures. In the television camera an electrical signal is formed to represent the brightness and in color television, also the color of each picture element. These signals are sent over the channel, one at a time, to the receiver. At the receiver the signals are transformed back into light, and the picture elements are assembled on the viewing screen in their proper relative positions. Persistence of Vision Essential to this process is visual memory: even though the picture elements are laid down on the screen one after the other, they all must be perceived at the same time. This requirement is met by persistence of vision, a property of the eye. When light entering the eye is shut off, the impression of light persists thereafter for about 0.1 second. If all the picture elements in the image are presented successively to the eye in a tenth of a second or less, the whole area of the screen appears to be illuminated, although in fact only one spot of light is present at any instant. In a television system a still picture is presented in less than a tenth of a second, so that a series of still pictures can be presented at a rate greater than ten pictures per second. Motion in the scene is represented, as in motion pictures by a series of still pictures, each differing slightly from those preceding and following it. Although ten still pictures per second is an adequate rate to convey the illusion of motion, for such motion to be depicted smoothly, a rate of at least 24 per second is necessary; this is the rate used in professional motion pictures.slightly different pictures are scanned each second, line by line. The picture tube produces an image that is composed of horizontal lines precisely like those used in the camera. As the camera examines the topmost line, a spot of light produced by the picture tube moves across the screen and produces the topmost line of light on the screen. The video signal causes the spot of light to become brighter or darker as it moves, and thus the picture elements scanned by the camera are reproduced line by line at the receiver, until the whole area of the screen is covered, completing the image. Then the process is repeated. The scanning motions in the camera and those in the receiver must keep in precise step. Otherwise the picture elements would appear in the wrong positions on the receiver screen, and the pattern of the image would be distorted or broken up entirely. Interlaced Scanning To avoid flicker, each still picture is presented twice by a process known as interlaced scanning. After the topmost line is scanned, an empty line is formed immediately below it, and the next scanned line appears just below the empty space. As the scanning proceeds, therefore, alternate lines are scanned, with empty spaces between the scanned lines. This represents the first showing of the still picture. The next image also consists of spaced lines, and its lines fall precisely in the empty spaces of the preceding image, so the whole screen is filled by the two sets of interlaced scanning lines. Picture Detail For fine details to be visible in television images, each picture element must be quite small. Experience with 16mm motion pictures has shown that the image should contain at least 100,000 picture elements, preferably 150,000 or more. This figure in turn determines how many scanning lines are required to cover the screen. In the United States the scanning pattern contains 525 lines from the beginning of one picture to the beginning of the next, and each line contains about 435 picture elements. Not all of the lines are actively used, and some details straddle two adjacent scanning lines and thus take two lines to be fully represented. With these effects taken into account, the total number of picture elements into which the scene is divided is about 150,000 elements. In Europe and many other parts of the world 625 lines are used, and the image contains about 200,000 picture elements.
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