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Color television employs the basic principles of black-and-white television. The essential difference is that a color broadcast is in reality three telecasts in one. The screen of a color receiver actually displays three images superimposed on each other; these images present, respectively, the red, green, and blue components of the colors in the scene. This use of three primary colors in the television follows the method used in color photography and color printing, in which three layers of colored dyes (in photography) or three interspersed sets of fine colored dots (in printing) give to the eye the impression of all the natural colors. 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 iscoated 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 ofthe 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.The scanning coils are arranged to move the electron beam over the stored charge image in the interlaced scanning pattern previously described. In this manner, the electron beam reads the stored charge image, line by line. By the time the scanning of the image is completed, the charge The electrons have negative electrical charge.When they hit a point on the antimony coating, they neutralize the positive charge stored at that point. This sudden change in charge is registered as a change in voltage on the metallic coating, which is connected to the camera terminal. As the electron beam scans the charge image, it thus produces a succession of voltage changes at this terminal, which constitute the video signal. The video signal at the camera terminal is weak, so it is amplified at once within the camera housing. After further processing, the amplified video signal is transmitted to the receiver, where it reaches the picture tube and re-creates the image. History of The Television 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 A few years earlier, in 1873, the fact that the electrical resistance of selenium (later used in early versions of the vidicon camera tube) was lowered when the material was illuminated was discovered by Louis May, an English telegrapher. In 1884 the German Paul Nipkow invented a mechanical system of scanning an image through holes in a rotating disk, but not until 1926 did the Englishman John Logie Baird and the American Charles F. JENKINS actually demonstrate the transmission of images in halftone using the Nipkow disk.The development of electronic methods began in 1897 when the German Ferdinand BRAUN produced the first CATHODE-RAY TUBE, the ancestor of the picture tube. In 1907 the Russian Boris Rosing suggested the use of Braun's tube to reproduce television images. Using a rotating mirror drum for scanning, Rosing actually transmitted crude geometrical shapes but was unable to reproduce halftone images.In 1908 the Scotsman A. A. Campbell-Swinton proposed that the image be stored in the form of electric charge in a camera tube and reproduced on a picture tube--the essential features of today's system. The crude techniques of that day did not permit the system to be realized in practice, and not until 1923 did the American Vladimir R. ZWORYKIN, who had studied in Russia under Boris Rosing, apply for a patent for a camera tube (iconoscope) that used a stored-charge image.Meanwhile, the techniques of signal amplification using vacuum tubes had advanced to the point that, wholly by electronic methods, signals from Zworykin's iconoscope could be transmitted by wire to a picture tube. The transistor, invented in 1948, later replaced the vacuum tube and in turn led to integrated circuits.By the early 1930s experimental broadcasts of black-and-white halftone images, composed of 343 scanning lines, were conducted by engineers of the Radio Corporation of America (now RCA). By 1935, English engineers had developed a broadcast system using 405 scanning lines. The first telecasts regularly scheduled for the public began in London in 1936 using this system. (A mechanical system using 240 lines was also broadcast but soon abandoned because the electronic system was clearly superior.) Experimental broadcasts using 441-line images began in New York that same year, but not until 1941 did the Federal Communications Commission authorize public broadcasting in the United States using 525-line images. This effort was held in abeyance during World War II. After the war black-and-white broadcasting developed rapidly in the United States, England, France, and Germany. One million receivers were in use in America by 1949, 10,000,000 by 1951, and more than 100,000,000 by 1975. One of the show's sponsors, Wonder Bread, put paper ballots in its loaves as a sales gimmick, and thousands of children mailed in their votes for Howdy. Rumors spread after Election Day that Howdy Doody had received thousands of actual write-in votes. If true, it's unlikely that Howdy's votes affected the outcome of the election. "Puppet Playhouse" was televised only on the East Coast, which Dewey carried anyhow. Truman won the election in the Midwest and West, which had few TV stations and no "Howdy Doody."
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