Television

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

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.

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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

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.

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.

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