Chapter 12: Electromagnetic Waves (545 KB) - D Cassidy Books

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3637_CassidyTX_12 6/13/02 11:34 AM Page 568■ TELEVISION (Continued)ing glass close to a television screen, you willprobably be able to see the individual pixelsthat make up the image.Through this process, a television setcomposes rapidly one still image after anotheron the screen at the rate of at least 15images, or frames, per second, which yourbrain perceives as a fluid moving scene.ThesubjectLensMotorNipkow diskLensLightsensorTELEVISION: THE INVENTIONThe research and development of televisiontechnology happened along two paths, mechanicaland electronic.FIGURE 12.13 Diagram of Nipkow disk apparatus.Mechanical TelevisionTelevision was inspired by human vision, inwhich thousands upon thousands of electricalcircuits are used to relay information inthe optic nerve from the retina to the brain.Designs based on so many electrical circuits,however, were far too complicated to everreach fruition. In 1880 Maurice Leblanc inFrance and W.E. Sawyer in the United Statessuggested instead that if each element of apicture could be rapidly scanned, electricaltransmission of pictures could be achievedusing only one circuit between the transmitterand receiver. Paul Nipkow of Germanyadopted this scanning technique in 1884.The picture to be televised is focused on arotating disk. When the first aperture in thedisk has scanned a line of the picture, the nextaperture scans the parallel line directly below.At each rotation of the disk another line isscanned until the whole picture has been examined.In this design, more apertures meanmore lines, and hence greater detail of thepicture. As it passes through each aperture,the light differs according to the light andshade of the picture, it then passes through aphotoelectric cell where it is changed into anelectrical image, and then translated into electricalimpulses. These impulses are then sentdown a circuit to a receiver, where they producelight in something like a gas-dischargelamp. The light from this lamp is then projectedonto another disk, similar to the transmitter,and by a reversal of the transmissionprocess, the brightness of each segment is reproducedand the original picture is reconstructed.In order for this system to work, therotation of the disks must be synchronized,and provided they are rotating at a sufficientspeed, persistence of vision allows the observerto see a whole image rather than a seriesof moving segments.Until the arrival of electronic scanning, itwas this mechanical system, characterized bythe Nipkow disk, which dominated the developmentof television. However, the systemwas crude, with the small number of lines resultingin poor definition and the small numberof rotations causing a flicker. Thereforeresearch turned toward electronic systems.Electronic SystemsOne of the leading players in the developmentof electronic television was VladimirKosma Zworykin. He was convinced thatelectronic television was the path he shouldfollow, and while working for the Radio Corporationof America (RCA), he developed theIconoscope which used a CRT for transmittingimages.At the wide end of the Ionoscope is a sheetof mica, on one side of which is a signal plate,and the other a silver mosaic treated with568
3637_CassidyTX_12 6/13/02 11:34 AM Page 56912.5 THE ELECTROMAGNETIC SPECTRUM 569cesium vapor and oxygen, in which each elementis encircled by an aluminum oxide insulator.This combination of elements providesa surface from which electrons are easilyliberated when exposed to light. When an imageis focused onto the mosaic surface, it takeson a positive charge that corresponds to thelight distribution. When the electron beampasses over the mosaic, the charge of each sectionchanges in proportion to the amount oflight that is falling on it. This change is transferredto the signal plate, which takes on a seriesof voltages corresponding to the lightalong the particular line.Although it marked a major breakthroughin electronic television, the Iconoscope was notsensitive enough to produce consistently clearimages, and in 1940 it was replaced by the Orthicon,based on a similar design but with arigid set of squares. The Orthicon, however,was made obsolete in the same year, by a tubedeveloped by Corning Glass Company, whichwas three to ten times more sensitive than theOrthicon and thirty to five hundred timesmore sensitive than the Iconoscope.At the beginning of the 1950s, televisiontechnology based on the electrical system hadadvanced so far that engineers began lookingtoward developing a color system. Therewere two possibilities. The first was a systembased on a frame-by-frame sequential transmissionof signals, each corresponding to theprimary colors, but this design was incompatiblewith the current black and whitetransmissions. The more complex systemproposed at the time was one in which signalsrepresenting the three primary colorswere transmitted simultaneously. This secondsystem was developed by the NationalTelevision Systems Committee (NTSC) inthe United States and is now the basis forcolor systems all over the world. By the early1960s, color television was becoming a consumersuccess in the United States.Recent developments in television technologyhave focused on the development ofHigh Definition (HDTV), in particular onthe use of digital technology. Instead of conventionalanalog technology, which transmitssignals in the form of waves, the new digitalHDTV system transmit pictures as digitaldata, which is then translated by computerswithin the digital television. As well as clearerpictures and better sound, digital televisionalso has the potential to transform and manipulatepictures as well as to receive them,and could perform the job of both a computerand a television set.FIGURE 12.14 Vladimir Zworykin, one of theinventors of television, is shown here holding acathode-ray tube.Further ReadingD.E. Fisher and M.J. Fisher, Tube: The Inventionof Television (New York: HarcourtBrace, 1996).
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Chapter 12: Electromagnetic Waves (545 KB) - D Cassidy Books

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