<text><span class="style10">adio, Television and Video (2 of 4)</span><span class="style7"></span><span class="style10">Television</span><span class="style7">The idea of using radio waves to carry visual information dates back to the early days of radio, but became practical only in 1926. The basic principle is to break up the image into a series of dots, which are then transmitted and displayed on a screen so rapidly that the human eye perceives them as a complete picture.In 1926 the Scottish inventor John Logie Baird (1888-1946) demonstrated television based on a mechanical method of scanning an image into lines of dots of light. Baird's system had little future, however, and was rapidly superseded by an all-electronic system. This was developed by Vladimir Zworykin (1889-1982), a Russian-born engineer working in the USA. His first practical camera, made in 1931, focused the picture onto a mosaic of photoelectric cells. The voltage induced in each cell was a measure of the light intensity at that point, and could be transmitted as a signal. A modern TV camera operates in essentially the same way, measuring the light intensity at each point of the image. This information is then encoded on the radio wave and transmitted.At the receiving end, the signal has to be decoded. A TV set is basically a </span><span class="style19">cathode-ray tube</span><span class="style7">, in which a 'gun' fires a beam of electrons at a luminescent screen. As they strike it, the screen lights up. To make up the whole picture the beam is scanned to and fro in a series of lines (625 in modern sets), covering the entire screen in 1/25 second.</span></text>
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<text><span class="style10">ight reflected from the scene being televised</span><span class="style7"> is focused by </span><span class="style19">lenses </span><span class="style7"> and split by means of </span><span class="style19">dichroic</span><span class="style7"> (color-separating) </span><span class="style19">mirrors </span><span class="style7">into three separate images, one in each of the three primary colors - blue, green and red. Each beam of colored light is then directed into one of three identical </span><span class="style19">vidicon tubes</span><span class="style7">. The pattern of light falling on a photo conductive layer within each tube causes a varying pattern of electrical resistance; as a electron beam scans the photo conductive area from behind, a varying electric current is induced in a circuit connected to the conductive layer. The pattern of dark and light in each primary-color image is thus converted into one of three varying electrical signals.A black-and-white </span><span class="style19">luminance </span><span class="style7">(brightness </span><span class="style19">signal </span><span class="style7">is created in the adding unit, by combining information from each of the three color signals. At the same time, the color encoder produces a single </span><span class="style19">chrominance signal, </span><span class="style7">which defines the hue and saturation of each primary color. The luminance and chrominance signals are combined into a composite </span><span class="style19">video signal. </span><span class="style7">Prior to transmission, the </span><span class="style19">audio signal </span><span class="style7">is incorporated, together with a </span><span class="style19">synchronizing pulse </span><span class="style7">('sync pulse'), which ensures that the electron scanning in the receiving system matches that of the transmitting system. </span></text>
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<text>ΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ELECTRICITY IN ACTIONΓÇó PHOTOGRAPHY AND FILMΓÇó SOUND RECORDINGΓÇó TELECOMMUNICATIONSΓÇó CINEMAΓÇó JOURNALISM</text>
