<text><span class="style10">adio, Television and Video (3 of 4)</span><span class="style7"></span><span class="style10">Satellite and cable TV</span><span class="style7">Because the VHF and UHF wavebands on which television signals are transmitted are not significantly reflected by the ionosphere, many transmitters are needed for complete coverage of all but the smallest countries. For intercontinental TV, some other way of bouncing the signal back from space is needed. This is achieved by satellites above the Earth that pick up the signals and broadcast them back to Earth. The satellite signals are not powerful enough to be picked up by ordinary TV aerials, so they are re-broadcast by ground transmitters. Special dish-shaped aerials can pick up satellite signals, however, making possible continent-wide direct broadcasting by satellite (DBS). Such satellite channels already operate in the USA and Europe. TV signals can also be fed down cables, making possible networks in which films, news, sport and other programs are available exclusively to those willing to pay a subscription.</span></text>
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<text><span class="style10">he composite signal picked up by the receiving aerial </span><span class="style7">is decoded, separating out the various constituent signals. The luminance signal controls the overall output of three electron guns in the cathode-ray tube of the receiving set, so determining the balance of light and shade in the final picture. The chrominance signal - now split into the three primary-color signals - regulates the relative strength of each electron beam. The syn pulse, divided into line and frame components, controls the deflection of the beams across and down the screen. The television screen is coated with stripes of different phosphors, which glow red, blue or green when struck by electrons. Immediately behind the screen is a </span><span class="style19">grille, </span><span class="style7">or</span><span class="style19"> shadow mask, </span><span class="style7">which contains many perforations. Traveling at slightly different angles as they pass through the perforations, the electron beams are caused to diverge before striking the screen, in such a way that the electrons from each gun can only reach phosphor stripes of the appropriate color. Each image on the screen thus consists of stripes of varying brightness and color that merge together to form the complete picture.</span></text>
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<text>ΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ELECTRICITY IN ACTIONΓÇó PHOTOGRAPHY AND FILMΓÇó SOUND RECORDINGΓÇó TELECOMMUNICATIONSΓÇó CINEMAΓÇó JOURNALISM</text>
