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TELEVISION AND RADIO TRANSMISSION 637
erally limited to 5,000 cycles top, although a few stations have special license to broadcast up to 8,000 cycles of sound current. The lower limit of sound frequencies is, as in the theatre, very approximately 50 cycles. These frequencies are impressed by modulation, as said, upon the r.f. carrier frequency.
(12) The television signal consists of several parts, since synchronizing impulses to keep the camera and cathode ray tube in step must be broadcast along with the signal proper. The signal alone occupies a vastly wider spectrum of frequencies than does sound current.
(13) If the television standard is 441 "lines" — meaning that the point of the electron beam scans the fluorescent material 441 times from top to bottom, and if the equipment can respond to 588 "points" along each "line," then the total frequency involved in scanning the fluorescent surface once must be 588x441, or approximately 260,000 pulsations. But this frequency would give only a still picture. Television utilizes 30 "frames" per second (instead of 24 frames per second as in the case of motion pictures). The total number of pulsations per second to permit 30 frame transmissions would then be 260,000 x 30 or very approximately 7,800,000. Counting each pulsation as one alternation, and two alternations to one cycle, the transmission frequency necessary in the case of 260,000 picture elements at 30 frames per second would be 7,800,000 ~ 2, or 3,900,000 cycles. In practice this maximum frequency is never realized nor needed because no image consists of contrasts at every point. There will be some areas which are all black or all white or all the same shade of gray and when the electron beam scans such areas there is no fluctuation in its strength. In practice a frequency of 3,000,000 cycles is adequate. It is called the video frequency.
(14) In addition to this video frequency, however, synchronizing impulses must be transmitted. These impulses control the action of the deflecting vanes of Figure 224 and hence keep the electron beam in Figure 224 in exact step with the electron beam in the television camera. They also increase the bias of the grid-cylinder