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RADIO RECEIVERS
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signal current of 0 to 3,000,000 cycles. The output of the video amplifier then goes to the cathode ray tube of Figure 224.
(12) Up to the second detector there is nothing about the television superheterodyne receiving system that is importantly different from any sound superheterodyne radio receiver. The television receiver works with a higher band of frequencies, but it works on the same basic principles. The video amplifier, in turn, is not particularly different from a sound amplifier ; the sound amplifier is designed to deal with frequencies from 50 to 5,000 or 8,000 cycles, whereas the video amplifier is designed to handle frequencies from 0 to 3,000,000 cycles, but this difference is a matter of the design of such component parts as transformers and choke coils, not a matter of important difference in circuits. The television circuits proper begin at the output of the television amplifier and are interposed between the output of that amplifier and the controls of the cathode ray tube.
(13) The signal output proper— that is, the 0 to 3,000,000 cycle current corresponding to differences in light and shade in the transmitted image— involves no complexities. This output is wired, as has been said, to the grid-cylinder of the cathode ray tube. It is essentially a voltage output rather than current output and is used to alter the voltage of the grid-cylinder, thereby modifying the intensity of the electron beam from moment to moment. The synchronizing impulse^ which accompany the signal proper involve some circuits new to the projectionist, which will now be described.
Scanning Impulses
(14) The alternating currents applied to the deflecting vanes of Figure 224 are generated in the receiver, by oscillator tubes incorporated for that purpose. The action of those tubes, however, is controlled _ by synchronizing impulses received from the transmitter, for the purpose of assuring that the electron beam in the cathode ray tube will be in perfect step with the electron beam of the television camera.