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CHAPTER XXVIII TELEVISION AND RADIO TRANSMISSION
( 1 ) The radio signal is simpler in nature than the television signal and therefore it will be helpful to outline first the transmission of sound radio and, thereafter, the transmission of television by radio.
(2) In any broadcasting station the sound current created by a microphone or phonograph pickup is amplified in the ordinary way to a volume corresponding with the power which the station is permitted to broadcast. If that power is 5 kilowatts or more, the output stages of the sound amplifier are commonly equipped with water-cooled tubes. However, there is nothing about the sound amplifier in the radio broadcasting station that differs essentially from sound amplifiers in theatres.
(3) The amplified sound current cannot be broadcast. If it were placed on the transmitting antenna, no receiver except one very close by could pick it up. The highest sound frequencies, those near 5000 cycles, might radiate to a very slight degree. The lower frequencies would not radiate at all, except by ordinary induction and therefore extremely short distances. Alternating currents of n;ore than 50,000 cycles are needed to obtain useful radiation. The sound to be broadcast must be transmitted on a "carrier" of radio frequency.
(4) Therefore, in every broadcasting station there is a device for generating radio frequency current. This generator is nothing but a small vacuum tube. Any three-element tube can be made to act as a generator of alternating current by introducing feedback between plate and grid. In Figure 201, a theatre amplifier, such feedback is introduced, but in reverse phase and is called inverse feedback. It operates to reduce volume and purify sound quality. But a polarity reversing switch
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