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652 RICHARDSON'S BLUEBOOK OF PROJECTION
horizontal-scanning mirror polygon has two windings, one being asynchronous and excited by a 500 cycle current generated by an oscillator tube and amplified. The other winding of this motor, which controls synchronous operation, is powered by the horizontal-scanning impulses as received from the transmitting station and amplified.
Methods of Transmission
(5) When television signals are received in the theatre by radio two different receivers tuned to two different frequencies are used. One is the television receiver, the other is a sound radio receiver which picks up the accompanying sound. Sound and television are not broadcast on the same frequency nor necessarily on related frequencies. However, they are broadcast in synchronism and therefore are received and reproduced in synchronism.
(6) Radio is not always employed for television transmission. Over very short distances, a half-mile or so, ordinary telephone lines may be used to carry the 0 to 3,000,000 cycle television signal and its accompanying synchronizing impulses. A second telephone line may be used to supply the sound. When the signal is transmitted by wire the superheterodyne receiver, of course, is not needed. The signal is the same as that which is delivered by the second detector of the superheterodyne radio receiver. All that is needed is the video amplifier and the synchronizing saw tooth frequency generators. On the sound end also no radio receiver is needed — only a sound amplifier and loud speakers.
(7) For wire transmission over distances of more than a half-mile or so a telephone line is impractical. Coaxial cable must be used. This is the same type cable used in many theatres to couple the photo-electric cell to its amplifier — one conductor inside insulation surrounded by the other conductor, the outer conductor being grounded. Television signals can be carried over long distances by means of coaxial cable, but very little coaxial cable is so far installed in the United States. There are a few lines between such cities as New York