RADIO TRANSMISSION SYSTEM FOR TELEVISION 643 



kilocycles, ± 20 kilocycles represents a 2.6 per cent band while at 5000 

 kilocycles the same side frequencies represent only a 0.8 per cent band. 

 In the latter case, therefore, it is possible to employ materially sharper 

 circuits without discriminating against the higher signal components. 

 The 5000-kilocycle circuits connected to the grid of a second detector 

 or modulator tube upon which suitable voltages from a 5120-kilocycle 

 oscillator were impressed. The 120-kilocycle components in the 

 output of this modulator were selected by means of a band-pass filter 

 which worked into a two-stage intermediate-frequency amplifier. A 

 second band-pass filter led to the third or final detector. A 20- 

 kilocycle low-pass filter was employed in the plate circuit of the latter. 

 This filter was designed for a low input impedance at 120 kilocycles in 

 order to meet the necessary condition for efficient rectifier action and 

 it also served as a coupling element for the audio stage which followed. 

 A special output transformer with a permalloy core was provided to 

 step down to the relatively low impedance of the line leading to the 

 television apparatus proper. 



A superheterodyne receiver of more conventional design was 

 employed for the speech receiver. The circuit arrangement involved 

 a double-tuned input circuit, one tuned radio-frequency stage, oscil- 

 lator and modulator, two intermediate-frequency stages, detector and 

 one audio stage. It was highly selective and afforded substantially 

 distortionless transmission for signal frequencies ranging from 50 to 

 5500 cycles. 



The transmitting equipment for the synchronizing channel consisted 

 of a Western Electric 6-A Radio Broadcasting Transmitter modified 

 to operate at 185 kilocycles. In order to avoid the necessity of trans- 

 mitting directly the 17.7-cycle component required for synchronizing 

 purposes, a 760-cycle carrier was modulated at 17.7 cycles by means 

 of a relay and impressed upon the input of the radio transmitter 

 together with the steady 2125-cycle component. At the receiving end, 

 the 2125- and modulated 760-cycle components were separated by 

 means of suitable filters, and the latter rectified to produce the desired 

 17.7-cycle current. 



Tests of the System 



As soon as the various apparatus units could be made ready for 

 service, a comprehensive series of transmission tests was undertaken. 

 In order to determine the relative suitability of the 1575- and 1750- 

 kilocycle bands disclosed by the preliminary survey, transmissions 

 from Whippany at intervals throughout the day were arranged. 

 Field strength measurements were taken at the receiving point 



