SYNCHRONIZATION OF TELEVISION 



611 



By means of a network this output was cut down to a level of 1 milli- 

 ampere into 600 ohms impedance, the output impedance also being 

 600 ohms. This is a satisfactory level at which to transmit the 

 high frequency, without inducing noise in adjacent wires in the 

 telephone cables. 



In the case of the low frequency interlock it was undesirable to 

 attempt to transmit 17.7 cycles over a long distance line. The 17.7 

 cycles was therefore used to operate a polarized relay, the contacts of 

 which modulated the output of a 760-cycle electro-mechanical oscil- 

 lator 2 as shown in Fig. 6. In other words, the relay short-circuited 



24 V 



Q 



ELECTRO- 5, 

 MECHANICAL4( 

 OSCILLATOR 3<> 



17.7 ~ 



Fig. 6 — Synchronizing modulator 



the output of the oscillator alternate half cycles before application to 

 the telephone line. Instead of using separate telephone pairs for the 

 2125-cycle and the modulated 760-cycle current, the two were com- 

 bined by passing them through the line filter (shown in Fig. 7), thereby 

 requiring only one pair for transmission of both frequencies. An 

 identical network was employed for the radio channel. The problem 

 of transmission of the synchronizing current is covered in the paper 

 by Messrs. Gannett and Green and in the case of radio transmission 

 in the paper by Mr. Nelson, 



Receiving and Amplifying Circuits 



Passing over this part of the problem, therefore, assume that the 

 synchronizing currents have been obtained at the receiving end of 

 the line. This power was delivered at a very low level, being about 



^ Described in the Bell Laboratories Record, March, 1927. 



