TRANSATLANTIC RADIO TELEPHONY 255 



could be done the entire system would utilize only about 3000 

 cycles. 



In this sort of arrangement, it is evident at once that selectivity 

 at the receiving station is of no further avail in preventing interference 

 from the local transmitter and that unless means are provided greatly 

 to reduce this crossfire or to set up the circuit in some fashion so that 

 it is harmless, the local circuit from transmitter to receiver with return 

 by wire will be in a singing condition, since it is not practicable to 

 obtain at the hybrid coil anything like a sufficient balance to prevent 

 this. 



It was found that with the American receiver in Maine some 500 

 miles from the transmitting station, the local signals were so reduced 

 by distance that the further reduction which could be obtained by 

 virtue of the antenna directional characteristics was sufficient to peimit 

 operation. At the English end, however, due to the proximity of the 

 receiving station to the transmitting station, this so-called "radio 

 balance" method of operating could not be employed. This difficulty 

 had been foreseen and there had been developed a switching device 

 based upon certain similar switching devices called echo suppressors 

 which are employed in long toll circuits.''* The function of this appa- 

 ratus was in part to supplement the hybrid coil in its office of pre- 

 venting received signals from getting into the transmitting line. The 

 arrangement is one in which switching means are employed alternately 

 to disable the transmitting or receiving side of the radio circuit 

 automatically in response to the voice currents produced by the 

 speakers at the two ends. Each end of the system was provided 

 with a device of this character operating on substantially the same 

 principles. Briefly, the functioning of the device is as follows: 



When no one is speaking on the circuit the transmitting voice paths 

 are blocked at both the New York and London ends of the system but 

 the receiving paths are open so that incoming radio signals pass freely 

 through to the ears of the subscribers. When a speaker, for instance, 

 in America, speaks, his voice currents actuate the device to block off 

 his receiving path and to open his transmitting path so that his voice 

 goes out. Since the other end of the circuit is in a receiving condition, 

 the voice currents travel through the entire system to the listener's 

 ear. When the American speaker has finished, his apparatus is 

 automatically restored to the receiving condition and the British 

 speaker is, by the functioning of the apparatus in London, able to 



'""Echo Suppressors for Long Telephone Circuits". A. B. Clark and R. C. 

 Mathes: Journal of the American Institute of Electrical Engineers, Vol. XLIV, June 

 1925, pp. 618-626. Also "Telephone Repeaters," C. Robinson and R. M. Chanine}': 

 The Electrician, December 12, 1924, pp. 665-667. 



