THE NEW YORK-LONDON TELEPHONE CIRCUIT 743 



England.^ Were it not for the use of the voice-operated devices, this 

 would result in a very strong echo being returned to the local talker 

 with disconcerting effects, and, dependent upon the adjustment of 

 other parts of the circuit, might even result in violent singing of the 

 circuit. 



Referring again to Fig. 2 {A), it will be seen that there are three 

 paths capable of giving rise to objectionable echoes or singing, one 

 path at each end of the circuit through the wire lines, radio transmitter 

 and local receiver, and a third path from end to end of the circuit and 

 back again. The first two paths are introduced by using the same 

 frequency band for transmission in both directions. The third path 

 which depends upon the impedance unbalance between the two-wire 

 lines at the terminals and their respective networks is similar to the 

 one which gives rise to echoes in long four-wire land telephone circuits. 

 All three paths are affected by the amplification adjustments. 



Suppression of echoes and singing in the circuit requires that all 

 three of these echo paths be kept blocked at all times against unwanted 

 transmission. Furthermore, since there is no single point common to 

 all the echo paths, the system for suppressing echoes comprises two 

 separate installations — one of which is located in New York and the 

 other in London. The devices used to control the echo paths are 

 operated by the voice currents of the two telephone subscribers, in 

 such a manner as to allow transmission to pass first in one direction 

 when one subscriber is talking, and then in the other direction when the 

 second subscriber replies. Transmission in the opposite direction to 

 that in which the waves are traveling is blocked. When no one is 

 talking, the outgoing transmission paths at both ends of the circuit are 

 blocked. The necessary functions at the New York end of the circuit 

 are performed by a combination of electro-magnetic relays, vacuum 

 tube detectors and delay circuits. A photograph of the installation is 

 shown in Fig. 4. At London a device performing similar functions 

 has been developed by engineers of the British General Post Office. - 



A schematic diagram of the device employed at the New York end 

 is shown in Fig. 5. By tracing the action of the relays it will be seen 

 that for all conditions of the relays, the echo paths shown are blocked 

 at the proper times. Thus, Fig. 5, which shows the conditions when 

 no one is talking, indicates that the circuit from the radio receiver to 

 the terminal is normally in a receiving condition but the transmitting 

 branch of the circuit is kept inoperative by relays ^^ and CS. 



1 Directive antenna systems with a blind spot might be used to overcome this 

 effect, buttheirdirectivepropertieswould not then be available for use against static 

 and other interference. The general directivity of the receiving systems used, 

 however, reduces the unwanted transmissions about lOO-fold. 



2 C. A. Beer and G. T. Evans, "The Post Office Differential \'oice-Operated 

 Anti-Singing Equipment," P. O. E. E. Jnl., April, 1927. 



48 



