740 BELL SYSTEM TECHNICAL JOURNAL 



made of the amplification in the wire links so that the radio 

 transmitter is fully loaded up. This permits radiation of full 

 power regardless of how loudly or weakly the subscriber talks, 

 and regardless of the length of the circuit between the subscriber 

 and the transatlantic terminals. This keeps the radio speech 

 waves as large as possible compared to the noise at all times. 

 These adjustments are made by the technical operators under 

 the guidance of the "volume indicators." 

 (3) To suppress echo effects, a system of voice-operated switching re- 

 lays has been devised whose function is to interrupt, when not 

 in use, any transmission path which may double back to its 

 source and give rise to echoes or singing in the circuit. 



The manual adjustments of controls required in (1) and (2) should 

 require no further explanation. 



Before describing the voice-operated switching system of (3), it will 

 be desirable to explain what this system is required to do. As pre- 

 viously stated, the adjustments employed to eliminate the two radio 

 effects— namely, variability and noise — tend to increase the severity 

 of echo effects. This follows from the fact that such adjustments 

 result in a net transmission loss from terminal to terminal which is 

 not constant as in ordinary telephone circuits, but which varies from 

 time to time depending on the loss in the ether path and the strength 

 of the voice currents which are delivered to the circuit terminal. The 

 overall transmission of the circuit may vary from a loss to a consider- 

 able gain. If means were not taken to prevent it, this gain would 

 set up between the two subscribers, circulating currents of rather large 

 amplitude producing either severe electrical echo effects or the totally 

 inoperative condition known as "singing." 



A further echo difficulty was brought about through the use of a 

 common frequency band or group of wave-lengths for transmitting in 

 both directions. This was highly desirable to reduce the amount of 

 frequency space occupied in the ether since there is but a limited suit- 

 able frequency space available. The radio waves at the frequencies 

 used (namely, 58.5 to 61.5 kilocycles) cannot be directed to a definite 

 point or confined to a single path. The radio receiver cannot, oi 

 course, when both transmitters are operating at the same frequency, 

 select one transmitter from the other by any ordinary tuning means. 

 Referring to Fig. 1, since the distance from the receiver at Houlton, 

 for example, to Rocky Point is much less than the distance from Houl- 

 ton to Rugby, the antenna at Houlton is exposed to a signal from the 

 transmitter in America which is much stronger than the signal from 



