1032 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



are given in Table V. The advantages of high velocity, four- wire cir- 

 cuits (carrier and radio) are obvious from these tables. 



ECHO SUPPRESSORS 



Even if the intertoU trunk plant of the Bell System were all carrier' 

 the length of some connections would be so great that some method of 

 controlling echo other than simply increasing circuit loss is desirable. 

 Lower losses can be obtained on such connections through the use of an 

 **echo suppressor," an electronic device which under control of the 

 talker's speech currents places a high loss in the return path at the right 

 time to intercept the return echo currents. 



Echo suppressors perform very well so long as not many circuits 

 equipped with them are connected in tandem and there is not too much 

 time delay between them. With manual operation the switching is so 

 limited that the chances of connecting circuits with echo suppressors 

 in tandem are small and it has been practicable to apply echo suppressors 

 on the basis of round-trip delay of the individual circuits. However, with 

 dial operation it will be possible to establish connections which are long 

 enough to require an echo suppressor but which are composed of cir- 

 cuits each too short to require an echo suppressor based on its round-trip 

 delay. For example, an echo suppressor would not normally be used on a 

 500-mile carrier circuit, but if eight such circuits were connected in tan- 



