522 BELL SYSTEM TECHNICAL JOURNAL 



respectively, of the bare cable, the near-end apparatus and the ter- 

 minating effect at the far end of the repeater section. 



C = apparatus loss at near end. 

 Sc — return loss of apparatus at near end. 



^7- — terminating effect of repeater and apparatus at far end of re- 

 peater section. 



L = loss of the line section at 1 ,000 cycles. 



The overall return loss of the complete repeater section, S, is as- 

 sumed equal to the combination of ^i, S2 and S3 as the sum of the 

 corresponding power ratios. 



Circuits With Echo Suppressors 



When echoes would otherwise be objectionable on a circuit, it may 

 be equipped with an echo suppressor. On a four-wire circuit equipped 

 with an ordinary echo suppressor, the currents which are strong 

 enough to operate the echo suppressor have their echoes suppressed. 

 When currents are too weak to operate the suppressor, echoes will be 

 returned, but, of course, will be much weaker than the loudest echoes 

 on the same circuit without an echo suppressor. The echoes on the 

 circuit with an echo suppressor will, therefore, generally be less objec- 

 tionable than those on the same circuit without an echo suppressor, 

 since those which get back to the talker are weaker in absolute volume, 

 while the noise and sidetone volume for a given speech volume are 

 unchanged. 



The more sensitive the echo suppressor is made, the weaker the 

 sounds will be which will just fail to operate the suppressor. Conse- 

 quently, the echoes will become less objectionable as the sensitivity is 

 increased. However, if the sensitivity is increased too much, the 

 suppressor may be falsely operated by noise currents, either from the 

 circuit, from room noise at the subscriber's premises which is picked 

 up through his transmitter, or from room noise picked up through 

 operators' sets. 



The process of determining the minimum echo net loss of a circuit 

 equipped with an echo suppressor has the following two steps: (1) 

 determine the zero level sensitivity ^ of the echo suppressor on the 

 circuit which is allowable with little or no false operation from noise 

 and (2) determine the minimum net loss from experimental curves. 



' The zero level sensitivity is defined as the amount of loss it is necessary to insert 

 between a 600-ohm source of one milliwatt of power and the 600-ohm input of the 

 circuit on which an echo suppressor is located in order to cause the echo suppressor 

 to be just operated. Unless otherwise specified, this is assumed to be at 1,000 

 cycles. 



