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BELL SYSTEM TECHNICAL JOURNAL 



mum echo net loss of a four-wire cable circuit as follows: Assume a 

 trial net loss and compute the loss in the echo path by adding the 

 loss from the toll switchboard to the point where the echo is reflected 



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5^20 

 UlO 



0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 20 



ROUND-TRIP CIRCUIT DELAY IN SECONDS 



Fig. 1 — Talker echo delay terms for 4-wire circuits — sidetone subsets. 



back, the terminal return loss (assumed 6 db for echo computations 

 in the Bell System) and the loss from that point back to the toll 

 switchboard. From this total, subtract the "delay weighting term" 

 from Fig. 1 for the corresponding round trip delay. If the resulting 

 weighted echo path loss is greater than or equal to zero, the circuit 

 will be satisfactory from an echo standpoint at this net loss without 

 variations. 



In the case of two-wdre circuits, the echo limitations are similar 

 to those for four-wire circuits except that echoes are also returned 

 from intermediate points in the circuit through the return paths at 

 the repeater hybrid coils. 



The general method of determining whether circuits are satisfactory 

 from an echo standpoint has been discussed in the paper entitled 

 "Telephone Transmission Over Long Cable Circuits" by A. B. Clark ^ 

 and later in a paper entitled "Echo Suppressors for Long Telephone 

 Circuits," by A. B. Clark and R. C. Mathes {A. I. E. E. Jour., June, 

 1925). It may be outlined briefly as follows: Determine the weighted 

 loss of each echo path by determining the actual loss from and to the 

 toll switchboard at the talker end (including the return loss at the 

 point where the echo is reflected back) and then subtract the "delay 

 weighting term" corresponding to the delay of each path as obtained 

 from the upper curve on Fig. L If any one of these w^eighted echo 

 path losses is reduced below zero db, the echo conditions will be unsat- 

 isfactory without regard to the efi'ect of the other paths, as outlined 

 above. However, if all these losses are positive, it is considered that 

 the net effect of all of the paths may be determined by adding the 



