TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1023 



Hence it seems sensible to compute the theoretical losses on the more 

 liberal basis of 1 in 100, relying on the excess loss in most connections to 

 reduce the over-all probability to the very small value desired. 



The echo problem with which we are concerned is illustrated in Fig. 2. 

 As shown there, part of the speech power which is being transmitted to 

 the listener "leaks" across the hybrid (or four- wire terminating set) at 

 the listener's end and returns to the talker. This is known as "talker 

 echo." Actually, of course, some of the echo which returns to the talker 

 can leak across the hybrid there and go back to the listener. This is known 

 as "listener echo." However, with modern plant listener echo will not 

 be important if talker echo is adequately controlled. 



J 



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4-WIRE TERMINATING SET 



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.HX, 



BALANCING 

 NETWORK 



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<h 



talker's speech path 

 talker echo path 

 listener echo path 



Fig. 2 — Echo paths. 



Considering the effect of echo on the talker, if the elapsed time before 

 the echo gets back to him is very short it is just like hearing his own voice 

 through the sidetone in his own set, and unless it is very loud he doesn't 

 notice it. If on the other hand, the elapsed time is long it sounds to him 

 very much like the familiar acoustical echoes arising from physical 

 obstacles. In extreme cases he may get the impression that the distant 

 party is trying to interrupt him. The over-all effect of echo then depends 

 on the following: 



1. How loud it is — which in turn depends on how much loss there is 

 in the echo path. 



2. How long it is delayed before it gets back to him. 



3. How easily he is annoyed by it (i.e., his "tolerance" to echo). 

 The factors involved are discussed in more detail in the following. For 



simplification four- wire circuits and four- wire switching are assumed; 

 two-wire circuits and two-wire switching are treated as variations and 

 are discussed later. 



