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BELL SYSTEM TECHNICAL JOURNAL 



terminal B, the heavy line in Figure 3-b shows the direct transmission, 

 which takes place over the top pair of wires in Figure 3-a. When this 

 current reaches the distant terminal, part of it goes to the listener 

 while another part, due to the imperfections of balance between the 

 line and network at that terminal, travels back through the pair of 

 wires at the bottom of Figure 3-a toward terminal A. The talker at 

 terminal A will hear this current as an echo if the four-wire circuit 

 is long enough so that the time lag is appreciable. This first echo 

 heard by the talker divides at terminal A in the same way as did the 

 direct transmission at terminal B, part of it taking the upper path of 

 figure 3-a back toward the listener. The listener will, therefore, first 

 receive the direct tansmission and then a little later an echo. This 

 process is repeated producing successive echoes which are received 

 at both terminals A and B as indicated. 



a-Four Wire Circuit 



a 



arxro 



stener 

 Station 



s 



b-Echo Paths 



Direct Transmissic 



Fig. 3 — Echo paths in four-wire circuit. 



A four-wire circuit 1000 miles (1600 kilometers) long has been set 

 up in which the balances at the two ends were deliberately made poor 

 so as to exaggerate the effects. More than a dozen successive echoes 

 could be heard before they became inaudible. Since for each echo 

 the voice energy traveled 2000 miles (3200 kilometers) this energy 

 must have travelled the distance around the world before becoming 

 inaudible. 



In order that a circuit will be satisfactory for regular telephone 

 use, the echoes must be kept small as compared to the direct transmis- 



