76 



BELL SYSTEM TECHNICAL JOURNAL 



this echo corresponds to the one labelled "1st echo talker". Sim- 

 ilarly for the echoes affecting the listener, the echo whose path is 

 longest in the two-wire circuit corresponds to a similar echo in the 

 four-wire circuit. Since many additional echo paths are present in 

 the two-wire circuit, it is evident that, other things being equal, the 

 overall transmission result obtainable from the two-wire circuit can- 

 not be made as good as that obtainable from the four-wire circuit. 



Two Wirt Circuit 





a 



- Paths of Echoes Affecting Talker 



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IcUia l Wtiohtt d 

 8 ?0 



35 160 



- Paths of Echoes (ffect.ng Lis 



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23 30 



45 63 



3Z » Z>ZOZ ' 3 ^- ZS 36 



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Fig. 6 — Echo paths in two-wire repeatered circuit with omitted loading coil. 



In a two-wire circuit it is, of course, obvious that any defect in the 

 lines which will cause a large irregularity will result in a considerable 

 impairment of the circuit. Figure 6 shows the effect of omitting a 

 loading coil at an intermediate point in a circuit, the conditions in 

 this circuit being assumed to be the same as those in Figure 4 with 

 the exception of the omitted loading coil. The omitted loading coil 

 introduces a large impedance irregularity which causes certain of the 

 echoes to be made much greater in comparative magnitude as indicated. 

 In order to reduce the echoes in the circuit with the omitted loading 

 coil sufficiently to make the circuit satisfactory for telephone use, it 

 is necessary to reduce the repeater gains. In this particular case it 

 is necessary to lower the total gain about 4 miles, which increases the 

 overall transmission equivalent of the circuit from about 10 miles 

 for the normal condition to about 14 miles for the condition with the 

 omitted loading coil. 



