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BELL SYSTEM TECHNICAL JOURNAL 



the figure. When subscriber E talks, transmission passes over the 

 lower path in a similar manner. 



Considering part b of the figure, it will be noted that when direct 

 transmission is received at the east end of the circuit, a portion of the 

 current passes to the opposite side of the four-wire circuit and is 

 transmitted to the subscriber at the west end as a talker echo. Sim- 

 ilarly, a portion of this talker echo is transmitted over the upper part 





TALKER 

 STATION 





fTWWJTpl LISTENER 

 vwvuvv STATION 



(a) 



DIRECT TRANSMISSION 



fb) 

 Fig. 6— Echoes in four-wire circuit. 



of the circuit to the listener at the east end of the circuit as a listener 

 echo. Successive talker and listener echoes follow this, as indicated 

 in the diagram. If the networks at the two ends of the circuit can be 

 made to simulate accurately the subscriber circuits, none of these 

 echoes will exist. A high degree of simulation, however, is impractic- 

 able in an economical telephone plant under usual conditions. In 

 two-wire circuits with many repeaters the echo paths may become very 

 complicated. 



An interesting case of "echoes" is that which may be produced 

 when two radio stations are sending out the same program at the same 

 wavelength. The program as received by one of these stations over 

 wire circuits is, of course, slightly delayed with respect to a station 

 nearer the source of the program. It is possible, then, for a receiving 

 set properly located to receive both of these stations, in which case if 



