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BELL SYSTEM TECHNICAL JOURNAL 



from noise. This is done by frequency discrimination as well as by 

 amplitude discrimination and the use of artificial delay. The detectors 

 have their input circuits arranged to keep out frequencies that are not 

 essential for speech operation. In addition, their sensitivity is made 

 adjustable. The transmitting detector is generally worked at a value 

 which results in no perceptible loss of intelligibility due to failure of the 

 transmitting relays to operate, at the same time allowing a maximum 

 amount of line noise to be applied without operating the relays falsely. 

 The receiving detector is adjusted frequently by the technical operator 

 so as to obtain the best operation on incoming speech without false 



OUT 



A. SUPPRESSED 



OUT 



B. UNSUPPRESSED 

 Fig. 3 — Repeating coil arrangement for suppressing echoes. 



operation from radio noise. When the incoming noise is low, relay 3 

 may be made very sensitive. Any incoming speech which does not 

 operate relay 3 is thus weak, and the receiving volume may be kept 

 high without danger of echoes operating the transmitting relays. 

 When the noise is high, relay 3 is made insensitive, requiring more loss 

 in the echo path and, consequently, lower volume to the listener. 



The method of suppressing transmission by opening a single relay 

 contact is illustrated in detail by Fig. 3. In A of this figure, the relay 

 (i? on the figure) is assumed to be operated so that transmission is 

 suppressed. The voltages induced in windings ^Si and 52 of the first coil 

 are opposed to each other in a circuit including P2 and P3 of the second 

 coil, the resulting flux in the core of this coil being very small. Losses 



