64 



BELL SYSTEM TECHNICAL JOURNAL 



part of the energy is diverted to short circuit the lower branch of the 

 circuit, as indicated. Meanwhile, the main transmission passes on to 

 the subscriber at the right. Echoes which return in the lower part of 

 the circuit are blocked as indicated. After the talker has ceased 

 speaking, the device remains operative for a time equal to the delay 





\-ny^ 



TALKER 



STATION 



W 



'T^yM 



E-W 



W-E 





r^y^J^J^^^ LISTENER 

 uvvvvv STATION 



■^z^ 



(a) 



DIRECT TRANSMISSION 



DIRECT 

 TRANSMISSION 



ECHO 



(b) 

 Fig. 7 — Echo suppressor cutting off echo in four-wire circuit. 



of the echo as measured from the input of the device to the disabling 

 point plus an additional time to take care of echoes in the circuit be- 

 tween the four-wire terminal and the subscriber. 



When the suppressor releases, the circuit is again free to transmit 

 in either direction. When the right-hand subscriber talks, the action is 

 similar except that the other half of the echo suppressor operates. 



In practical use the echo suppressors are so carefully controlled that 

 telephone users are generally unable to tell whether a suppressor is on 

 the circuit or not. This is due to the short delays and careful adjust- 

 ments of the time functions of the device. It is possible, however, if 

 the delays are longer and the adjustments are made with less care, to 

 introduce two types of difficulty. If one subscriber talks fairly steadily 



