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BELL SYSTEM TECHNICAL JOURNAL 



It may be said in passing that by an increase in sensitivity it is per- 

 fectly possible, with the extremely fast relays used, to omit this delay 

 circuit and obtain satisfactory operation. This would, however, 

 make the device more subject to noise effects. 



Referring to Fig. 5, a delay circuit is 

 also included in the receiving branch of 

 the circuit which, however, performs a 

 somewhat different function from that 

 in the transmitting branch. This delay 

 circuit serves to delay transmission 

 across the hybrid coil, thereby permit- 

 ting the relay RES to operate and apply 

 its protecting short-circuit before the 

 echo from the connected circuit arrives 

 at the input to the transmitting detector. 

 In suppressing the echo from the radio 

 receiver by relay TES a similar action is 

 performed by the delay in transmission 

 over the wire lines to the radio stations. 

 The type of delay circuit used in the 

 voice-operated device just described is 

 shown in Fig. 1 {A). This consists of 

 an electrical network by means of which 

 transmission sent into it is received at 

 its output after a finite time interval. 

 To obtain this delay action a loaded 

 artificial line having low attenuation is 

 employed, in conjunction with a network 

 which balances its "surge" impedance, 

 and a hybrid coil. An interesting fea- 

 ture of the arrangement is that the prin- 

 ciple of reflection, which tends to cause 

 objectionable echoes in telephone circuits, 

 is here usefully employed to pass the 

 voice currents through the artificial line 

 twice. This results in a considerable sav- 

 ing of apparatus. Fig. 7 {B) shows the 

 path of transmission through the delay 

 circuit. Alternating current entering the 

 hybrid coil divides equally between the 

 delay circuit and the balancing network. 

 That part which enters the balancing 



