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BELL SYSTEM TECHNICAL JOURNAL 



nected and the transmitter disconnected. Speech coming from the 

 wire line connects the transmitter and disconnects the receiver. The 

 positive switching action is, therefore, dependent upon the impulses 

 of speech from the land line. This arrangement is preferred to the 

 reverse one of depending upon impulses of speech receiver over the 

 radio channel. This is because the system must operate on speech 

 only and not noise, and the speech-to-noise-ratio is usually higher and 

 more dependable on the wire line than on the incoming radio channel. 

 This single function of switching-over in response to the subscriber's 

 voice is the principal and basic function of such devices. There are, 

 however, many auxiliary features incorporated to guard against false 



TERMINAL office: 



RADIO 

 ~ PECFIVER 



J 



J 



TRAMS- 

 WITTER 



Fig. 6 — Circuit diagram illustrating operation of voice-operated switching device, 

 Note: Voice currents coming from the line, rectified in the transmitting detector, 

 clear the transmitting path by removing short circuit at 55 and short-circuiting 

 receiving path at TES. Switch at RES is operated by received radio speech or 

 noise to prevent echoes in the wire lines from reaching transmitting detector. 



operation by noise currents and speech current echoes which greatly 

 increase the ability of the arrangement to operate satisfactorily under 

 conditions of severe noise or weak speech. These have been described 

 elsewhere * more completely than would be appropriate in this dis- 

 cussion. 



Viewed from the radio standpoint these voice-operated devices are 

 of great importance since they permit radio links to be used as trunks 

 in wire networks without their having to meet the requirements which 

 wire line trunks must meet. At the present stage of development it 

 would be practically impossible to provide radio circuits meeting wire 

 line standards. 



* See bibliography 7. 



