TWO-WAY RADIO TELEPHONE CIRCUITS 375 



Operation of relay 1 clears a path for the speech waves after they 

 pass through the transmitting delay circuit. From there they pass 

 into the privacy switching circuit traveling downward to the privacy 

 hybrid set. Here they divide and the useful portion travels through 

 the privacy device. At its output the waves divide, part being ampli- 

 fied by the transmitting and part by the receiving privacy repeater. 

 The subscriber at A is prevented from hearing echo by the operation of 

 relay 2 which disables the receiving transmission path in a way to be 

 described later. The portion of the waves which travels upward 

 through the transmitting privacy repeater, however, is now free to 

 pass on to the wire line and radio link due to the operation of relay 4. 

 Operation of relay 5 prevents any echo due to direct transmission from 

 the local radio transmitter to the radio receiver from being passed into 

 the privacy device and thus causing distortion of the outgoing waves. 



Action of this device on incoming speech waves is as follows: The 

 waves after being detected by the radio receiver travel over the wire 

 line and, provided the transmitting relays have not been previously 

 operated, pass through the first repeating coil combination and into 

 the lower side of the privacy hybrid. There they divide and the useful 

 part passes through the privacy device where it is restored to an intel- 

 ligible form. At the output of the privacy device, the waves are ampli- 

 fied by both transmitting and receiving privacy repeaters. Any re- 

 transmission of these waves from the local transmitter is prevented by 

 relay 4 which is now released. At the output of the receiving privacy 

 repeater the waves pass through the second repeating coil combination 

 and thence to the receiving delay circuit. A part of these waves, if 

 they are strong enough, may operate the receiving detector and thus, 

 relay 3. 



After passing through the receiving delay circuit the speech waves 

 travel on through a receiving potentiometer, receiving repeater, hybrid 

 set, and to the subscriber at A. 



A small part of these waves may be reflected due to the difference in 

 impedance between the subscriber's line and the network and return 

 over the transmitting branch of the circuit. However, if these waves 

 are strong enough to operate relay 3, they are prevented from operating 

 the transmitting relays. If they are too weak to operate relay 3, an 

 adjustment of receiving volume is made so that they will be too weak 

 to operate the transmitting relays. This adjustment is accomplished 

 by the potentiometer ahead of the receiving repeater. The waves 

 are prevented from passing through the privacy device by relay 1 which 

 is released. 



In practice, it is necessary to protect the vodas against operation 



