nVO-lVAY RADIO TELEPHONE CIRCUITS 373 



The vodases in Fig. 6 are shown for the condition when no speech is 

 being transmitted. Relay 1 keeps the transmitting circuit blocked so 

 that singing cannot occur around the complete circuit or through a 

 local radio path and terminal. Transmission is free to pass the con- 

 tacts of relay 2. When the United States subscriber speaks, voice cur- 

 rents go into the transmitting detector and delay circuit. While they 

 are traversing the delay circuit, relays 1 and 2 become operated pro- 

 vided relay 3 has not been operated previously. The operation of relay 

 1 permits the voice currents to travel on to the radio transmitter. The 

 operation of relay 2 blocks the receiving path and prevents echoes and 

 singing that might otherwise occur when relay 1 is operated. 



Upon being received at the distant end, the voice currents operate 

 relay 3 from the receiving detector, thus protecting the transmitting 

 detector and relays against operation by echoes of received speech 

 currents. These echoes are returned from unbalances in the two-wire 

 portion of the connection beyond the terminal. The receiving delay 

 circuit delays- the speech long enough to insure complete operation of 

 relay 3 before the echoes return. When the subscriber stops speaking, 

 the relays return to normal. 



By adding two more relays to the transmitting side of the vodas, it 

 is possible to save part of the apparatus which is used to increase pri- 

 vacy on the circuit. This saving is made by using the same privacy 

 device for both transmitting and receiving. This is possible provided 

 the action of the privacy device is the same for distorting the voice 

 waves at the transmitting end as it is for restoring them at the receiving 

 end of the circuit. An arrangement of the vodas having this feature, 

 which is now in general use, is illustrated in Fig. 2. The apparatus 

 additional to the simple vodas consists of relays 4 and 5, the privacy 

 device, a hybrid set, and two one-way repeaters. In Fig. 2 this 

 apparatus is labeled "Privacy Switching Circuit." 



The action of the device shown in Fig. 2 on transmitting speech 

 waves is as follows : Useful waves from subscriber A are impressed on a 

 potentiometer ahead of the transmitting repeater, which is kept ad- 

 justed by the technical operator to maintain constant volume at the 

 output of the transmitting repeater. The waves then pass into the 

 vodas where they first reach the transmitting delay circuit and are 

 stored for a short interval. A small part of these waves enters the 

 transmitting detector and operates relays 1,2,4, and 5, provided relay 

 3 has not been operated by the receiving detector previously. The 

 interval of the transmitting delay circuit is several times as great as the 

 operating time of relays 1, 2, 4, and 5 so that initial weak parts of 

 speech syllables may be stored in this delay circuit until stronger parts 

 arriving later have had a chance to operate the relays. 



