378 BELL SYSTEM TECHNICAL JOURNAL 



as high as 75 db are produced by this arrangement by proper design of 

 coils and adding a small condenser (C) to balance the capacity (C) of 

 the leads to the relay contacts. In practice, a pair of wires is cut off to 

 give the right capacity, and then laced into the cable form. In B of the 

 figure, the circuit is shown in the normal condition. Transmission 

 through the coils is affected only by their normal transition loss. While 

 the windings ^i and S2. in A are effectively in series opposing, in B they 

 are connected by separate circuits to the corresponding windings Pi and 

 Pz, due to the extra path through the relay contact. 



Operation of a Radio Telephone Circuit 



Having in the vodas a means for suppressing echoes and singing 

 under extreme conditions (with the additional advantages of suppress- 

 ing intermediate "cross-transmission" paths), it is important to con- 

 sider the broader application of such a device to radio telephone cir- 

 cuits. Three cases of operation with anti-singing devices are of 

 interest: 



1. Vodas at one end, plain hybrid set at the other. 



2. Vodas at one end, echo suppressor (without anti-singing relay) 

 at the other. 



3. Vodases at both ends. 



1. Vodas at One End, Plain Hybrid Set at the Other 



This arrangement is shown in Fig. 4. In this and the next figures, 

 the privacy switching circuit has been omitted for simplicity. The 

 vodas prevents singing and echo effects from unbalances at the A end 

 and also prevents the A subscriber from hearing echoes. 



The disadvantages of these arrangements may be understood by 

 considering the transmission received at the B end which has no voice- 

 operated relays. Speech received over the circuit would be returned to 

 the local radio transmitter as an echo or echoes. If a weak talker were 

 connected at the B end, the volume control device would amplify these 

 echoes to an appreciable extent. In addition to overloading the radio 

 transmitter, such echo would permit both sides of the conversations to 

 be broadcast from the same station, thus reducing privacy. Radio 

 noise might also be received at B and transmitted as echoes to the A 

 end of the circuit. In addition, line noise from a two-wire circuit at 

 the B end would be freely transmitted to the A end, causing a limitation 

 of the sensitivity of relay 3 and consequently a reduction of volume 

 at the A end. 



