THE VODAS 461 



the direction in which the normally blocked path is exposed to the 

 better signal-to-noise ratio and the normally activated path is exposed 

 to the poorer signal-to-noise ratio. The vodas is, of course, arranged so 

 that the normally blocked (transmitting) side is exposed to the land 

 lines, which are usually quieter than the radio links. In the receiving 

 side, the device can be less sensitive because there is no need for having 

 it completely operated under control of the voice waves. All that is 

 necessary is to have this side sensitive enough to operate in response to 

 comparatively large voice or noise waves which might otherwise, after 

 reflection and passage into the outbound path, result in false operation 

 of the more sensitive side associated with this path. 



In the vodas the principle of balance is used to keep the reflected 

 currents small and thus allow the sensitivity of the normally activated 

 device to be further reduced if necessary. Where a high degree of 

 balance is not obtained and when noise from the radio limits the 

 sensitivity of the receiving device it is sometimes necessary, particularly 

 for weak outgoing volumes, to reduce the incoming volume so as to 

 prevent echoes from operating the normally blocked transmitting side. 



This echo limitation is primarily due to noise in the radio link, 

 reflections from the two-wire plant and weak volumes from the 

 subscribers. It is difficult to produce any large improvement in talker 

 volumes and balance; so it would appear that the solution of the 

 difficulty would probably come from the direction of improving radio 

 transmission. Some benefit has also been obtained by reducing the 

 effect of radio noise on the vodas with special devices of which the 

 "Compandor" '''• ^^ and the "Codan" ^^'2" are examples. More re- 

 cently, use has been made of a new voice-controlled device called a 

 "Noise Reducer" -^' '^^ which reduces the received noise between speech 

 sounds. 



Vodas Design — Type A Control Terminal 



Figure 2 shows a schematic diagram of a vodas * arranged to use the 

 same privacy device for both transmitting and receiving. This is 

 the type used on transatlantic and other long routes. Since the 

 operation of this arrangement has been described before, ^^ it will not 

 be repeated here. 



The diagram of the relay circuit in Fig. 3 shows how various time 

 actions are obtained. Relays 1, 2, 4 and 5 are operated from battery 

 Bi when the ground contact of relay TM is opened. Thus the travel 

 time of any relay armature is not a factor in securing fast initial 



* The vodas apparatus, together with the volume control devices and technical 

 operator's circuits, go to make up what is called a Type A Control Terminal. 



