THE VODAS 459 



In wire circuits, simple hybrid coils and echo suppressors ^ are 

 usually adequate to prevent such effects because the gains are not 

 increased to provide for loading the circuit with energy when speech is 

 weak, and also because the cross-transmission paths are absent. In 

 long radio circuits, however, singing may result from the adjustments 

 of amplification made to load the radio transmitter in case of weak 

 speech and thus override noise, even though separate frequency bands 

 are used in the two directions. Moreover, it is desired that the users of 

 the service have as good transmission over the entire connection, 

 including these radio links, as that to which they are accustomed in 

 their own wire telephone systems, and even better transmission may 

 be desired owing to differences in the language habits of the sub- 

 scribers. Consequently, the overall transmission efficiencies of inter- 

 continental radio circuits are sometimes better than those of the best 

 land lines in the areas to be interconnected. 



Fundamentals of Vodas Operation 



A voice-operated device to suppress singing effects can be designed 

 to have three possible arrangements: 



1. The terminal can normally be blocked in one direction and con- 

 nected through in the other. 



2. Both directions of transmission can normally be blocked and 

 activated in either direction but not both directions by the voice 

 waves. 



3. The circuit can remain activated in the last direction of speech 

 and blocked in the other direction. 



Where there is no noise on the transmission system under con- 

 sideration any of these three arrangements will give satisfactory opera- 

 tion as there is then nothing to prevent making the voice-operated 

 devices as sensitive as may be necessary to obtain full operation on 

 weak as well as on strong voice waves. If there is any noise on the 

 system which tends to operate the device it is necessary to make it less 

 sensitive to avoid false operation. A point may be reached where the 

 sensitivity is so low that the weakest parts of speech will not cause 

 operation, and the weak consonants will be lost. The reduction in 

 articulation has been found to be proportional to the time occupied by 

 these lost or "clipped" sounds.^ 



If the device is located at a point in the circuit where the signal-to- 

 noise ratio coming from one direction is poorer than that coming from 

 the opposite direction it is obvious that a considerable advantage will 

 be gained by using arrangement 1, since the device may be pointed in 



^ See references at end of text. 



