THE VODAS 467 



increasing the sensitivity of relay {K), when speech is present. Even 

 if the noise is strong enough to operate relay {K) over the upper 

 branch when the gain is inserted, the release of relay {F) at the end 

 of a speech sound will remove the gain and permit {K) to fall back. 

 Thus, it is possible to work relay {K) more sensitively on weak speech 

 than would be possible without the syllabic device. 



Figure 6 shows a photograph of a B-type terminal in ship-to-shore 

 service at Forked River, New Jersey. The vodas and volume control 

 apparatus are in the left-hand cabinet. The right-hand cabinet 

 contains privacy apparatus, a signaling oscillator and a vodas relay 

 test panel. 



Performance 



In any system employing voice-operated devices it is necessary for 

 the time actions to provide for to-and-fro conversation with a minimum 

 of difficulty when the subscribers desire to reverse the direction. 

 The electromagnetic relays used in the vodas have advantages over 

 other types of switching arrangements which have been proposed in 

 that they (1) operate and release at definite current values, (2) have 

 fast operating and constant releasing times, (3) have their windings 

 and their contacts electrically separated, thus simplifying the circuits, 

 and (4) operate in circuits having low impedances. 



The operating times of the two types of vodas are shown in Fig. 7 

 as a function of the strength of suddenly-applied single-frequency sine 

 waves in the voice range. These measurements were made with a 

 capacitance bridge.^ The sensitivities of the two types were adjusted 

 so that observers noted an equivalent amount of clipping. The Type 

 A vodas was provided with a 20-millisecond delay circuit; the Type B 

 had no delay. For the Type A vodas, the operating time is quite 

 small and constant just above the threshold of operation. 



For weak inputs the operating time of the syllabic device is de- 

 termined by relay (/) and the filter, as shown in Fig. 7. As the 

 suddenly-applied input is increased, a point is reached where the less 

 sensitive detector operates relay {K), reducing the operating time 

 from around 20 milliseconds to values comparable to those of the 

 Type A. 



The operation was also tested on waves formed by applying simul- 

 taneously two sine waves of equal amplitude but slightly different 

 frequencies. These waves were recorded on an oscillograph, together 

 with a d-c. indication of the operation of each of the vodas relays, with 

 the sensitivities adjusted the same as for Fig. 7. The time from the 

 beginning of a beat wave (null point) to the time of operation was 

 measured from these oscillograms and plotted against various values of 



