THE VODAS 473 



This figure also shows the relatively narrow frequency range passed 

 by the repeating coil and syllabic frequency filter of the Type B vodas. 



Operating Attendance 

 To insure proper operation of a vodas a technical operator ^ is in 

 attendance. He is provided with circuits which enable him to talk 

 and monitor on the circuit as indicated in Figs. 2 and 5. His duties 

 include adjusting the sensitivity of the receiving relays for the par- 

 ticular value of radio noise existing and adjusting the transmitting and 

 receiving speech volumes by the aid of potentiometers and volume 

 indicators. He selects the proper hangover time and coordinates the 

 operation of the circuit as a whole with the distant end. At times, 

 he may be required to increase the sensitivity of the transmitting side 

 of the vodas in the case of talkers with poor ability to operate relays 

 or to decrease the sensitivity when weak volumes are supplied from 

 land lines with more than the usual amount of noise. 



Summary 



The vodas is used in radio telephony to switch the voice paths 

 rapidly to and fro, and thus prevent echoes and singing that would 

 otherwise occur at unpredictable times. It is also used to save privacy 

 apparatus by permitting the use of the same apparatus for both 

 directions of transmission. The performance characteristics of the 

 electromagnetic relays used in the vodas are very suitable in that 

 they have small operating and constant releasing times. 



Improved performance of the voice-operated relays in the presence 

 of line noise can be secured by the use of a syllabic type of vodas 

 which discriminates between the characteristic voltage-time envelopes 

 of the noise and speech waves. Laboratory and field tests indicate 

 that this device, even without delay circuits, gives slightly better 

 performance on most conditions than the original vodas with delay. 

 When provided with a transmitting delay circuit, the syllabic device is 

 decidedly better than the older vodas. 



References 

 The International Bibliography on the Coordination of Radio 

 Telephony and Wire Telephony is given in the C.C.I.F. Green Book 

 Volume I of the Proceedings of the Xth Plenary Meeting, held at 

 Budapest, September 1934. Below is a chronological list of Bell 

 System papers relating to the vodas. 



1. "The Limitation of the Gain of Two-Way Telephone Repeaters by Impedance 

 Irregularities," George Crisson, Bell Sys. Tech. Jour., Vol. 4, No. 1, January, 

 1925, pp. 15-25. 



