THE VODAS 457 



Historical Background 



The two-way problem in telephony began with the invention of the 

 telephone itself, and was the subject of considerable pioneering activity 

 during the latter part of the nineteenth century. The invention of the 

 amplifier brought about new problems when applied in a repeater for 

 two-way operation. Even before a practical repeater had been devised, 

 inventors visualized controlling the direction of transmission through 

 amplifiers in a line by relays controlled from switches associated with 

 the subscribers' instruments, an idea which is in use today on airplanes 

 and small boats and in special circuits where this type of two-way 

 operation is practicable. It is also used by amateur radio telephone 

 operators. But for public telephone service more rapid and automatic 

 control of two-way conversation is preferable. 



To control the direction of transmission in a manner that would meet 

 public convenience, invention progressed through the early part of the 

 twentieth century toward devices for switching the speech paths 

 automatically by voice waves. During this period, long distance radio 

 telephony was first demonstrated to be practical on a one-way basis. 



From that time until the first transatlantic radio telephone circuit 

 was placed in service on January 7, 1927, anti-singing voice-operated 

 devices underwent a process of development aimed at meeting the 

 requirements of two-way radio telephone service. The vodas was one 

 result. Since 1927, improvements have been made in cheapening and 

 simplifying the equipment and in making a vodas that will operate 

 better on speech and not so frequently on noise. It has also been 

 possible to arrange a vodas so as to permit using the same privacy 

 apparatus for both directions of transmission, thereby saving the cost 

 of duplicate apparatus. 



The Radio Telephone Problem 



The conditions encountered when joining two-wire two-way circuits 

 by radio links are illustrated in Fig. 1 in which (o) shows a connection 

 between two subscribers, W and E, while {b) shows the paths of direct 

 transmission and echo when E talks. In addition to the talker and 

 listener echoes which arise in such a connection, singing can occur 

 around the closed circuit CAFGDBC if the amplification is great 

 enough. Also, when the same frequency band is used to transmit in 

 both directions, two cross-transmission paths AB and DF are set up, 

 and echoes and singing can take place around the end paths ABC and 

 DFG. Any echoes or singing are of course primarily due to reflections 

 of energy at points of impedance irregularities in the two-wire plant, 

 including the subscribers' telephones themselves. 



