TRANSOCEANIC RADIO TELEPHONE DEVELOPMENT 561 



radio signal strength and radio noise. The employment of long dis- 

 tance radio telephony in commercial use was preceded by experimental 

 operation and tests which gave a considerable fund of statistical in- 

 formation covering the cyclical changes characteristic of overseas radio 

 transmission. 



The realization that a relatively high degree of reliability was essen- 

 tial to success discouraged any attempt at commercial service until 

 high-power transmission on a practical basis was assured by the inven- 

 tion of a method of making water-cooled tubes. 



In searching for the most efficient way of applying the power made 

 available by water-cooled tubes telephone engineers were led to the 

 employment of a method which had already been successfully used in 

 high-frequency wire telephony. This method, now well known to 

 radio engineers, is called single-sideband suppressed-carrier transmis- 

 sion. As compared with the ordinary modulated carrier transmission, 

 it increases the effectiveness of a radio telephone system by about 10 

 to 1 in power. This accrues partly because none of the power capacity 

 of the transmitter is used up in sending the non-communication bearing 

 carrier frequency and partly because the narrower band width permits 

 greater selectivity and noise exclusion at the receiver. 



A very important final element was also necessary to prevent voice- 

 frequency singing through residual unbalances and around the entire 

 radio link when wire circuits and radio channels are connected together. 



Recourse was again had to a device newly worked out for wire 

 telephone transmission. By associating together and electrically inter- 

 locking several of the voice current operated switching devices which 

 had been developed for suppressing echoes on long wire lines, an ar- 

 rangement now commonly known as a "vodas"^ was developed. 

 When the subscriber talks, his own speech currents, acting on the 

 vodas, cause it to connect the radio transmitter to the wire line and at 

 the same time to disconnect the radio receiver. When the same sub- 

 scriber listens the connection automatically switches back to the re- 

 ceiver. No singing path ever exists. The amplification, in the two 

 oppositely directed paths can be adjusted substantially independently 

 of each other, and constant full load output from the radio transmitters 

 is secured. With this device it became possible to connect almost any 

 telephone line to a radio system and to adjust amplification so that a 

 weak talker over a long wire line could operate the radio transmitter as 

 effectively as a strong local talker. 



2 This word, "vodas," Is synthesized from the initial letters of the words "voice- 

 operated device, anti-singing." 



