Carrier Systems on Long Distance Telephone Lines ^ 



By H. A. AFFEL, C. S. DEMAREST and C. W. GREEN 



Synopsis: Two previous papers before the American Institute of Elec- 

 trical Engineers discussed the activities of the Bell System in the develop- 

 ment of multiplex telephone and telegraph systems using carrier current 

 methods. The present paper describes developments which have resulted 

 in improvements in the carrier telephone art during the past few years. A 

 new, so-called type "C" system is described in detail, together with suitable 

 repeaters and pilot channel apparatus for insuring the stability of operation; 

 the line problems are considered and typical installations pictured. The 

 growth of the application of carrier telephone systems and their increasingly 

 important part in providing long distance telephone service on open-wire 

 lines are shown. 



Introduction 



AT the 1921 Midwinter Convention of the American Institute of 

 Electrical Engineers, Messrs. Colpitts and Blackwell presented a 

 paper entitled "Carrier Current Telephony and Telegraphy." This 

 described the development work of the Bell System and the resulting 

 commercial types of multiplex telephone and telegraph systems using 

 carrier current methods. The paper also gave a brief historical 

 summary and included a theoretical discussion of the methods in- 

 volved. 



The carrier current art had at that time emerged from the laboratory 

 to play its part in meeting the practical requirements of telephone 

 service in the field. This step was made possible largely by two 

 tools, now indispensable to the communication engineer, the thermionic 

 tube and the wave filter. 



In an ordinary telephone circuit, each frequency component in 

 the voice of the speaker is transmitted by an electrical current of 

 the same frequency. In most cases the electrical equipment of the 

 circuit is not called upon to transmit frequencies above about 3,000 

 cycles per second. In carrier current operation, however, the voice- 

 frequency currents are caused to modulate a high-frequency current 

 which thus serves as a "carrier" for the message. In this way, an 

 additional telephone channel is obtained, using frequencies entirely 

 above those transmitted in connection with the ordinary voice fre- 

 quency channel. By using other high frequencies, several additional 

 messages may be transmitted simultaneously on the same pair of 

 wires. Each channel occupies a certain range of high frequencies. 

 For example, the words of one speaker may be conveyed by a channel 

 employing frequencies from about 23,500 to about 26,000 cycles per 



' Presented before the Summer Convention of the American Institute of Electri- 

 cal Engineers, June 29, 1928. 



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