A Carrier Telephone System for Toll Cables * 



By C. W. GREEN and E. I. GREEN 



A new 12-channel carrier telephone system for existing cables 

 is described. This system, which incorporates a number of inter- 

 esting departures from the previous carrier art, is now being 

 manufactured in considerable quantities to meet increased traffic 

 requirements. 



AN important advance in the art of carrier telephony has been made 

 by the development of a new 12-channel system, known as the 

 type K, for toll telephone cables of existing type. It is applicable 

 both to cables installed underground, and also to aerial cables, for 

 which the wide range of temperature variation introduces quite dif- 

 ficult transmission problems. Field trials on cables previously 

 installed between Toledo and South Bend have been successful, and 

 the system is now being manufactured to meet field demands. 



This new development is an outgrowth of the experiments at Mor- 

 ristown, New Jersey, described by Messrs. Clark and Kendall before 

 the American Institute of Electrical Engineers in 1933,^ and the essen- 

 tial principles of the new system were included in those experiments. 

 The earlier work dealt, however, with cable specially designed for 

 carrier operation, and only underground cable was experimented with. 

 As that work drew to a close, it became clear that because of general 

 economic conditions several years would elapse before the Bell System 

 would require any substantial increase in toll facilities. Hence this 

 early system was not put into commercial form, but work was con- 

 tinued to determine the extent to which carrier could be applied to 

 existing cables, of which more than 15,000 miles were available for 

 such use. Serious problems of cross-talk at high frequencies had to 

 be reckoned with. A more serious problem, however, was that of 

 maintaining stability of transmission, since with aerial cable, which 

 comprises about two-thirds of the existing cable mileage, the total 

 variation in attenuation, due to temperature variation, is about three 

 times that for underground cable, and the rate of variation not infre- 

 quently is several hundred times as great. 



In spite of these and other difificulties, the capabilities of the 

 present system go far beyond those of previous systems. As a develop- 



* Presented at Winter Convention of A.I.E.E., New York, N. Y., Jan. 24-28, 1938. 

 ' For references see end of paper. 



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