Telephone Transmission Over Long Cable Circuits 



By A. B. CLARK 



Synopsis: The application of telephone repeaters has made it possible to 

 use small gauge cable circuits to handle long distance telephone service over 

 distances up to and exceeding 1,000 miles. A general picture of the long 

 toll cable system which is being projected for use in the northeastern sec- 

 tion of the United States was presented recently by Mr. Pilliod and 

 published in the July number of the Technical Journal. 



Many of the circuits in these toll cables are so long electrically that a 

 number of effects, which are comparatively unimportant in ordinary tele- 

 phone circuits, become of large and sometimes controlling importance. 

 For example, the time required for voice energy to traverse the circuits 

 becomes very appreciable so that reflections of the energy may produce 

 "echo" effects very similar to echoes of sound. The behavior of the 

 circuits under transient impulses, even when two-way operation is not in- 

 volved so that "echoes" are not experienced, is very important. In order 

 to keep within proper limits of variation of efficiency with frequency over 

 the telephone range special corrective measures are necessary. Owing to 

 the small sizes of the conductors, the attenuations in the longer circuits 

 are very large. Special methods are, therefore, required to maintain the 

 necessary stability of the transmission, including automatic means for 

 adjustment of the repeater gains to compensate for changes in the resis- 

 tance of the conductors caused by temperature changes. 



THIS paper aims to present an idea of what is involved in the 

 transmission of voice currents over long toll cable circuits. 

 Because of the breadth of the subject covered, no attempt has been 

 made to make the discussions of the various items complete, or to 

 include many of the results of the experimental and theoretical work 

 which contributed to a solution of the problems and which has involved 

 the cooperative efforts of a large number of engineers and investi- 

 gators. This paper should be considered merely as an introduction 

 to the subject. It is hoped that subsequent papers will be presented 

 dealing with these matters in more detail. 



For the benefit of those who are not intimately in touch with tele- 

 phone transmission work, the different types of circuits used in toll 

 cables are first briefly reviewed. The important characteristics of 

 the loading systems are then presented. Following this, various 

 important effects encountered in long cable circuits are discussed 

 and their reactions on the design of cable systems indicated. 



In view of the discussion on telephone repeaters given in the 

 Gherardi-Jewett paper, 1 which was presented before this Institute 

 on October 1, 1919, it will be assumed that the reader of the present 

 paper is tamiliar with the general features possessed by the various 

 types of such devices and, accordingly, no descriptions of them are 

 given, their overall performance only being of interest in the present 

 connection. 



'Transactions of A. I. E. E., Vol. XXXYIII Part 2 Page 1287. 



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