Certain Factors Limiting the Volume Efficiency of 

 Repeatered Telephone Circuits 



By LEONARD GLADSTONE ABRAHAM 



Vacuum tube amplifiers are now regularly built into long distance tele- 

 phone circuits where required to maintain their volume efficiency. Con- 

 sequently, the overall volume efficiency of these circuits no longer depends 

 to any important extent on the loss per unit length of the line wires. In- 

 stead, the efficiency is controlled by certain factors which, before amplifiers 

 were introduced, had negligible effect. Among these factors are echo, 

 singing or "near singing," and crosstalk. The stability of the lines and 

 amplifiers also becomes very important. 



This paper sets forth the methods now in use in the Bell System for 

 computing the highest volume efficiencies at which telephone circuits may 

 be worked without causing echo, singing or crosstalk effects to become too 

 serious. The matter of making proper allowance for the normal variability 

 of the circuits is also included. Specific references are made to various 

 sources of published data which permit the methods to be applied to obtain 

 practical working figures for cable circuits. The fundamentals, however, 

 are also applicable to open-wire circuits. 



THE excellence of transmission over a toll telephone circuit is 

 determined by its overall volume efficiency (including the effect 

 of variations from time to time) , by distortion of the waves, by various 

 delay effects and by the masking effect of noise. The term "net 

 loss" ^ is commonly used to more specifically designate the overall 

 volume efficiency as limited by the factors which will be discussed 

 herein. It is equal to the total loss introduced by the toll lines and 

 all associated apparatus minus the total gain introduced by all of the 

 amplifiers. In the United States the net loss is usually given for the 

 single frequency of 1,000 cycles and is expressed in decibels. 



To avoid producing an undue amount of echo, singing (or near 

 singing), or crosstalk in repeatered circuits, the net loss must be kept 

 above certain minimum figures. The net loss which safely meets 

 requirements for echo, singing and crosstalk after making due allow- 

 ance for transmission variations in the circuit is called the "minimum 

 working net loss." This paper discusses the methods used in the Bell 

 System for predetermining the minimum working net losses of tele- 

 phone circuits, particularly those in cable, for which references to 

 published data are made which will enable telephone transmission 

 engineers to readily carry out the required computations. 



A telephone circuit may be used for terminal business only (i.e., 



^ The net loss of a circuit is the insertion loss of the circuit between 600-ohni 

 impedances. 



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