82 BELL SYSTEM TECHNICAL JOURNAL 



is shown between the repeaters transmitting in the two directions, 

 since to keep the conductors carrying weak power separated from 

 those carrying strong power, it is merely necessary to keep the ap- 

 paratus and cabling connected to the inputs of the repeaters separated 

 from the apparatus and the cabling connected to the repeater outputs. 



V. Steady State Distortion 



The possible sources of distortion may be divided broadly into (1) 

 repeaters and auxiliary apparatus and (2) the lines. 



With reference to the distortion introduced by the repeaters, the 

 vacuum tube is fortunately very nearly perfect, at least in so far as 

 concerns practical telephony. At one time, for purposes of test, a 

 circuit was set up containing 32 vacuum tubes in tandem. On this 

 circuit the distortion was so small that when listening to ordinary 

 conversation it was difficult to detect any difference in the quality 

 of transmission before and after traversing the 32 vacuum tubes. 



It is beyond the limits of this paper to enter into the problems of 

 design which were encountered in the development of the repeater 

 circuits. For the present purpose of considering the overall per- 

 formance of repeatered circuits in cable no serious error will be made 

 if it is assumed that the complete repeater circuits meet the require- 

 ments for an ideal repeater as set up in the Gherardi-Jewett paper. 



Considering next the lines, it is necessary to make the loading very 

 regular so that balance difficulties will not cause an undue amount 

 of trouble on two-wire circuits. Regularity of the loading is also 

 essential in order to avoid irregular transmission of different fre- 

 quencies. In order to secure this regularity of loading, it is necessary 

 that the spacing between loading points be made very uniform and 

 that the cable be so manufactured that the electrostatic capacity 

 of its circuits be held within close limits. The loading coils them- 

 selves must be closely alike in their electrical properties and further- 

 more, the coils must be stable, i. e. these electrical properties must 

 not change appreciably due to the passage of voice currents or other 

 currents required for cable operation through them. 



Next, it is necessary to design the repeaters and associated apparatus 

 used on the longer circuits, particularly the four-wire circuits, so as 

 to put in different amounts of gain at different frequencies, thereby 

 making the overall transmission at different frequencies approximately 

 constant in spite of the fact that the loss introduced by the cable 

 circuits at different frequencies is not constant. Figure 9 shows the 

 overall or net transmission equivalent plotted against frequency for 



