A CARRIER TELEPHONE SYSTEM FOR TOLL CABLES 



89 



complement of carrier pairs becomes quite large. With 40 carrier 

 pairs, for example, the number of coils required is 780. The balancing 

 coils are mounted on panels as shown in Fig. 7 and are connected 

 together in a crisscross arrangement. Each repeater section is 

 balanced separately, the balancing panels being located in the repeater 

 station. 



REPEAT- 

 ERS 



INTERMEDIATE LINE 



SHOWING PATHS OF 



CROSSTALK 



r 



T — TT 

 ^ U- 



:u: 



; \ I — 1- 



■' '- J L 



BALANCING 

 COILS 



Fig. 6 — Method of balancing out crosstalk. 



Other measures are necessary to supplement this balancing tech- 

 nique. To reduce the crosstalk coupling, and also to average the 

 transmission characteristics of different pairs, the carrier pairs in dif- 

 ferent quads of an existing cable are respliced about every mile so as 

 to approach random splicing. The crosstalk coupling between the 

 two sides of a quad is reduced by test splicing at the middle of a 

 repeater section and the quads are split at repeater points. In a new 

 cable, of course, the desired splicing arrangements are introduced at 

 the time of installation. The carrier pairs are also transposed from 

 one cable to the other as indicated in Fig. 1. This avoids interaction 

 crosstalk that would take place, through the medium of the voice- 

 frequency pairs, between the high-level carrier outputs on one side of 

 a repeater station and the low-level inputs on the other side. There 

 is of course, a similar effect between carrier pairs at any point in a 

 repeater section. This is much less serious since no level difference is 

 involved, but it does tend to limit the effectiveness of the balancing 

 over a range of frequencies. 



Reflections resulting from impedance irregularities reverse the direc- 

 tion of propagation and therefore produce far-end crosstalk from 

 near-end crosstalk. This crosstalk cannot readily be balanced out 



