TOLL TRANSMISSION IN THE UNITED STATES 



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only up to a few thousand cycles, that is, these cables were not designed 

 to meet crosstalk requirements at high frequencies. Crosstalk between 

 pairs in a cable arises from a lack of geometrical symmetry between 

 each pair and every other. As a result of very careful research, we 

 have developed a method of connecting small mutual inductance 

 coils, or condensers, or both, between all the pairs concerned, and in 

 this way reducing crosstalk to a satisfactory degree. At present coils 

 alone are being used. Figure 10 shows schematically the balancing 

 method. 



I REPEAT-I 

 I ERS 



INTERMEDIATE LINE SHOWING 

 PATHS OF CROSSTALK 



BALANCING COILS 





-1 r 1 ' 







J u 



-J L^ 



REPEAT- 

 ERS 





Fig. 10 — Diagrammatic representation of method of balancing out cross-talk. 



It is obvious that in this process of balancing out the crosstalk, 

 the value given to the adjusting coil or condenser must be determined 

 for each particular unit involved. When the balancing units have 

 been installed and once adjusted, however, we feel that they will 

 remain permanent. Present indications are that by adopting this 

 procedure, we can employ frequency ranges up to 60,000 cycles upon 

 our toll cables, and this will permit us to secure 12 one-way channels 

 on each pair of conductors. With at least our present type of cables, 

 we anticipate that it will be necessary to use separate cables in opposite 

 directions to avoid the additional crosstalk that arises when adjacent 

 pairs are used to transmit in opposite directions. Referring to our 

 present cables, if the pairs which it is desired to utilize are loaded, 

 it is necessary first to remove the loading coils. Repeaters will be 

 spaced about 17 miles (27 km.) apart. Since the present repeater 

 points on cables are about 50 miles (80 km.) apart, it will be necessary 

 to add on the average two new repeater points between existing 

 repeaters. The total amount of apparatus at these new repeater 

 points, however, does not bulk very large, because a repeater handling 

 12 channels will be no larger than the older type voice repeaters 



