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BELL SYSTEM TECHNICAL JOURNAL 



ponent is apparent from Fig. 5, which shows the net loss per mile at 

 temperatures of 0° F., and 110° F., assuming that the attenuation has 

 been equalized so as to obtain a flat characteristic at 55° F., and that 

 the gain is then adjusted so as to hold the transmission constant at 28 

 kilocycles as the temperature varies. Although the twist is small 

 enough so that it need not be corrected at each repeater, it is too large 

 to be allowed to accumulate over a very long distance. 



0.06 



0.04 



1 ^ 0.02 



O I/) 

 QO 

 -^0.04 



0.06 



5 10 15 20 25 30 35 40 45 50 55 60 65 70 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 5 — Twist characteristics of 19-gauge non-loaded cable pair. 



Crosstalk 



As noted above, crosstalk between opposite directions of transmission 

 is avoided by using two separate cables (or shielded compartments in 

 the same cable). To prevent crosstalk in offices, special measures are 

 employed. There remains the problem of "far-end " crosstalk between 

 pairs in the same cable which transmit in the same direction. The 

 pairs are packed closely together, and substantial crosstalk occurs 

 between them because of small departures from symmetry and slight 

 imperfections of twisting. However, by abandoning the use of 

 phantoms and by connecting small adjustable mutual inductance coils 

 between each carrier pair and every other carrier pair, sufficient cross- 

 talk reduction is obtained to permit transmission up to 60 kilocycles 

 on a substantial number of pairs.^ The scheme is illustrated in Fig. 6. 



In the original Morristown cable, the crosstalk was reduced in part 

 by separating the 16-gauge carrier pairs from one another by 19-gauge 

 quads which served as spacers. With existing cables, however, the use 

 of spacers would be impracticable since this would require resplicing 

 the cable at every joint, and therefore reliance must be placed largely 

 on balancing. Since the number of combinations to be balanced 

 increases approximately as the square of the number of pairs employed 

 for carrier, the number of balancing coils required for even a moderate 



