128 BELL SYSTEM TECHNICAL JOURNAL 



frequency range 36 to 140 kilocycles from the type C and other lower 

 frequencies on the line. The second kind is the directional filters of 

 the type J system itself. These separate a twelve-channel band of 

 frequencies lying below 84 kilocycles used for west to east transmission 

 from the twelve-channel group lying above 92 kilocycles which is 

 transmitted from east to west. These directional filter sets are care- 

 fully designed to equalize any non-uniformity of loss in both the 

 directional and the line filters. As this equalization involves a con- 

 siderable loss over a large part of the filter band it is provided entirely 

 in the receiving directional filters where the transmission is at a low 

 level and the loss can readily be made up by amplification. In this 

 way nearly the full energy output of the transmitting or repeater 

 amplifier is available for line transmission. 



Line Crosstalk Problems 



As noted previously, type J systems will, in general, be applied on 

 pairs on which type C systems are already operating. Such pairs 

 have already been arranged to transmit frequencies up to 30,000 

 cycles, and transposed in such a manner as to perform satisfactorily as 

 regards crosstalk to and from nearby pairs on which similar carrier 

 systems are operating. In addition, on most modern lines the spacing 

 between wires of a pair has been reduced from twelve to eight inches; 

 and, on many of the lines, in order further to reduce crosstalk by 

 increasing the spacing between pairs, the number of pairs on a cross- 

 arm has been limited to four instead of five, omitting the pole pair. 

 Now, by applying a new transposition system designed for type J 

 operation up to 140,000 cycles, an eight-inch spaced four-crossarm 

 line may be arranged to transmit type J frequencies on at least ten 

 pairs out of sixteen. Type C systems may, of course, be used on all 

 of the pairs. Finally by using the most advanced transposition de- 

 sign methods, and increasing the crossarm spacing, in addition to the 

 features noted above, a new line may be constructed to permit the 

 operation of sixteen channels on all pairs. 



To make the pairs of wires good for type J systems, more than a four- 

 fold increase in frequency range, was difficult. The natural tendency 

 of the circuits to crosstalk is increased even more than the frequency 

 ratio, so that in addition to applying a new transposition design it is 

 necessary that the transposition poles be more accurately located, and 

 that the sags of the two wires of each pair be kept more nearly alike. 

 On lines which already have eight-inch spaced wires, no major struc- 

 tural changes are necessary. However, on lines which have only 

 twelve-inch spaced wires and where it is desired to make available a 



