144 BELL SYSTEM TECHNICAL JOURNAL 



removed from the carrier pairs. In most existing voice frequency toll 

 cables the 19-gauge quads were spliced as three groups, one a two- wire 

 circuit group, one an east bound four-wire circuit group and the third a 

 west bound four-wire circuit group. Ordinarily, the carrier pairs will 

 be selected from the four-wire groups because these groups are usually 

 larger than the two-wire group and since the quads within a group are 

 spliced at random there is less chance of a large value of coupling be- 

 tween pairs of different quads, i.e., two pairs are less apt to be recur- 

 rently in a relation of high coupling. The carrier pairs are divided 

 equally between the two four-wire groups, in order that the least number 

 of four-wire voice circuits will be lost. 



In cables with large four-wire groups it is satisfactory to maintain 

 the grouping arrangement on the pairs converted to carrier. In such 

 cables, however, one four-wire group is in the center or core of the 

 cable and the other group in the outer periphery. In order that all 

 circuits will have about the same velocity and attenuation and be sub- 

 jected to about the same temperature conditions for both transmission 

 and crosstalk reasons, one (four-wire) carrier group in these cables 

 will be spliced to the other (four-wire) carrier group and vice versa at 

 the 6000 -foot intervals. 



In cables with relatively small four-wire groups, there is more chance 

 of two pairs being recurrently in a relation of high coupling. To re- 

 duce this chance, a special splicing arrangement has been devised for 

 use at the 6000-foot intervals. With existing splicing the maximum 

 coupling in cables with small groups is about 2.5 times that for cables 

 with large groups. This ratio is appreciably reduced by the special 

 splicing, likewise reducing the maximum mutual inductance that must 

 be supplied by the balancing unit. 



The foregoing was with particular reference to crosstalk between 

 pairs in different quads. Crosstalk between pairs in the same quad 

 (side-to-side crosstalk) is an additional problem. A quad consists of 

 two twisted pairs of wires which are twisted together to permit the use 

 of voice frequency phantom circuits. Since the two sides of a quad 

 are so closely associated, side-to-side crosstalk is generally much greater 

 that than between pairs of different quads. The electrical size of the 

 balancing unit, therefore, is determined by the side-to-side crosstalk, 

 which is reduced by "poling." 



To apply poling, the quads are carried through as quads for an entire 

 carrier repeater section. From measurements of side-to side crosstalk 

 in phase and magnitude, quads in one half repeater section are chosen 

 and spliced to quads in the other half in such manner as to partially 

 neutralize the side-to-side crosstalk. In effect, quads in one half-sec- 

 tion serve as balancing units for the other half. 



