140 BELL SYSTEM TECHNICAL JOURNAL 



Interaction Crosstalk 



The crosstalk currents from a carrier repeater output into voice 

 frequency circuits in the same cable must be limited, since they cross- 

 talk again into carrier circuits near repeater inputs and, consequently, 

 are amplified by the high gain repeaters. Intermediate circuits most 

 responsible for crosstalk of this type are made up of combinations of 

 pairs and phantoms and the sheath, i.e., longitudinal paths. 



One case of crosstalk of this kind would occur if the same cable were 

 used for carrier pairs transmitting in the same direction on both sides of 

 a repeater. This is prevented by transposing carrier pairs from one 

 cable to the other at each repeater point, as shown on Fig. 1. 



A second interaction crosstalk problem is encountered at the com- 

 mon voice and carrier repeater points and involves coupling between 

 cables as well as in the same cable. Here the coupling path is from 

 carrier repeater outputs to intermediate circuits in the same outside 

 cable, back into the common office over these intermediate circuits and 

 then via office coupling to intermediate circuits in a second outside 

 cable and from there to carrier repeater inputs connected to pairs in 

 the second cable. Referring to Fig. 1, a set of noise (and crosstalk) 

 suppression coils is encountered in this path. The high longitudinal 

 circuit impedance of these coils minimizes this interaction crosstalk. 



Far-End Crosstalk 



Far-end crosstalk currents are subjected to line attenuation and 

 amplification similarly to the main transmission currents, and do not 

 have extra amplification as in the case of near-end crosstalk. Further- 

 more, far-end crosstalk currents due to couplings at different points 

 along the line tend to arrive at the distant end of the disturbed circuit 

 at the same time. Hence a considerable portion of the far-end cross- 

 talk over the type K frequency range, which occurs between circuits 

 transmitting in the same direction in the same cable, can be neutralized 

 by introducing compensating unbalances at only a comparatively few 

 points, such as one per repeater section. The far-end crosstalk reduc- 

 tion problem is greatly simplified because phantom circuits are not 

 used for carrier operation. 



Theoretically, for the same precision of match between the im- 

 pedances in the two directions at the balancing point, the crosstalk re- 

 duction would be about the same whether the balancing is done at an 

 intermediate point or at either end of a repeater section. Balancing 

 will be done at repeater inputs rather than at an intermediate point, 

 such as the middle, because it is practicable to obtain repeater im- 



