LO.iniXG FOR TI'.I.P.PIIOXi: CIRCrnS 267 



Considering now the cross-talk between four-wire circuits in the 

 same quad, it is to he noted that the cHrectional effects of the tele- 

 phone repeaters block the transmission of cross-ialk in the one-wa\- 

 path l)ack to the near end of the circuit, and consequently the special 

 cross-talk adjustments on the coils for four-wire H-41-2o circuits are 

 made primarily for reducing far-end cross-talk. 



In two-wire circuits, near-end and far-end cross-talk both occur, 

 and generalK' near-end cross-talk is much greater because its "axerage"' 

 cross-talk })ath has less attenuation than that of the fai-end cross-talk. 

 ConsequentK', the special cross-talk adjustments made in the two- 

 wire circuit coils are for the purpose of reducing the near-end cross- 

 talk to a minimum. 



In the foregoing connection, it is to be noted that the cross-talk 

 current caused by electromagnetic unbalances flows around the two 

 ends of the disturbed circuit in series. On the other hand, the cross- 

 talk current caused by electrostatic unbalances divides and flows 

 from its point of origin in opposite directions around the tw^o ends of 

 the circuit in parallel. Consequently, when electrostatic and electro- 

 magnetic cross-talk currents are in ])hase at one end of the circuit, 

 they will be practically in phase opposition at the other end of the 

 circuit. The special cross-talk adjustments are made in such a way 

 as to get the maximum benefit from the phase opposition at the 

 particular end of the circuit where the reduction is more important. 



In the four-wire type of circuit used in very long cable circuits, 

 relatively large amplification gains are possible in the repeaters 

 because of the characteristic circuit feature which allows the re- 

 peaters to act as one-way amplifiers. As a result of these high am- 

 plifications, there are large differences in power level on the input 

 and output sides of the repeaters. This fact has made it desirable 

 for cross-talk reasons to segregate the oppositely transmitting branches 

 of the four-wire circuits. In the cables, the "east-bound" and 'Svest- 

 bound" branches of the four-wire circuits are in different groups. 

 This segregation is also carried out in the loading coil pots, and in the 

 office cabling.^'' 



With loading coils as manufactured at present, the cross-talk 

 unbalances in the loaded cables are such that the resultant o\-er-all 

 cross-talk is expected to be tolerable for the longest circuits now 

 definiteK' planned in cable. The margin below commercial limits is 

 much less in two-wire circuits than in four-wire circuits. /\t present, 

 there is a growing tendency to use two- wire circuits for longer distances 



26 U. S. Patent No. 1,394,062—0. B. Blaokwi'll. 



