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BELL SYSTEM TECHNICAL JOURNAL 



two telephone lines parallel each other, as in Fig. 3, they will be electri- 

 cally coupled through mutual inductance and capacity. Currents 

 flowing in one will consequently produce crosstalk currents in the other. 

 When the subscriber at the west end of the line {A) is talking, waves 

 initiated by his voice will cross from line {A) to line {B) at adjacent 

 points along the entire length of the lines. 



Crosstalk entering line {B) at a typical point may traverse four chief 

 paths. It may (1) flow directly back to the west end, (2) flow onward 

 to B" and be reflected back to the west end, (3) flow directly onward to 

 the east terminal, and (4) flow backward to B' and thence, by reflection, 

 to B". There are of course an infinite number of other paths involving 

 multiple reflections but the reduction in amplitude caused by successive 



west east 

 terminals terminals 

 key: — 

 typical path of "far end" crosstalk 



TYPICAL PATH OF REFLECTED "NEAR EN D" CROSSTALK 



Fig. 3 — Diagram illustrating relation between impedance mis-matches and crosstalk 



in carrier systems. 



reflection and line attenuation makes these negligible in comparison 

 with the others. The first two of these four possibilities of crosstalk 

 production can be eliminated immediately. Modern carrier systems 

 are so designed that conversations going in one direction are carried by 

 one band of frequencies and those travelling in the opposite direction by 

 a different band. Currents entering the west terminal, whether they 

 follow a direct path such as AefB', or are first reflected at B", making 

 the typical path A'efB"B', are therefore eliminated by the filters in the 

 terminal office. Crosstalk currents of the third type ("far end cross- 

 talk") following the typical path A'efB" cannot be eliminated since 

 they fall within the frequency band used by the subscriber at B" for 

 listening. We may observe, however, that these currents traverse the 

 same length of line in travelling from A' to B" no matter what the 

 point (such as ef\n the diagram) at which they cross from one line to the 

 other. Since both lines are alike crosstalk currents will be attenuated 

 and shifted in phase by the attenuation and phase shift of a single 



circuit which, as far as crosstalk is concerned, constitutes a third circuit. The effect 

 of this circuit on the crosstalk of the other two is by no means negligible but con- 

 sideration of it is omitted herein in order to simplify the presentation of other im- 

 portant relations fundamental to crosstalk. 



