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BELL SYSTEM TECHNICAL JOURNAL 



Interaction Crosstalk at Repeater Points 

 Another type of problem was introduced by what is known as 

 interaction crosstalk. This is the crosstalk which occurs from one side 

 to the other of a J repeater station. Figure 9 illustrates two paths 



S -INDICATES A SUPPRESSION DEVICE 



Fig. 9 — Interaction crosstalk at a J repeater station. 



which it may take. Path A shows the crosstalk from a system to itself 

 which may cause transmission distortion or repeater singing while 

 Path B is the path of crosstalk between different circuits. The 

 essential feature of this interaction crosstalk is that, as Fig. 9 shows, 

 the crosstalk path at a repeater station passes through the J repeater 

 and hence the crosstalk is amplified by the repeater gain. 



The new problems of controlling this crosstalk were the result of 

 larger magnitudes of crosstalk at the higher frequencies, the larger 

 repeater gains and the fact that with more repeaters there were more 

 points on a system where it could occur. Magnitudes of interaction 

 crosstalk which had previously been thought of as inconsequential 

 assumed a new importance. For instance, with the gain of about 75 db 

 proposed for the repeater for use in sleet areas, an initial value of 

 unamplified interaction crosstalk as low as 0.25 crosstalk unit would be 

 magnified to 1400 units, which might considerably exceed the far-end 

 crosstalk existing at the same time in one repeater section. 



Several new methods for reducing this interaction crosstalk were 

 devised. In the first place, in order to prevent direct coupling between 

 the wires of the open-wire line on the two sides of the station, it was 

 found necessary to cut a gap in the line. With the wires entirely 

 removed for a distance usually of about eighty feet, the line is brought 

 into the station from the two terminal poles by means of the lead-in 

 cables. 



