138 BELL SYSTEM TECHNICAL JOURNAL 



stantially the same distance. This makes it possible to greatly reduce 

 the total effect of these distributed couplings by the use of small ad- 

 justable mutual inductance coils connected between pairs at one point 

 in each repeater section. 



If nothing more were done, there would still be objectionable cross- 

 talk since currents from the outputs of carrier repeaters could crosstalk 

 into voice frequency circuits and these circuits could then again cross- 

 talk into other carrier frequency circuits at points near their repeater 

 inputs. This effect is minimized by transposing the carrier pairs from 

 one cable to the other at carrier repeater points. 



At common voice frequency and carrier frequency repeater points 

 there would be an unsatisfactory crosstalk path from a carrier repeater 

 output into all the wires not used for carrier frequencies and from them 

 through coupling between office wiring into similar wires in the other 

 cable and finally into carrier repeater inputs in the second cable. This 

 crosstalk is minimized by the use of carrier frequency suppression coils 

 in the voice frequency circuits. These coils also serve the purpose of 

 preventing carrier frequency noise originating in voice frequency cir- 

 cuits from being transmitted into the cables and inducing noise at 

 points near carrier repeater inputs. 



Near-End Crosstalk 



Near-end crosstalk is the result of coupling between circuits trans- 

 mitting in opposite directions, while far-end crosstalk is the result of 

 coupling between circuits transmitting in the same direction. Near- 

 end crosstalk coupling between different carrier circuits of the same 

 frequency must be kept very small, particularly near a repeater point, 

 since crosstalk from the output of a repeater into an opposite directional 

 pair near the input of its repeater will be greatly amplified by this 

 repeater. 



Crosstalk between carrier circuits within the offices is kept low by 

 careful shielding, segregation, suppression of spurious paths through 

 battery supply, common grounding arrangements, etc. 



Since the type K system operates on a "four-wire" basis, different 

 electrical paths are used for opposite directions of transmission. 

 Satisfactory near-end coupling in the outside plant is obtained, there- 

 fore, by placing east bound pairs in one cable and west bound pairs in 

 another. When two cables have relatively heavy sheaths as in the 

 larger Bell System cables, their coupling is sufficiently small even with 

 the two cables in close proximity. 



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