Crosstalk and Noise Features of Cable Carrier Telephone 



System * 



By M. A. WEAVER, R. S. TUCKER and P. S. DARNELL 



CROSSTALK and noise are important factors in cable carrier 

 transmission as outlined in the paper "A Carrier Telephone 

 System for Toll Cables" by Messrs. C. W. Green and E. I. Green. 

 Crosstalk and noise limit the number of carrier channels which can be 

 utilized in any one cable, not only by limiting the number of channels 

 which can be placed on a single pair, but by limiting the number of 

 pairs which can be used. Noise also controls the transmission loss 

 which can be permitted between repeaters. Without the crosstalk 

 and noise reduction measures described in this paper, the number of 

 carrier channels per cable would be so few and the spacing between re- 

 peaters so short, that the type K carrier system would be impracticable. 



Crosstalk 



To utilize existing toll cables in the Bell System for frequencies up 

 to 60 kilycycles required the solution of many new crosstalk problems 

 because: (1) Crosstalk increases rapidly with the frequency, (2) Non- 

 loaded carrier pairs due to their high speed of propagation are especially 

 suitable for very long distances and hence the crosstalk requirements 

 per unit length are relatively severe, (3) The large gains of the carrier 

 repeaters amplify certain crosstalk currents much more than in the 

 case of voice frequency circuits. 



Two general effects need to be considered: intelligible crosstalk must 

 be prevented; and, a large number of circuits crosstalking into a par- 

 ticular circuit must not contribute an undue amount of noise. The 

 second effect is called babble, since it consists of a multiplicity of un- 

 related voice sounds which, in the aggregate, are unintelligible. 



An important feature is the use of different cables for opposite direc- 

 tions of transmission. This makes the major crosstalk problem the 

 reduction of crosstalk between pairs in the same cable used for trans- 

 mission in the same direction. The crosstalk currents due to trans- 

 mission at one end of a disturbing circuit through the distributed 

 couplings with a disturbed circuit tend to arrive at the distant end at 

 the same time since the currents via any of the couplings travel sub- 



* Presented at Winter Convention of A. I. E. E., Jan. 24-28, 1938. 



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