4 BELL SYSTEM TECHNICAL JOURNAL 



cycles wide (possibly wider), can be transmitted over separate properly 

 arranged pairs in the same direction in a single existing cable without 

 serious disturbance due to crosstalk. 



With respect to noise in existing cables, the matter of principal con- 

 cern is noise produced in telephone offices by apparatus working on 

 other circuits, since the natural shielding afforded by the cable sheath 

 largely eliminates noise from outside sources. Two methods are avail- 

 able for control of the noise produced in telephone offices: (1) Intro- 

 duction of high-frequency choking devices in all wires not assigned to 

 carrier service at the points where the cables enter offices in which noise 

 is produced; (2) Attack on noise at points where it is produced by in- 

 troduction of spark-killers and individual high-frequency choking 

 devices. The lenient noise-to-signal ratio requirement for television 

 mentioned above makes high-frequency television application much 

 easier than message telephone. 



While a million-cycle frequency range over more than one pair in an 

 existing cable seems unlikely for telephone message purposes, there are 

 interesting possibilities in the use of lower maximum frequencies. For 

 example, it seems likely that twelve same-directional telephone chan- 

 nels may be obtained from each one of a large fraction of the pairs in 

 existing toll cables and that the crosstalk between the pairs may be kept 

 within proper bounds by simple balancing methods previously de- 

 scribed.^ 



Open Wire 



With open wire, conditions are just about the reverse of those with 

 cable. A mile of open wire has an attenuation of only about 1 db at a 

 million cycles as compared to 6 db for the small-sized coaxial. How- 

 ever, overcoming crosstalk between different pairs of wires on a pole 

 line presents very formidable problems while avoiding interference from 

 and to radio systems may be even more formidable. 



The attenuation of open-wire pairs has been checked up to several 

 million cycles and it has been found that it behaves as expected. When 

 there is little crosstalk from the high-frequency band to other wires in 

 the lead the attenuation-frequency characteristic is smooth; when 

 there is severe crosstalk, the characteristic is bumpy. For a given 

 length of circuit the attenuation is small compared with that of the 

 small-sized coaxial, and the variation due to changing weather condi- 

 tions is also small — about one-third that of the coaxial. It is interest- 

 ing to note, however, that the percentage change in attenuation for the 

 coaxial is less than half that for the open-wire line. While it is, of 

 course, evident that the open-wire transmission variations depend in 

 part on changes in the series resistance of the wires due to changing 



