CARRIER IN CABLE 255 



the terminal apparatus used in present open-wire systems is evident 

 so no further discussion of this seems required. Fig. 4 shows five 

 relay rack bays carrying terminal equipment (exclusive of line 

 amplifiers) for one system terminal yielding nine telephone circuits. 

 Important problems in cable carrier transmission are: 



1. Keeping circuits electrically separated from each other, i.e., pre- 



venting troublesome crosstalk. 



2. Maintaining stability of transmission. 



Crosstalk 



With respect to crosstalk, the first and most important requirement 

 is to secure a very high degree of electrical separation between paths 

 transmitting in opposite directions. Careful crosstalk tests demon- 

 strated that by placing east-going circuits in one cable and west-going 

 circuits in another, the necessary degree of separation could be ob- 

 tained even though the two cables were carried in adjacent ducts. 

 Tests on short cable lengths indicate that adequate separation can 

 probably be secured by means of a properly designed shield; one 

 practical form of such a shield consists of alternate layers of copper 

 and iron tapes. With such a shield a cable may be divided into two 

 compartments and thus carry both directions of transmission. 



Having thus separated opposite bound transmissions there is left 

 the problem of keeping the crosstalk between same direction trans- 

 missions within proper bounds. In the cable used for the Morristown 

 trial the 16 A.W.G. pairs used for the carrier were separated from each 

 other by sandwiching them in between No. 19 A.W.G. (.9 millimeter 

 diameter) quads of the usual construction. These quads served as 

 partial shields between the carrier circuits and would in a commercial 

 installation have been suitable for regular voice-frequency use. Thus 

 a considerable reduction in the crosstalk between the carrier pairs was 

 effected. 



When the problem of keeping crosstalk between circuits trans- 

 mitting in the same direction within proper bounds is examined it 

 becomes evident that no matter how high the line amplifier gains may 

 be, these gains do not augment this crosstalk since if all of the circuits 

 are alike transmission remains at the same level on all circuits. Not 

 so evident perhaps is another fact that crosstalk currents due to un- 

 balances at different points tend to arrive at the distant end of the 

 disturbed circuit at the same time. This makes it possible to neutralize 

 a good part of the crosstalk over a wide range of frequency by intro- 

 ducing compensating unbalances at only a comparatively few points. 



