AN IMPROVED THREE-CHANNEL CARRIER TELEPHONE SYSTEM 51 



tubes required in the system has been reduced, which results in a 

 material saving in power. 



Certain features of the improved equipment, notably the automatic 

 regulation, can also be used on the older types of systems and the 

 design objectives were set up with this in view. 



Frequency Allocation 



The frequency range employed by the system and the allocation of 

 channel bands within that range are shown in Fig. 2. The allocations 

 used in the older systems are also shown for comparison. 



6.3 



Xl 



: ic 



NEW SYSTEMS 



J .IZZ 



7.6 10.6 



r-TTi! c 



1 20.7 



3 tc 



I32: 



24.4 



in 



ARROWS DENOTE CARRIER 

 ; - EAST TO WEST 

 f - WEST TO EAST 



17.7 214 



OLDER SYSTEMS 



28.4 

 25.4 



XZI c 



nn 



9.4 



ic 



Xi 



19. S 



24.4 

 4|— 



r~5~i! 



FREQUENCr- KILOCYCLES 



Fig. 2 — Frequency allocation of the new systems relative to that of the older systems. 



The original selection of the frequency range for the Type C system 

 was the result of many different factors. Foremost among these was 

 the desire to keep the frequencies low in order to minimize line crosstalk 

 and attenuation and changes in the latter due to weather and tempera- 

 ture. On the other hand was the greater filter cost that results from 

 crowding the channels close together. Different frequency bands 

 were used for transmitting in opposite directions in order to avoid 

 the problem of near-end crosstalk and to give the advantages of four- 

 wire transmission. Although consideration was given to the general 

 desirability of increasing the band of frequencies effectively transmitted 

 by the individual channels, the requirement for coordinated operation 

 with older systems already in use precluded any material increase in 

 the frequency space allocated to each channel of the new system. 

 Nevertheless, as will be seen from Fig. 4, the channels show very little 

 distortion within the transmitted band and represent a material 

 improvement over the older systems. 



