A CARRIER TELEPHONE SYSTEM FOR TOLL CABLES 87 



range of temperature variation (and attenuation variation) for an 

 aerial cable may be half as much in one day as in an entire year. For 

 an underground cable, changes of temperature occur quite gradually 

 and the total annual variation is about one-third of that for an aerial 

 cable. 



These relations between the attenuation of a cable pair and the 

 frequency and temperature are of fundamental importance in the 

 design of the type K system. First of all, since the attenuation at 60 

 kilocycles is about 4 db per mile, the total attenuation for a cable 

 circuit of the length used in designing the type K system, i.e., 4000 

 miles, would be approximately 16,000 db. This must be offset by a 

 corresponding gain. 



In the next place, differences in the attenuation at the different fre- 

 quencies would, if uncorrected, become so great that signals of the less 

 attenuated channels would overload the repeaters, while those of the 

 more attenuated channels would drop down into the noise region. 

 Hence, each repeater must be given a gain-frequency slope which is 

 complementary to the attenuation slope of the line. 



Finally, the changes of transmission due to temperature variations 

 and other causes must be compensated so precisely that the net vari- 

 ation in each channel is held within very narrow limits. The method 

 of doing this is explained later. Here it is interesting merely to con- 

 sider the magnitude of the problem. For the top channel, assuming 

 a 4000-mile circuit, the annual variation in attenuation of an aerial 

 cable pair might be approximately 2000 db. The systems thus far 

 installed have, of course, been limited to much shorter distances than 

 this. 



Even if the change of attenuation with temperatures were related 

 to frequency by a simple law, correct compensation over the frequency 

 range would be far from easy. To a casual inspection the differential 

 between any two curves of Fig. 4, for example those for 55° F., and 

 110° F., will not appear serious. This differential, which becomes very 

 large for a long circuit, is a complicated function of the frequency. 



The attenuation differential with temperature can be considered as 

 made up of two components, one which is independent of frequency 

 and another which varies with frequency. The former component, 

 which is much the larger, requires a gain adjustment which is uniform 

 or flat over the frequency range of the system. The latter component 

 is frequently referred to as the "twist." For the range from 12 to 60 

 kilocycles, the maximum change of attenuation with temperature 

 occurs near 28 kilocycles. Hence this frequency has been used as a 

 datum point in determining the twist. The shape of the twist com- 



