136 BELL SYSTEM TECHNICAL JOURNAL 



Equalization 



Equalization is necessary in each direction of transmission at a 

 repeater point and in the receiving direction at a terminal, to compen- 

 sate for frequency distortion produced by the preceding section of line. 

 Fortunately, the attenuation frequency curves for the usual open-wire 

 circuits, that is, 104, 128 and 165-mil wire, have nearly the same shapes 

 for section lengths giving the same attenuation at the maximum fre- 

 quencies for the two directions of transmission, so that these various 

 circuits can be equalized alike. 



As is well known, the transmission frequency characteristic of an 

 amplifier with large feedback is almost the inverse of that of the 

 feedback circuit itself, so that the insertion in the feedback circuit of a 

 network having the same characteristics as a line section will provide 

 equalized transmission over the amplifier and section combined. In 

 the outer feedback circuit of the line repeater is included an equalizer 

 which has a characteristic sloping with respect to frequency in the 

 same way as the variation in loss under wet weather conditions of the 

 longest open-wire section likely to be used. Thus, there is provided in 

 the repeater a basic equalization for this longest wet weather line. 

 At a receiving terminal a basic equalizer is provided which performs 

 this same compensation, but in this case the slope of the curve must 

 necessarily be opposite to that of the line attenuation and of the 

 equalizer in the feedback path of the line repeater. 



Line sections, however, vary in length and in the amount of en- 

 trance cable included. In order that they may be properly corrected 

 by this basic equalization, they must be built out to equal this longest 

 wet weather section. For this purpose there are provided flat loss 

 pads and building-out networks whose losses have the same frequency 

 shapes as the losses of short lengths of open-wire circuit. These pads 

 and networks can be inserted or omitted by simple changes in strapping. 

 They suffice to build out the shortest section which is expected to be 

 used. 



Pilot Currents 



For a satisfactory system, arrangements must be provided to correct 

 automatically for the effects on line attenuation due to changes in 

 weather, by adjusting the amplification at each repeater point and in 

 the receiving terminal circuit. To permit measuring these effects 

 a pilot current of fixed frequency, near the middle of the transmitted 

 band, and of constant amplitude, is supplied from each terminal. 

 This is applied to the transmitting side of the terminal circuit between 

 the twelve-channel terminal and the first group modulator, where the 



