DISTORTION CORRECTION 439 



Introduction 



EVERY actual electrical circuit or transmission system distorts 

 transmitted signals; that is to say, the received signal, regarded 

 as a time-function, differs in shape from the impressed signal. Heavi- 

 side studied in detail the distorting action of the transmission line 

 itself and indicated the necessary electrical properties of the distor- 

 tionless line.^ The distortionless line of Heaviside was approximately 

 realized in the loaded line ^ in which similar lumped inductances are 

 inserted in series with the line at uniform intervals. While this loading 

 has the effect of partially correcting distortion of the lower frequency 

 components of the signal, it also tends to increase the distortion of the 

 higher frequency components and so limit somewhat the useful fre- 

 quency range. More recently the transmission characteristics of some 

 newly installed submarine cables have been greatly improved by means 

 of continuous loading with the new magnetic material permalloy.^ 



The methods mentioned above are directed to rendering the line 

 itself more nearly perfect. The method of distortion correction pre- 

 sented here may be used to supplement them and is that of pas- 

 sive terminal networks; more particularly networks whose iterative 

 impedances are a pure constant resistance at all frequencies.^ These 

 networks are, however, not limited in their use to any particular type 

 of transducer or transmission system but have general applicability. 

 For this reason the general problem of distortion correction by this 

 method resolves itself principally into a study of the transmission 

 properties of these networks together with systematic methods of 

 design to meet specified requirements. 



This paper takes up first the characteristics necessary for no dis- 

 tortion in an electrical circuit; then, an extended study of constant 

 resistance networks which can be used for distortion correction ; finally, 

 several applications to important practical problems. In addition. 

 Appendix IV gives a considerable number of network structures and 



1 "Electrical Papers," Vol. II, p. 123, 1892; "Electromagnetic Theory," Vol. I, 

 p. 445, 1893, Oliver Heaviside. 



2 U. S. Patent No. 652,230 to M. I. Pupin, dated June 19, 1900. See also "On 

 Loaded Lines in Telephonic Transmission," G. A. Campbell, Phil. Mag., March, 

 1903. Later a loading system more specifically directed to reducing distortion per se 

 was disclosed in U. S. Patent No. 1,564,201 to J. R. Carson, A. B. Clark and J. Mills, 

 dated December 8, 1925. 



^ "The Loaded Submarine Telegraph Cable," O. E. Buckley, B. S. T. J., July, 

 1925. 



^ The equalization of the attenuation of certain transmission lines has for some 

 time been obtained by means of comparatively simple series or shunt terminal net- 

 works. See, for example, U. S. Patent No. 1,453,980 to R. S. Hoyt, dated May 1, 

 1923. Such networks necessarily produce total terminal impedances which vary 

 with frequency. 



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