444 BELL SYSTEM TECHNICAL JOURNAL 



The total attenuation, a', and time-of-transmission, t, are somewhat 

 at our disposal ; it will be found that their best choice is usually guided 

 by experience. The transducer will often consist of a number of sec- 

 tions, not necessarily alike. This distortion correcting process may 

 be called "equalizing both the attenuation and the time-of-phase- 

 transmission." 



The idea of altering circuit transmission characteristics by means of 

 one or more sections of constant resistance recurrent networks forms the 

 fundamental basis of the method of distortion correction presented here. 

 It is, of course, dependent for its application upon the physical possi- 

 bility of designing recurrent networks whose iterative impedances are 

 a constant resistance at all frequencies and whose propagation con- 

 stants have the desired characteristics. 



Another method by which distortion correction has sometimes been 

 obtained is by means of terminal thermionic distortion circuits wherein 

 networks of particular frequency characteristics are placed in the plate 

 circuits of successive thermionic tubes. In it any reaction of one 

 stage upon a preceding stage or upon the original circuit is prevented 

 by the unilateral property of the tubes, whereas in the method given 

 here this same result is obtained by the property of a constant resist- 

 ance iterative impedance and the use of a resistance termination. 

 While from the standpoint of the original circuit both methods give 

 the resultant effect of a terminal unilateral device, one very practical 

 advantage of the constant resistance method over the thermionic 

 tube method appears to be that it corrects distortion before any 

 amplification is added and hence with it there would be less tendency 

 to cause tube distortion or modulation. Another advantage is that 

 the distortion correcting networks can be designed independently of 

 the amplifying device. A description of this other method appeared 

 in the last number of the Journal? 



Before taking up specific types of constant resistance structures, let 

 us consider some of the inherent limitations of certain transducers as 

 are brought out by the following theorems. 



2.2. Linear Transducer Theorems 



These theorems relate to the variation with frequency over the 

 entire frequency range of the iterative parameters, that is, the propaga- 

 tion constants and iterative impedances, of certain passive linear 

 transducers. In symmetrical transducers we could as well employ the 

 image parameters which are of such utility in a study of electric wave- 



^ "Phase Distortion and Phase Distortion Correction," Sallie Pero Mead. B. 5. 



r. J., April, 1928. 



