Phase Distortion and Phase Distortion Correction 



By SALLIE PERO MEAD 



Synopsis: The importance of the role played by the steady state phase 

 characteristics of long cable circuits has recently been emphasized in 

 telephone and telegraph transmission. In this paper an analytical exposi- 

 tion of the theory of phase distortion is followed by a consideration of 

 various methods of phase distortion correction with particular referenceto 

 terminal phase compensating networks and to the application of the lattice 

 network to the loaded line as a terminal phase corrector. 



1. Introduction 



FROM the standpoint of Ideal quality, a transmission system 

 must be so designed that the received currents, which represent 

 the transmitted signal, shall be a faithful copy of the corresponding 

 currents which enter the transmission system at the sending end; 

 that is, the transmission system must be distortionless. For relatively 

 short distances the deleterious effects of phase distortion are not 

 appreciable but as the range of transmission is increased a point may 

 be reached where the impairment of quality becomes so serious as to 

 reduce the commercial efficiency of the circuits. This fact was first 

 recognized on long submarine telegraph circuits. With regard to 

 telephony, the importance of the question of the quality of received 

 speech was initially emphasized by the advent of the efficient telephone 

 repeater,^ making possible the increased length of the modern telephone 

 system. This increased length, involving the necessity for circuits of 

 higher quality, led to the development of improved loading designs." 



It has long been recognized as a principle of good telephone or 

 telegraph transmission that the variation of attenuation over the 

 range of speech or signal frequencies should be minimized. With the 

 increased length of circuits, however, this requirement alone was 

 found insufficient to insure good quality and phase distortion over the 

 range of essential frequencies was found to play an important role. 

 Thus steady state phase characteristics have attained prominence in 

 the engineering of long cable circuits and in the application of this 

 technique to telegraph, telephone and picture transmission. 



Distortion in the variation with frequency of the phase difference 

 between the current at the receiving end and that at the sending end 

 of the transmission line, as well as distortion of the amplitude char- 

 acteristic of the received as compared to the initial current, gives rise 



^ See reference 1. 

 2 See reference 2. 



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