494 BELL SYSTEM TECHNICAL JOURNAL 



made to phase distortion in systems for transmitting other than tele- 

 phone signals. 



Interpretation of Phase Distortion 



Telephone systems must be so designed that the received signal 

 approximates in wave form the sent signal within limits found by exper- 

 ience to be tolerable. We are here concerned primarily with the de- 

 parture of the received signal wave from the sent signal * which may be 

 attributed to the phase characteristic * of such networks as lines, 

 filters, repeaters, etc. which go to make up the complete system. Such 

 distortion is called phase or delay distortion. The reason for the term, 

 delay, will appear later. We shall summarize here some of the more 

 general conclusions of the effect on signals of certain phase characteris- 

 tics and discuss the validity of these conclusions in Appendix II. 



If the phase characteristic of any network is of the type shown by any 

 of the dotted lines in Fig. 1 the received wave will be an exact copy of 

 the sent or reference wave (assuming no amplitude distortion). In 

 the case of Fig. 2 the received wave differs from the reference wave 

 only in that it is reversed in sign which is equivalent simply to reversing 

 the terminals of the load. In both cases the received wave is delayed 

 ivith respect to the reference wave by a time interval that is given by the 

 slope of the phase characteristic or dBjdw. If B is in radians, and co 

 = 2irf, where/ is the frequency in cycles per second, the delay will be 

 in seconds. There is no distinction in effect between the phase char- 

 acteristics for any of the dotted lines and furthermore they are identical 

 with any such solid broken line as that shown. Since this is true we 

 may completely represent any of the phase characteristics of Fig. 1 

 if we choose by the single line passing through zero in which case the 

 delay is 5/co. 



If the phase characteristics are straight lines and intersect the verti- 

 cal axis at odd multiples of 7r/2 the received wave may be obtained from 

 the reference wave first by delaying it by dB/dco and then shifting the 

 phases of all its steady state sinusoidal components as obtained by 

 Fourier Integral or Series analysis by 7r/2. 



If the straight line phase characteristics intersect the ordinate at 

 intermediate values the received wave may be said to be the sum of 



* See Appendix I. If one desires to be specific the sent or reference wave in the 

 case of considering image transfer phase shift may look upon as the current entering 

 the network and in the case of insertion phase shift the current through the load with 

 the network omitted; and the received wave the current through the load in either 

 case with the network in place. 



* In actual apparatus certain general types of phase characteristics are associated 

 with certain attenuation characteristics. For this reason it is difficult to separate 

 to the extent one might sometimes desire the effects of the two types of distortion, 

 attenuation and phase. 



