Measurement of Phase Distortion * 



By H. NYQUIST and S. BRAND 



This paper deals with the measurement of phase distortion or delay distor- 

 tion and is particularly concerned with measurements on telephone circuits. 

 For this purpose, use is made of a quantity defined as "envelope delay," 

 which is the first derivative of the phase shift with respect to frequency. 

 Various methods for measuring this quantity and the principles on which 

 they are based are discussed, the details of the measuring circuits being 

 omitted and sources of further information referred to when possible. Data 

 are included which give the measured envelope delay-frequency charac- 

 teristics of several kinds of telephone circuits. 



AT an early date in the use of long loaded telephone circuits, certain 

 disturbing effects at high frequencies were noticed which have 

 been known as transients.^ It was found that on such circuits, even 

 when the attenuation was very carefully equalized within the trans- 

 mitted range, these transient effects still persisted and were made worse. 

 It was realized that these effects were due to phase distortion or delay 

 distortion, that is, the resultant effect of phase shift varying with 

 frequency in a peculiar manner. It was also determined that these 

 effects resulted largely from the loading associated with the circuit 

 pairs and that the effect could be considerably reduced by using a 

 much lighter loading for the circuits.- Lighter loading systems were 

 applied to telephone circuits so that as a result these transient effects 

 were minimized to such an extent as to make circuits commercial for 

 telephone use. 



Recent developments in telephone transmission and in special ser- 

 vices requiring the use of telephone circuits have emphasized these 

 high-frequency effects due to phase distortion and have indicated a 

 similar effect at low frequencies which results from the equipment 

 associated with the circuit. The use of loaded cable circuits in place 

 of open-wire circuits, with a corresponding increase in the number 

 of repeaters, has increased the phase distortion considerably on tele- 

 phone circuits. This is particularly true when very long telephone 

 circuits in cable result so that these effects are quite disturbing. 

 Certain special uses to which circuits have been put within the last 

 five or six years require either a much wider band of frequencies for 

 transmission, or can allow only a very small amount of distortion 

 within the required band. Circuits which would ordinarily be satis- 

 factory for telephone use are not good enough for these purposes. 



* Presented at New York Section, A. I. E. E., May 1930. 



1 "Telephone Transmission of Long Cable Circuits," A. B. Clark, Jour. A. I. E. E., 

 Vol. XLII, p. 1, 1923, and B. S. T. J., Vol. II, p. 67, 1923. 

 -J. R. Carson, A. B. Clark, J. Mills, U. S. Patent 1,564,201. 



522 



