Effects of Phase Distortionon Telephone Quahty* 



By JOHN C. STEINBERG 



This paper discusses the effects of the type of phase distortion found in low 

 pass filters and the loaded line on telephone quality. The effects are ascribed 

 to three factors; the first involves the slopes of the phase characteristic at 

 various frequencies in the range of interest, the second involves the intercept 

 values on the phase shift axis of the tangents to the phase curve, the third 

 involves the interference caused by portions of one sound overlapping por- 

 tions of a succeeding sound. The first factor appears to be the one of most 

 importance. 



IN the engineering of telephone systems it is convenient to define their 

 transmission properties in terms of the changes that occur in trans- 

 mission in the amplitude and the phase of steady state sinusoidal waves 

 of different frequency. The terms attenuation characteristic and 

 phase characteristic refer, respectively, to the amplitude change, 

 usually expressed in decibels, and to the phase shift, expressed in 

 radians or degrees, as functions of frequency. That distortion which 

 is attributable to the attenuation characteristic is spoken of as attenua- 

 tion distortion, and that attributable to the phase characteristic, as 

 phase distortion. 



To be of greatest use in evaluating a system the steady state proper- 

 ties must be experimentally correlated with the satisfactoriness, or 

 quality in its broad sense, of the system from the viewpoint of the 

 individual receiving the signals. If the signals are speech, quality 

 involves the recognizability of the speech sounds and their naturalness. 

 If the signals are music, the second factor is the one of chief concern. 

 A reasonably quantitative measure of the recognizability of the re- 

 ceived speech sounds may be obtained by means of the articulation test 

 which is described in a later paragraph. Naturalness is considerably 

 less definite, and the procedure in this case has been to compare the 

 distorted signals, speech or music, with the original or undistorted 

 signals and obtain the amounts of distortion that cause just noticeable 

 differences. 



The purpose of this paper is to discuss the effects of phase distortion 

 on the quality of speech.^ Brief reference will be made also to a small 

 amount of data that have been obtained for music. 



* Presented at New \'ork Section, A. I. E. E., May 19.^0. 



1 A companion paper by C. E. Lane on " Phase Distortion in Telephone Apparatus" 

 shows the types of phase characteristics found in various networks and discusses their 

 relation to the transmission properties. For a discussion of methods of measuring 

 phase characteristics the reader is referred to a companion paper on "Measurement 

 of Phase Distortion" by H. Nyquist and S. Brand. 



550 



