Effects of Phase Distortion on Telephone Quality

N the engineering of telephone systems it is convenient to define their transmission properties in terms of the changes that occur in transmission 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. T h a t distortion which is attributable to the attenuation characteristic is spoken of as attenuation distortion, and that attributable to the phase characteristic, as phase distortion. To be of greatest use in evaluating a system the steady state properties 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 received 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.1 Brief reference will be made also to a small amount of data that have been obtained for music.