526 



BELL SYSTEM TECHNICAL JOURNAL 



T^^^ = 



Aco ~ A/ ' 



where A(3 represents a finite difference in /3, etc. 



The use of steady-state conditions for the measurements of envelope 



delay * has quite evident advantages practically over a method which 



would tend to measure the delay of the envelope itself in a transient 



state. 



Methods of Measurement 



In making measurements of phase shift or envelope delay for the 

 purpose of determining the phase distortion of a telephone system, it 

 should be borne in mind that the absolute value is usually of small 

 importance and that the chief purpose is to determine the relative 

 values from one frequency to another; that is, the characteristic of the 

 phase shift or delay with frequency is the desired information as re- 

 gards phase distortion. 



1. Measurement of Phase Shift 



The first method of measurement described here will be one which 

 may be used to measure the phase shift itself.^" Fig. 1 shows schemati- 



Fig. 1 — Arrangement for measuring ])liase shift. 



cally an arrangement of apparatus for measuring phase shift. Current 

 of the frequency of measurement is sent through two paths, one con- 



* The envelope delay is generally different from the phase delay which is the ratio 

 of the phase shift to the frequency being considered, and should not be confused with 

 it. 



'0 W. P. Mason, U. S. Patent 1,684,403. 



I 



