384 BELL SYSTEM TECHNICAL JOURNAL 



by a filter when its loss is not the same at all frequencies in the trans- 

 mitted band, currents of some frequencies being attenuated more than 

 others. The effect of amplitude distortion on the program is to change 

 the relative intensities, or volumes, of tones of the frequencies at which 

 distortion occurs, thus impairing the naturalness of the program. 

 Amplitude distortion ordinarily can be corrected without much diffi- 

 culty by means of suitable attenuation equalizers. 



Delay distortion is introduced by a filter when different frequency 

 components of a signal require different lengths of time for propagation 

 through the filter. This type of distortion is related directly to the 

 shape of the phase shift-frequency characteristic. The slope of this 

 phase shift curve usually is taken as a measure of the delay introduced 

 by the filter. Stated mathematically, the delay in seconds is taken as 

 dB/do), where B is the phase shift in radians and w is 2-n-f, f being the 

 frequency in cycles per second. Thus if the phase shift of the filter 

 is proportional to frequency, dBJdoo, or the delay, is constant and 

 there is no delay distortion. In this case the wave form of a signal 

 transmitted through the filter remains unchanged, the signal being 

 delayed in transmission an interval of time corresponding to the slope 

 of the phase shift curve. If the slope of this curve is not constant 

 over the transmitting band of the filter, however, delay distortion is 

 introduced. In low-pass filters, the difference between the slope of 

 the phase shift curve at a given frequency and the minimum slope of 

 the curve is a measure of the delay distortion at that frequency. 



A discussion of delay distortion in telephone apparatus, including 

 filters, as well as a discussion of the effect of delay distortion on tele- 

 phone quality, may be found in two recent articles on these subjects.^' ^ 

 Whereas the effect of amplitude distortion is to weaken or strengthen 

 some of the tones in the sound being transmitted with respect to the 

 other component tones, the effect of delay distortion is to introduce 

 unnatural audible effects which may become so pronounced as to be 

 annoying if the delay distortion be great enough. 



Delay distortion is present in most filters used in communication 

 work, but ordinarily not in such magnitude that its effect is noticeable. 

 As a rule, it need be considered only when a large number of filters is 

 used in a single circuit, as in the case of the program systems. Delay 

 distortion is in general more difficult to correct than amplitude distor- 

 tion. One of the unusual features of the low-pass line filter used in 

 the wide-band program circuits is the means employed to keep it 

 free from delay distortion. 



^" Phase Distortion in Telephone Appiiratus" by C. E. Lane, Bell. Sys. Tech. 

 Jour., July, 1930. 



* "Effects of Phase Distortion on Telephone Quality," by J. C. Steinberg, Bell 

 Sys. Tech. Jour., July, 1930. 



