Phase Delay

The phase response of an LTI filter gives the radian phase shift added to the phase of each sinusoidal component of the input signal. It is often more intuitive to consider instead the phase delay, defined as

The phase delay gives the time delay in seconds experienced by each sinusoidal component of the input signal. For example, in the simplest lowpass filter of Chapter 1, we found that the phase response was , which corresponds to a phase delay , or one-half sample. Thus, we can say precisely that the filter exhibits half a sample of time delay at every frequency. (Regarding the discussion in §1.3.2, it is now obvious how we should define the filter phase response at frequencies 0 and .)

From a sinewave-analysis point of view, if the input to a filter with frequency response is

then the output is

and it can be clearly seen in this form that the phase delay expresses the phase response as a time delay in seconds.

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.