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## Phase Delay and Group Delay

The phase response of a filter gives the

radianphase shift experienced by each sinusoidal component of the input signal. Sometimes it is more meaningful to considerphase delay[Papoulis 1977].

Definition.Thephase delayof an LTI filter with phase response is defined by

The phase delay gives the

time delayin seconds experienced by each sinusoidal component of the input signal. For example, in filter , the phase response is which corresponds to a phase delay which is one-half sample.More generally, if the input to a filter with frequency response is

then the output is

and it can be seen that the phase delay expresses phase response as time delay.In working with phase delay, care must be taken to ensure all appropriate multiples of have been included in . We defined simply as the complex angle of the frequencyresponse , and this is not sufficient for obtaining a phase response which can be converted to true time delay. By discarding multiples of , as is done in the definition of complex angle, the phase delay is modified by multiples of the sinusoidal period. Since LTI filter analysis is based on sinusoids without beginning or end, one cannot in principle distinguish between ``true'' phase delay and a phase delay with discarded sinusoidal periods. Nevertheless, it is convenient to define the filter phase response as a

continuousfunction of frequency with the property that (for real filters). This specifies a means of ``unwrapping'' the phase response to get a consistent phase delay curve.

Definition.A more commonly encountered representation of filter phase response is called thegroup delay, and it is defined by

For linear phase responses, the group delay and the phase delay are identical, and each may be interpreted as time delay.

For any phase function, the group delay may be interpreted as the time delay of the

amplitude envelopeof a sinusoid at frequency [Papoulis 1977]. The bandwidth of the amplitude envelope in this interpretation must be restricted to a frequency interval over which the phase response is approximately linear. While the proof will not be given here, it should seem reasonable when the process of amplitude envelope detection is considered. The narrow ``bundle'' of frequencies centered at the carrier frequency is translated to Hz. At this point, it is evident that the group delay at the carrier frequency gives the slope of the linear phase of the translated spectrum. But this is a constant phase delay, and therefore it has the interpretation of true time delay for the amplitude envelope.