The relationship between phase shift, phase delay, and group delay of a system are reviewed. The differential time-delay distortion and differential phase-shift distortion are defined. A rationale is presented for the use of these as indicators of phase linearity of audio amplifier circuits. The differential time-delay and phase-shift distortions are calculated for several low-pass and high-pass filter functions. The calculations are used to illustrate the use of the differential phase-shift distortion as a criterion for calculating minimum amplifier bandwidths for a specified phase linearity.
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