Editing Group delay and phase delay (section)

=== Phase delay ===
A linear time-invariant system or device has a [[phase response]] property and a phase delay property, where one can be calculated exactly from the other. Phase delay directly measures the device or system time delay of individual ''sinusoidal'' frequency components. If the phase delay function at any given frequency&mdash;within a frequency range of interest&mdash;has the same constant of proportionality between the phase at a selected frequency and the selected frequency itself, the system/device will have the ideal of a flat phase delay property, a.k.a. [[linear phase]].<ref name="RabinerGold1975" /> Since phase delay is a function of frequency giving time delay, a departure from the flatness of its function graph can reveal time delay differences among the signal’s various sinusoidal [[#Frequency components of a signal|frequency components]], in which case those differences will contribute to signal distortion, which is manifested as the output signal waveform shape being different from that of the input signal.

The phase delay property in general does not give useful information if the device input is a [[Modulation|modulated]] signal. For that, group delay must be used.