What is minimum phase system?

Answer 1

The relation of "minimum" to "phase" in a minimum phase system or filter can be seen if you plot the unwrapped phase against frequency. You can use a pole zero diagram of the system response to help do a incremental graphical plot of the frequency response and phase angle. This method helps in doing a phase plot without phase wrapping discontinuities.

Put all the zeros inside the unit circle (or in left half plane in the continuous-time case) where all the poles have to be as well for system stability. Use the angles from all the poles, and the opposite of the angles from all the zeros, to a point on the unit circle to calculate phase, as that frequency response reference point moves around the unit circle. Now compare this plot with a similar plot for a pole-zero diagram with any of the zeros swapped outside the unit circle (non-minimum phase). The overall average slope of the line with all the zeros inside will be lower than the average slope of any other line representing the same LTI system response (e.g. with a zero reflected outside the unit circle). This is because the "wind ups" in phase angle are all mostly cancelled by the "wind downs" in phase angle only when both the poles and zeros are on the same side of the unit circle line. Otherwise, for each zero outside, there will be an extra "wind up" of increasing phase angle that will remain mostly uncancelled as the plot reference point "winds" around the unit circle from 0 to PI. (...or up the vertical axis in the continuous-time case.)

This arrangement, all the zeros inside the unit circle, thus corresponds to the minimum increase in total phase, which corresponds to minimum average total phase delay, which corresponds to maximum compactness in time, for any given (stable) set of poles and zeros with the exact same frequency magnitude response. Thus the relationship between "minimum" and "phase" for this particular arrangement of poles and zeros.