When designing an FIR filterDesigner the phase response has discontinuities of PI (3.142) radians in the stopband.
These discontinuities should give a PhaseDelay function that has discontinuities also, but filterDesigner plots the filter's phasedelay function as a constant (a horizontal line).
Why is that so?
The same thing happens when using the MATLAB phasedelay function. For example consider a length 10 moving average filter that has the FIR 0.1*[1 1 1 1 1 1 1 1 1 1]. The figure below shows the phasedelay I obtained for it using the MATLAB phasedelay() function in red and the phase delay I obtained from first principles in blue. The blue one has the discontinuities while her red one stays constant at a value of 4.5 samples.
The two plots give the same value for the passband frequency range (which is the more important frequency range), but differ after that.
Is this just a convention used by the MATLAB filterDesigner tool and the MATLAB phasedelay function, or is there some Mathematical reason for it?
Thanks,
Jason