Linear phase IIR filter with equiripple stopband

A simple technique of introducing transmission zeros into a low pass all-pole filter function to make its stopband transmission equiripple is described. The zeros are introduced through a transformation which maps the stopband of the filter onto the unit circle and the passband onto a segment of the real axis in the transformed plane. The technique is used to improve the loss characteristic of all-pole constant delay filters. Examples are included to illustrate the design procedure.