Almost linear-phase polyphase IIR lowpass/highpass filter approach

The direct design of a linear phase FIR filter allows one perfectly linear phase, but at the expense of the high filter order, N, and consequently high computational burden for the given specification. IIR filters, however, can be designed with much smaller orders than their FIR counterparts, but at the expense of the non-linear phase. It is, however, possible to design almost linear phase IIR filters for a given specification of the allowed phase or group delay ripples. There are few approaches for the design of such filters. This paper provides an improvement on the existing algorithms and gives a different viewpoint on these design methods showing a considerable improvement in both stopband attenuation (as well as passband ripples) and maximum group delay ripples over the methods reported previously.