Design and realization of variable IIR digital filters as a cascade of identical subfilters

A new approach to design variable IIR digital filters by using a cascade of N identical individual filter, of any order n is proposed in this paper. F irst, the approximation method for lowpass filter specifications is outline d, then the general limitations of the new method are investigated and a co mpact formula is derived. Next, the limitations for the main canonic approx imations (Butterworth, Chebyshev and Elliptic) are investigated and compare d and convenient expressions for design and evaluation are obtained. New fi rst- and second-order filter sections, permitting very easy tuning of the c utoff frequency by recalculating and reprogramming of a single multiplier c oefficient value, are developed and the design and tuning strategies for hi ghpass, bandpass and bandstop filters are proposed. Finally design examples are given and the sound superiority of the new method compared to other kn own is demonstrated experimentally.