Improved tuning accuracy design of parallel-allpass-structures-based variable digital filters

The best and simplest method to realize variable IIR digital filters is to apply frequency transformations on a parallel-allpass-structures-based circuits, followed by truncated Taylor series expansion of the allpass sections coefficients. In this work an approach to minimize the two main disadvantages of this method is proposed. It is shown first, after analyzing the sensitivity of the realization, that the degradation of the stopband characteristics could be considerably decreased by minimizing the sensitivities of the allpass sections in the parallel structure over some critical frequency ranges. Then the tuning accuracy is improved by minimizing the sensitivities of the allpass sections used to apply the frequency transformations. Appropriate first- and second-order allpass sections are proposed for the most typical cases. The effectiveness of the proposed approach is demonstrated by simulation examples.