Implementation of frequency-warped recursive filters

Frequency-warped filters are beneficial in many audio applications because they inherently use nonuniform frequency resolution which may be matched so that it approximates the frequency representation of hearing. Warped transversal filters are immediately obtained by replacing the unit delays of a conventional filter with first-order allpass blocks. Implementation of warped recursive filters is a problem since those filters contain delay-free loops. A technique to implement any such filter without changing the coefficients or the structure of the filter is presented in this paper. In addition, a method to modify such a filter to a new structure where the delay-free loops are eliminated is introduced.