Robust IIR Digital Filter Sharpening

Digital filter sharpening aims to improve the performance of a prototype filter by cascading it with a "partial compensator" incorporating multiple uses of that same coefficient set. Although the Kaiser & Hamming FIR sharpening structure has long enjoyed popularity, there has been a widespread mistaken impression that no corresponding structure exists for IIR sharpening. In this paper we review the features of three available methods and also introduce a new sharpener that operates with no restrictions on phase conditions for prototypes and without requiring root-finding. The use of conjugate-reversal of numerator coefficient vectors, along with prototype pairing and allpass filtering, combine to facilitate robust internal sharpener delay alignment. Performance of the new structure is highlighted here in one complex and three real filter sharpening examples.