Multirate approximately linear-phase IIR filter structures for arbitrary bandwidths

This paper introduces new multirate approximately linear-phase IIR digital filter structures for arbitrary bandwidths. The overall filters have the same input and output sample rates. Multirate techniques are only used internally in order to reduce the computational complexity. The overall filters make use of an IIR filter for the actual filtering, delays for constructing complementary filters, and linear-phase FIR filters for the sampling rate alterations. The main advantages of the new filters over multirate FIR filters are that they generally have shorter delays and require fewer delay elements. In many cases, their complexities are also lower. The overall filters are designed by separately optimizing a number of linear-phase (possibly half-band) FIR filters, and one IIR filter being realizable as a parallel connection of two allpass filters, one of which is a pure delay. Several design examples are included illustrating the usefulness of the proposed filters.