Digital Two-Rate IIR and Hybrid IIR/FIR Filters for Sampling Rate Conversion

The paper discusses structures, design methods, and efficiency of digital two-rate filters for sampling rate alteration by integer or rational ratios. Four canoic filter structures are demonstrated: the direct form, the parallel form, the ladder/lattice forms, and a structure based on canonic all-pass sections. In addition, a structure exploiting numerator coefficient symmetry is presented. An extended class of filters (hybrid IIR/FIR) where the passband equalization is performed by a combination of poles and off-the-unit-circle zeros is introduced. The author also describes a design method which iteratively utilizes the Remez algorithm. This simple extension of the McClellan et al. (1973) program facilitates the design of virtually any kind of filter including hybrid filters and/or two-rate filters. Several design examples are presented to visualize the different aspects of two-rate filtering.