DSP System Design presents the investigation of special type of IIR polyphase filter structures combined with frequency transformation techniques used for fast, multi-rate filtering, and their application for custom fixed-point implementation. Detailed theoretical analysis of the polyphase IIR structure has been presented for two and three coefficients in the two-path arrangement. This was then generalized for arbitrary filter order and any number of paths. The use of polyphase IIR structures in decimation and interpolation is being presented and performance assessed in terms of the number of calculations required for the given filter specification and the simplicity of implementation. Specimen decimation filter designs to be used in Sigma-Delta lowpass and bandpass A/D converters are presented which prove to outperform other traditional approaches. DSP System Design may be of interest to graduate students, researchers, and professionals circuit designers, who would require fast and low-complexity digital filters for both single and multi-rate applications, especially those with low-power specification.