Optimal IIR filters with variable delay

A new method of designing IIR filters with tuneable delay is presented. The proposed variable delay filters consist of a number of parallel fixed-coefficient IIR filters whose outputs are weighted and added up. The weights are affected only by the required delay. In this paper they are chosen as simple polynomial functions of the delay but other selection are also possible. The fixed-coefficient filters are designed using Weighted Least Squares (WLS) approach. In order to guarantee stability of the designed filters the WLS optimisation criterion has been modified by linearly combining it with some time-domain components. These components are selected in such a way that all the local minima of the modified cost correspond to stable filters. The proposed method is illustrated with a design of a bandpass filter whose bulk delay can be modified by up to ±0.5 of the sample time.