Fractional Delay (FD) Filters allow shifting of a given signal by a fraction of the sample interval. In the literature, a variety of approaches exist to design such filters, some of which are completely analytic. In this paper, we discuss an alternative analytic formulation for FIR-FD filters, which in one special case coincides with the widely known so-called maximally flat FIR FD filters. However, by variation of the free parameter, significant extension of the bandwith of (approximately) constant group delay is achieved. By deriving the same analytic form by a variable substitution, a way is found to apply this kind of optimization to IIR Allpass FD filters as well. Comparing the optimized FIR and IIR filters, it is shown that the FIR filters reach equivalent bandwidths and are thus attractive with regard to computational load.
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