Adjustable Fractional-Delay Filters Utilizing the Farrow Structure and Multirate Techniques

The Farrow structure can be used for efficient realization of adjustable fractional-delay FIR filters, but despite its efficiency compared to other approaches, its implementation complexity grows rapidly as the bandwidth approaches π. To reduce the complexity, a multirate approach has been proposed. In this approach, the input signal is first interpolated by a factor of two via the use of a fixed half-band linear-phase FIR filter. Then, the actual fractional-delay filtering takes place. Finally, the so generated signal is downsampled to retain the original input/output sampling rate. In this way, the bandwidth of the fractional-delay filter used is halved compared to the overall bandwidth. Because the complexity of half-band linear-phase FIR filter interpolators is low, the overall complexity can be reduced. In this paper, we give further details of the multirate approach that have not been published before. In addition, we introduce the use of an approximately linear-phase IIR filter instead of a linear-phase FIR filter in the interpolation process in order to reduce the complexity even further. Design examples are included demonstrating this point.