Fractional Delay Digital Filters

The chapter goal is focused to introduce the concept of fractional delay filters (FDF), as well as a concise description of most of the existing design techniques. For this purpose, several illustrative examples are presented, where each design method is implemented by MATLAB programs. A fractional delay filter is a filter of digital type having as main function to delay the processed input signal a fractional of the sampling period time. There are several applications where such signal delay value is required, examples of such systems are: timing adjustment in all-digital receivers (symbol synchronization), conversion between arbitrary sampling frequencies, echo cancellation, speech coding and synthesis, musical instruments modelling etc. (Laakson et al., 1996). In order to achieve the fractional delay filter function, two main frequency-domain specifications must be met by the filter. The filter magnitude frequency response must have an all-pass behaviour in a wide frequency range, as well as its phase frequency response must be linear with a fixed fractional slope through the bandwidth. Several FIR design methods have been reported during the last two decades. There are two main design approaches: time-domain and frequency-domain design methods. In first one, the fractional delay filter coefficients are easily obtained through classical mathematical interpolation formulas, but there is a small flexibility to meet frequency-domain specifications. On the other hand, the frequency-domain methods are based on frequency optimization process, and a more frequency specification control is available. One important result of frequency-domain methods is a highly efficient implementation structure called Farrow structure, which allows online fractional value update. The chapter is organized as follows. Next section gives the formal definition of fractional delay filter. In the third section, some design methods are briefly described. Two efficient implementation structures for wideband fractional delay filter, as well as description of recently reported design methods for such structures, are illustrated in fourth section. MATLAB designed examples and concluding remarks are presented in fifth and sixth sections, respectively.