Simple design of fractional delay allpass filters

A novel closed-form method for designing fractional delay allpass filters is proposed. The design uses closed-form formulas and is based on truncating the coefficient vector of a Thiran allpass filter. While the resulting filters are non-optimal, they allow a wider approximation bandwidth than the Thiran allpass filter, which yields a maximally flat delay approximation at the zero frequency. Formulas have been derived to assist in choosing the two parameters, order and prototype order, for the new design. There is practically no upper limit for the filter order, since the method is not prone to numerical problems.