Fractional delay FIR filter design with the Darlington transform

In this brief, we present a method for the design of finite-impulse response (FIR) digital filters with an approximately constant group delay in a lowpass interval of the frequency axis. Our approach is based on a transform, that was introduced by S. Darlington. The approximation is performed in a complex ζ-plane, where an infinite-impulse response transfer function with a prescribed denominator polynomial has to be found, that approximates a given FIR transfer function in the least-squares sense. The optimal solution to this approximation problem is obtained in a straightforward manner. The filters we get, are very close to the filters that can be designed with an approach, based on the Remez algorithm. However, the proposed method is more general