On design techniques for approximately linear phase recursive digital filters

Digital filters made from a parallel combination of two allpass branches have been of considerable interest for more than a decade. The design of such filters can be viewed as a phase approximation problem. In this contribution, the behaviour of such filters is examined and several new design techniques are proposed which exploit the known relationships for the magnitude, phase and group delay. The motivation for this study is to explore these relationships more closely with a view to determining the link between various parameters such as filter order, passband and group delay ripple and stopband attenuation. An example is given which illustrates one of the design approaches taken