Variable digital filters

Different methods and practical experiences for the design and implementations of variable digital filters are presented with emphasis on the case of a selective filter with cutoff frequencies subject to change. It is shown that a combination of a coefficient calculator and a digital filter can overcome difficulties which arise if the allpass transformation is directly applied for the implementation. Higher flexibility is achieved when a microcomputer is used to design the filter for given specifications, and when its speed is sufficiently high to carry out the filtering operation in real-time afterwards.