Variable digital filters

IIR prototype filters cannot be used for the design of variable filters through Schüssler´s all-pass transformation because of the occurrence of delay-free loops; no such problem arises with FIR prototypes. In this paper, the drawbacks of the latter type, with regard to computational efficiency and output roundoff noise, have been brought out, An approach is suggested, wherein the delay-free loops with IIR prototypes are eliminated by modifying the coefficients of the first- and second-order sections of the prototype filter. The computation of modified coefficients In this approach requires fewer multiplications as compared to an existing method.