A SYNTHESIS OF VARIABLE WAVE DIGITAL-FILTERS

It is often desired to change the cutoff frequencies of digital filter s in some applications like digital electronic instruments. This paper proposes a design of variable lowpass digital filters with wider rang es of cutoff frequencies than conventional designs. Wave digital filte rs are used for the prototypes of variable filters. The proposed desig n is based on the frequency scaling in the s-domain, while the convent ional ones are based on the z-domain lowpass-to-lowpass transformation s. The first-order approximation by the Taylor series expansion is use d to make multiplier coefficients in a wave digital filters obtained f rom a frequency-scaled LC filter become linear functions of the scalin g parameter, which is similar to the conventional design. Furthermore this paper discusses the reduction of the approximation error. The cur vature is introduced as the figure of the quality of the first-order a pproximation. The use of the second-order approximation to large-curva ture multiplier coefficients instead of the first-order one is propose d.