Construction of variable cutoff digital filter based on a changeover of taps

This paper shows that by taking every M‐th impulse response from the impulse responses of FIR low‐pass digital filters and constructing a new digital filter based on them, a filter is obtained with low‐pass characteristic such that the passband width is M times that of the original filter. Next, taking advantage of this characteristic, a method is proposed for designing a low‐pass digital filter with variable cutoff frequency based on the same circuit by first designing a basic digital filter and varying M, that is, changing over taps. This principle is applicable not only to FIR filters but similarly also to IIR filters if some restrictions are allowed for denominator polynomials. Further, it is shown that by applying this theory to IFIR (Interpolated FIR) digital filters and multirate digital filters, filters can be realized which are more effective with respect to their cutoff characteristic and operational points.