Conformal mapping is a well-known technique for modifying the cut-off or resonant frequencies of filters. Conformal mapping is applied to the problem of digital filters for professional audio production. Methods are examined for computing the coefficients of presence filters, such as parametric equalizers with or without shelving, and of band-pass or band-reject filters. Formulas are given for the coefficients of these filters in terms of the specifications (center frequency, boost/cut, and so on), which are exact and relatively straightforward to compute. A new kind of shelving filter is introduced that is equiripple in the passbands and can have an arbitrarily narrow transition band (with increasing filter order).
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