Basically, if you want to preserve the order, you probably want to move your zeroes down in frequency ie morph the filter into a shelf (ie change the Nyquist gain). You can do this using various constraints or heuristics and they all lead to slightly different trade-offs, but that's the general idea anyway. There's also at least one paper about just fitting a filter into the desired response but I can't seem to remember suitable search terms right now (and don't seem to have a local copy for the exact name of the paper; edit: I think I was thinking about "A Generalization of the Biquadratic Parametric Equalizer" by Knud Bank Christensen, edit2: not, actually that wasn't the one I think.. relevant anyway).camsr wrote:If you guys could figure out how to remove the distortion from the bottom side of the bell, that would be great.
Alternatively if you don't care about preserving order (or well, don't mind some additional FIR work), then http://www.lce.hut.fi/~ssarkka/pub/eq-article.pdf basically works by band-limiting the analog filter in continuous time, such that you can then use impulse-invariant (or whatever) on it (without the aliasing you'd normally get).
). It rather "generates" up to three bell-shaped spectra (formants) at a fixed frequency or at a (not necessarily integer) multiple of the fundamental frequency.