[music-dsp] Re: Revisiting C++ Filtering Classes

Vinnie thevinn at yahoo.com
Thu Jun 4 07:03:13 EDT 2009

I rewrote each of the response types to represent an analog prototype of a low-pass filter with a fixed angular frequency pi/2 (normalized frequency).

Then I use the low-pass to low-pass transformation on the roots:

c' = (1+kc)/(1-kc)

Where c is the complex root (i.e. pole or zero), k is a frequency transformation coefficient k=tan(w/2) with w=new cutoff frequency.

This is working great for Butterworth and Chebyshev Type I. But it is failing for the inverse Chebyshev.

Is this technique generalizable to any analog prototype filter or did I just get lucky?

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