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I need to calculate frequency response, phase response and apply to signals the Butterworth, Chebychev1 and Chebychev2 band-pass filters.

I'm developing in C++ with Qt, and I'm looking for algorithms or a (free) library to do that. I have my plotting solution, I just need the formulas.

The input values I have are the time/amplitude signal, the order of the filter and the low and high cut-off frequencies.

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Is this just a google question? – Assad Ebrahim Jan 21 '13 at 21:59
Nope, I searched and found nothing relevant (either not band-pass or implementations without algorithms provided) – gregseth Jan 22 '13 at 9:02

This site gives formulas for butterworth and various common filters.

And here's some C code to try.

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