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copied!<p>I know this is a post on an old thread, and I would usually leave this as a comment, but I'm apparently not able to do that.</p> <p>In any case, for people searching for similar code, I thought I would post the link from where this code originates (it also has C code for other types of Butterworth filter coefficients and some other cool signal processing code).</p> <p>The code is located here: <a href="http://www.exstrom.com/journal/sigproc/">http://www.exstrom.com/journal/sigproc/</a></p> <p>Additionally, I think there is a piece of code which calculates said scaling factor for you already.</p> <pre><code>/********************************************************************** sf_bwbp - calculates the scaling factor for a butterworth bandpass filter. The scaling factor is what the c coefficients must be multiplied by so that the filter response has a maximum value of 1. */ double sf_bwbp( int n, double f1f, double f2f ) { int k; // loop variables double ctt; // cotangent of theta double sfr, sfi; // real and imaginary parts of the scaling factor double parg; // pole angle double sparg; // sine of pole angle double cparg; // cosine of pole angle double a, b, c; // workspace variables ctt = 1.0 / tan(M_PI * (f2f - f1f) / 2.0); sfr = 1.0; sfi = 0.0; for( k = 0; k < n; ++k ) { parg = M_PI * (double)(2*k+1)/(double)(2*n); sparg = ctt + sin(parg); cparg = cos(parg); a = (sfr + sfi)*(sparg - cparg); b = sfr * sparg; c = -sfi * cparg; sfr = b - c; sfi = a - b - c; } return( 1.0 / sfr ); } </code></pre>

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