/* libSoX Bandpass effect file.     July 5, 1991 * Copyright 1991 Lance Norskog And Sundry Contributors * * This source code is freely redistributable and may be used for * any purpose.  This copyright notice must be maintained. * Lance Norskog And Sundry Contributors are not responsible for * the consequences of using this software. * * Algorithm:  2nd order recursive filter. * Formula stolen from MUSIC56K, a toolkit of 56000 assembler stuff. * Quote: *   This is a 2nd order recursive band pass filter of the form. *   y(n)= a * x(n) - b * y(n-1) - c * y(n-2) *   where : *        x(n) = "IN" *        "OUT" = y(n) *        c = EXP(-2*pi*cBW/S_RATE) *        b = -4*c/(1+c)*COS(2*pi*cCF/S_RATE) *   if cSCL=2 (i.e. noise input) *        a = SQT(((1+c)*(1+c)-b*b)*(1-c)/(1+c)) *   else *        a = SQT(1-b*b/(4*c))*(1-c) *   endif *   note :     cCF is the center frequency in Hertz *        cBW is the band width in Hertz *        cSCL is a scale factor, use 1 for pitched sounds *   use 2 for noise. * * * July 1, 1999 - Jan Paul Schmidt <jps@fundament.org> * *   This looks like the resonator band pass in SPKit. It's a *   second order all-pole (IIR) band-pass filter described *   at the pages 186 - 189 in *     Dodge, Charles & Jerse, Thomas A. 1985: *       Computer Music -- Synthesis, Composition and Performance. *       New York: Schirmer Books. *   Reference from the SPKit manual. */   p->a2 = exp(-2 * M_PI * bw_Hz / effp->in_signal.rate);  p->a1 = -4 * p->a2 / (1 + p->a2) * cos(2 * M_PI * p->fc / effp->in_signal.rate);  p->b0 = sqrt(1 - p->a1 * p->a1 / (4 * p->a2)) * (1 - p->a2);  if (p->filter_type == filter_BPF_SPK_N) {    mult = sqrt(((1+p->a2) * (1+p->a2) - p->a1*p->a1) * (1-p->a2) / (1+p->a2)) / p->b0;    p->b0 *= mult;  }