ref: c4a08032074cdf097cfb13683efb4cfad74b96c6
dir: /src/band.c/
/* * 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. */ /* * Sound Tools Bandpass effect file. * * 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. */ #include <math.h> #include <string.h> #include "st_i.h" /* Private data for Bandpass effect */ typedef struct bandstuff { float center; float width; double A, B, C; double out1, out2; short noise; /* 50 bytes of data, 52 bytes long for allocation purposes. */ } *band_t; /* * Process options */ int st_band_getopts(eff_t effp, int n, char **argv) { band_t band = (band_t) effp->priv; band->noise = 0; if (n > 0 && !strcmp(argv[0], "-n")) { band->noise = 1; n--; argv++; } if ((n < 1) || !sscanf(argv[0], "%f", &band->center)) { st_fail("Usage: band [ -n ] center [ width ]"); return (ST_EOF); } band->width = band->center / 2; if ((n >= 2) && !sscanf(argv[1], "%f", &band->width)) { st_fail("Usage: band [ -n ] center [ width ]"); return (ST_EOF); } return (ST_SUCCESS); } /* * Prepare processing. */ int st_band_start(eff_t effp) { band_t band = (band_t) effp->priv; if (band->center > effp->ininfo.rate/2) { st_fail("Band: center must be < minimum data rate/2\n"); return (ST_EOF); } band->C = exp(-2*M_PI*band->width/effp->ininfo.rate); band->B = -4*band->C/(1+band->C)* cos(2*M_PI*band->center/effp->ininfo.rate); if (band->noise) band->A = sqrt(((1+band->C)*(1+band->C)-band->B * band->B)*(1-band->C)/(1+band->C)); else band->A = sqrt(1-band->B*band->B/(4*band->C))*(1-band->C); band->out1 = band->out2 = 0.0; return (ST_SUCCESS); } /* * Processed signed long samples from ibuf to obuf. * Return number of samples processed. */ int st_band_flow(eff_t effp, st_sample_t *ibuf, st_sample_t *obuf, st_size_t *isamp, st_size_t *osamp) { band_t band = (band_t) effp->priv; int len, done; double d; st_sample_t l; len = ((*isamp > *osamp) ? *osamp : *isamp); /* yeah yeah yeah registers & integer arithmetic yeah yeah yeah */ for(done = 0; done < len; done++) { l = *ibuf++; d = (band->A * l - band->B * band->out1) - band->C * band->out2; band->out2 = band->out1; band->out1 = d; *obuf++ = d; } *isamp = len; *osamp = len; return(ST_SUCCESS); } /* * Do anything required when you stop reading samples. * Don't close input file! */ int st_band_stop(eff_t effp) { return (ST_SUCCESS); /* nothing to do */ }