/* * Butbr * * This is an implementation of a 2nd-order butterworth * band-reject filter, discretized using the bilinear transform. * * For more information on using the BLT on 2nd-order * butterworth filters, see: * * https://ccrma.stanford.edu/~jos/filters/Example_Second_Order_Butterworth_Lowpass.html */ #include <math.h>#include <stdlib.h>#include "soundpipe.h" #ifndef M_PI#define M_PI 3.14159265358979323846#endif int sp_butbr_create(sp_butbr **p){    *p = malloc(sizeof(sp_butbr));    return SP_OK;} int sp_butbr_destroy(sp_butbr **p){    free(*p);    return SP_OK;} int sp_butbr_init(sp_data *sp, sp_butbr *p){    p->freq = 1000;    p->bw = 1000;    p->pidsr = M_PI / sp->sr * 1.0;    p->tpidsr = 2 * M_PI / sp->sr * 1.0;    p->a[5] = p->a[6] = 0.0;    p->lfreq = 0.0;    p->lbw = 0.0;    return SP_OK;} int sp_butbr_compute(sp_data *sp, sp_butbr *p, SPFLOAT *in, SPFLOAT *out){    USED(sp);     SPFLOAT *a;    SPFLOAT t, y;    SPFLOAT bw, freq;     if (p->bw <= 0.0) {      *out = 0;      return SP_OK;    }     a = p->a;    bw = p->bw;    freq = p->freq;     if (bw != p->lbw || freq != p->lfreq) {        SPFLOAT c, d;        p->lfreq = freq;        p->lbw = bw;        c = tan((SPFLOAT)(p->pidsr * bw));        d = 2.0 * cos((SPFLOAT)(p->tpidsr * freq));        a[0] = 1.0 / (1.0 + c);        a[1] = -d * a[0];        a[2] = a[0];        a[3] = a[1];        a[4] = (1.0 - c) * a[0];    }     t = *in - a[3]*a[5] - a[4]*a[6];    y = t*a[0] + a[1]*a[5] + a[2]*a[6];    a[6] = a[5];    a[5] = t;    *out = y;     return SP_OK;}