/* * Butlp * * This is an implementation of a 2nd-order butterworth * lowpass 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 <stdint.h>#include <stdlib.h>#define ROOT2 (1.4142135623730950488) #ifndef M_PI#define M_PI 3.14159265358979323846#endif #include "soundpipe.h" static int filter(SPFLOAT *in, SPFLOAT *out, SPFLOAT *a){    SPFLOAT t, y;    /* a5 = t(n - 1); a6 = t(n - 2) */    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;} int sp_butlp_create(sp_butlp **p){    *p = malloc(sizeof(sp_butlp));    return SP_OK;} int sp_butlp_destroy(sp_butlp **p){    free(*p);    return SP_OK;} int sp_butlp_init(sp_data *sp, sp_butlp *p){    p->freq = 1000;    p->pidsr = M_PI / sp->sr * 1.0;    p->a[5] = p->a[6] = 0.0;    p->lfreq = 0.0;    return SP_OK;} int sp_butlp_compute(sp_data *sp, sp_butlp *p, SPFLOAT *in, SPFLOAT *out){    USED(sp);     if (p->freq <= 0.0) {        *out = 0;        return SP_NOT_OK;    }     if (p->freq != p->lfreq) {        SPFLOAT *a, c;        a = p->a;        p->lfreq = p->freq;        /* derive C constant used in BLT */        c = 1.0 / tan((SPFLOAT)(p->pidsr * p->lfreq));         /* perform BLT, store components */        a[0] = 1.0 / (1.0 + c*ROOT2 + c*c);        a[1] = 2*a[0];        a[2] = a[0];        a[3] = 2.0 * (1.0 - c*c) * a[0];        a[4] = (1.0 - c*ROOT2 + c*c) * a[0];    }     filter(in, out, p->a);    return SP_OK;}