biquad.h

/* Simple implementation of Biquad filters -- Tom St Denis
 *
 * Based on the work

Cookbook formulae for audio EQ biquad filter coefficients
---------------------------------------------------------
by Robert Bristow-Johnson, pbjrbj@viconet.com  a.k.a. robert@audioheads.com

 * Available on the web at

http://www.smartelectronix.com/musicdsp/text/filters005.txt

 * Enjoy.
 *
 * This work is hereby placed in the public domain for all purposes, whether
 * commercial, free [as in speech] or educational, etc.  Use the code and please
 * give me credit if you wish.
 *
 * Tom St Denis -- http://tomstdenis.home.dhs.org
*/

/* this would be biquad.h */
#include <math.h>
#include <stdlib.h>

#ifndef M_LN2
#define M_LN2	   0.69314718055994530942
#endif

#ifndef M_PI
#define M_PI		3.14159265358979323846
#endif

/* whatever sample type you want */
typedef double smp_type;

/* this holds the data required to update samples thru a filter */
typedef struct {
    smp_type a0, a1, a2, a3, a4;
    smp_type can_a0, can_a1, can_a2 ; /* canonical a0,a1,a2 */
    smp_type can_b0, can_b1, can_b2 ; /* canonical b0,b1,b2 */
    smp_type x1, x2, y1, y2;
}
biquad;

extern smp_type BiQuad(smp_type sample, biquad * b);
extern biquad *BiQuad_new(int type, smp_type dbGain, /* gain of filter */
                          smp_type freq,             /* center frequency */
                          smp_type srate,            /* sampling rate */
                          smp_type bandwidth);       /* bandwidth in octaves */

/* filter types */
enum {
    LPF, /* low pass filter */
    HPF, /* High pass filter */
    BPF, /* band pass filter */
    NOTCH, /* Notch Filter */
    PEQ, /* Peaking band EQ filter */
    LSH, /* Low shelf filter */
    HSH /* High shelf filter */
};

double BiQuad_response(double freq, double srate, biquad *p, double *from_formula) ;