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/*
These filter coefficients computations are taken from
http://www.harmony-central.com/Computer/Programming/Audio-EQ-Cookbook.txt
written by Robert Bristow-Johnson
*/
#ifndef __GGEE_FILTERS_H__
#define __GGEE_FILTERS_H__
#ifndef M_PI
#define M_PI 3.141593f
#endif
#include <math.h>
#define LN2 0.69314718
#define e_A(g) (pow(10,(g/40.)))
#define e_omega(f,r) (2.0*M_PI*f/r)
#define e_alpha(bw,omega) (sin(omega)*sinh(LN2/2. * bw * omega/sin(omega)))
#define e_beta(a,S) (sqrt((a*a + 1)/(S) - (a-1)*(a-1)))
typedef struct _rbjfilter
{
t_object x_obj;
t_float x_rate;
t_float x_freq;
t_float x_gain;
t_float x_bw;
} t_rbjfilter;
static int check_stability(t_float fb1,
t_float fb2,
t_float ff1,
t_float ff2,
t_float ff3)
{
float discriminant = fb1 * fb1 + 4 * fb2;
if (discriminant < 0) /* imaginary roots -- resonant filter */
{
/* they're conjugates so we just check that the product
is less than one */
if (fb2 >= -1.0f) goto stable;
}
else /* real roots */
{
/* check that the parabola 1 - fb1 x - fb2 x^2 has a
vertex between -1 and 1, and that it's nonnegative
at both ends, which implies both roots are in [1-,1]. */
if (fb1 <= 2.0f && fb1 >= -2.0f &&
1.0f - fb1 -fb2 >= 0 && 1.0f + fb1 - fb2 >= 0)
goto stable;
}
return 0;
stable:
return 1;
}
#endif
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