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/*
* Copyright © 2009 Keith Packard <keithp@keithp.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; version 2 of the License.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
*/
#include "cc.h"
#include "cephes.h"
#include <math.h>
#include <stdlib.h>
static inline double sqr (double x) { return x * x; }
/*
* Kaiser Window digital filter
*/
#if 0
/* not used in this program */
static double highpass (double n, double m, double wc)
{
double alpha = m/2;
double dist;
dist = n - alpha;
if (dist == 0)
return (M_PI/2 - wc) / M_PI;
return -sin(dist * (M_PI/2-wc)) / (M_PI * dist);
}
#endif
static double lowpass (double n, double m, double wc)
{
double alpha = m/2;
double dist;
dist = n - alpha;
if (dist == 0)
return wc / M_PI;
return sin (wc * dist) / (M_PI * dist);
}
static double kaiser (double n, double m, double beta)
{
double alpha = m / 2;
return i0 (beta * sqrt (1 - sqr((n - alpha) / alpha))) / i0(beta);
}
static double beta (double A)
{
if (A > 50)
return 0.1102 * (A - 8.7);
else if (A >= 21)
return 0.5842 * pow((A - 21), 0.4) + 0.07886 * (A - 21);
else
return 0.0;
}
static int M (double A, double delta_omega)
{
if (A > 21)
return ceil ((A - 7.95) / (2.285 * delta_omega));
else
return ceil(5.79 / delta_omega);
}
struct filter_param {
double omega_pass;
double delta_omega;
double A;
double beta;
int M;
} filter_param_t;
static struct filter_param
filter (double omega_pass, double omega_stop, double error)
{
struct filter_param p;
p.omega_pass = omega_pass;
p.delta_omega = omega_stop - omega_pass;
p.A = -20 * log10 (error);
p.beta = beta (p.A);
p.M = M (p.A, p.delta_omega);
if ((p.M & 1) == 1)
p.M++;
return p;
}
static double *
make_low_pass_filter(double omega_pass, double omega_stop, double error, int *length_p)
{
struct filter_param p = filter(omega_pass, omega_stop, error);
int length;
int n;
double *lpf;
length = p.M + 1;
lpf = calloc (length, sizeof(double));
for (n = 0; n < length; n++)
lpf[n] = lowpass(n, p.M, omega_pass) * kaiser(n, p.M, p.beta);
*length_p = length;
return lpf;
}
static double *
convolve(double *d, int d_len, double *e, int e_len)
{
int w = (e_len - 1) / 2;
int n;
double *con = calloc (d_len, sizeof (double));
for (n = 0; n < d_len; n++) {
double v = 0;
int o;
for (o = -w; o <= w; o++) {
int p = n + o;
double sample = p < 0 ? d[0] : p >= d_len ? d[d_len-1] : d[p];
v += sample * e[o + w];
}
con[n] = v;
}
return con;
}
double *
cc_low_pass(double *data, int data_len, double omega_pass, double omega_stop, double error)
{
int fir_len;
double *fir = make_low_pass_filter(omega_pass, omega_stop, error, &fir_len);
double *result;
result = convolve(data, data_len, fir, fir_len);
free(fir);
return result;
}
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