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/* cutoff filter (biquad) based on Nigel Redmon (www.earlevel.com)
*
* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
* All Rights Reserved
*
* 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, either version 3 of the License, or
* (at your option) any later version.
*
* 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, see <http://www.gnu.org/licenses/>.
*/
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include "sample.h"
#include "iir_filter.h"
#define PI M_PI
void iir_lowpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
if (iterations > 64) {
fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
abort();
}
memset(filter, 0, sizeof(*filter));
filter>iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
filter>a0 = K * K * norm;
filter>a1 = 2 * filter>a0;
filter>a2 = filter>a0;
filter>b1 = 2 * (K * K  1) * norm;
filter>b2 = (1  K / Q + K * K) * norm;
}
void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
memset(filter, 0, sizeof(*filter));
filter>iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
filter>a0 = 1 * norm;
filter>a1 = 2 * filter>a0;
filter>a2 = filter>a0;
filter>b1 = 2 * (K * K  1) * norm;
filter>b2 = (1  K / Q + K * K) * norm;
}
void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
memset(filter, 0, sizeof(*filter));
filter>iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
filter>a0 = K / Q * norm;
filter>a1 = 0;
filter>a2 = filter>a0;
filter>b1 = 2 * (K * K  1) * norm;
filter>b2 = (1  K / Q + K * K) * norm;
}
void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations)
{
double Fc, Q, K, norm;
memset(filter, 0, sizeof(*filter));
filter>iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate;
K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K);
filter>a0 = (1 + K * K) * norm;
filter>a1 = 2 * (K * K  1) * norm;
filter>a2 = filter>a0;
filter>b1 = filter>a1;
filter>b2 = (1  K / Q + K * K) * norm;
}
void iir_process(iir_filter_t *filter, sample_t *samples, int length)
{
double a0, a1, a2, b1, b2;
double *z1, *z2;
double in, out;
int iterations = filter>iter;
int i, j;
/* get states */
a0 = filter>a0;
a1 = filter>a1;
a2 = filter>a2;
b1 = filter>b1;
b2 = filter>b2;
z1 = filter>z1;
z2 = filter>z2;
/* process filter */
for (i = 0; i < length; i++) {
in = *samples;
for (j = 0; j < iterations; j++) {
out = in * a0 + z1[j];
z1[j] = in * a1 + z2[j]  b1 * out;
z2[j] = in * a2  b2 * out;
in = out;
}
*samples++ = in;
}
}
