forked from cellular-infrastructure/osmocom-analog
82 lines
1.9 KiB
C
82 lines
1.9 KiB
C
/* cut-off 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 "filter.h"
|
|
|
|
#define PI M_PI
|
|
|
|
//#define CASCADE
|
|
|
|
void biquad_init(biquad_low_pass_t *bq, double frequency, int samplerate)
|
|
{
|
|
double Fc, Q, K, norm;
|
|
|
|
memset(bq, 0, sizeof(*bq));
|
|
Q = sqrt(0.5); /* 0.7071... */
|
|
Fc = frequency / (double)samplerate;
|
|
K = tan(PI * Fc);
|
|
norm = 1 / (1 + K / Q + K * K);
|
|
bq->a0 = K * K * norm;
|
|
bq->a1 = 2 * bq->a0;
|
|
bq->a2 = bq->a0;
|
|
bq->b1 = 2 * (K * K - 1) * norm;
|
|
bq->b2 = (1 - K / Q + K * K) * norm;
|
|
}
|
|
|
|
void biquad_process(biquad_low_pass_t *bq, double *samples, int length, int iterations)
|
|
{
|
|
double a0, a1, a2, b1, b2;
|
|
double *z1, *z2;
|
|
double in, out;
|
|
int i, j;
|
|
|
|
if (iterations > 10) {
|
|
fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
|
|
abort();
|
|
}
|
|
|
|
/* get states */
|
|
a0 = bq->a0;
|
|
a1 = bq->a1;
|
|
a2 = bq->a2;
|
|
b1 = bq->b1;
|
|
b2 = bq->b2;
|
|
|
|
z1 = bq->z1;
|
|
z2 = bq->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;
|
|
}
|
|
}
|
|
|