82 lines
1.9 KiB
C
82 lines
1.9 KiB
C
/* cut-off filter (biquad) based on Nigel Redmon (www.earlevel.com)
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*
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* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <stdio.h>
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#include <stdint.h>
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#include <string.h>
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#include <stdlib.h>
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#include <math.h>
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#include "filter.h"
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#define PI M_PI
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//#define CASCADE
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void biquad_init(biquad_low_pass_t *bq, double frequency, int samplerate)
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{
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double Fc, Q, K, norm;
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memset(bq, 0, sizeof(*bq));
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Q = sqrt(0.5); /* 0.7071... */
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Fc = frequency / (double)samplerate;
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K = tan(PI * Fc);
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norm = 1 / (1 + K / Q + K * K);
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bq->a0 = K * K * norm;
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bq->a1 = 2 * bq->a0;
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bq->a2 = bq->a0;
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bq->b1 = 2 * (K * K - 1) * norm;
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bq->b2 = (1 - K / Q + K * K) * norm;
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}
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void biquad_process(biquad_low_pass_t *bq, double *samples, int length, int iterations)
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{
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double a0, a1, a2, b1, b2;
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double *z1, *z2;
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double in, out;
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int i, j;
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if (iterations > 10) {
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fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
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abort();
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}
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/* get states */
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a0 = bq->a0;
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a1 = bq->a1;
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a2 = bq->a2;
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b1 = bq->b1;
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b2 = bq->b2;
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z1 = bq->z1;
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z2 = bq->z2;
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/* process filter */
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for (i = 0; i < length; i++) {
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in = *samples;
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for (j = 0; j < iterations; j++) {
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out = in * a0 + z1[j];
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z1[j] = in * a1 + z2[j] - b1 * out;
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z2[j] = in * a2 - b2 * out;
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in = out;
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}
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*samples++ = in;
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}
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}
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