2020-02-27 17:47:54 +00:00
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/* FIR filter
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*
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* (C) 2017 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 "../libsample/sample.h"
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#include "fir_filter.h"
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//#define DEBUG_TAPS
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static void kernel(double *taps, int M, double cutoff, int invert)
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{
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int i;
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double sum;
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for (i = 0; i <= M; i++) {
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/* gen sinc */
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if (i == M / 2)
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taps[i] = 2.0 * M_PI * cutoff;
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else
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taps[i] = sin(2.0 * M_PI * cutoff * (double)(i - M / 2))
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/ (double)(i - M / 2);
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/* blackman window */
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taps[i] *= 0.42 - 0.50 * cos(2 * M_PI * (double)(i / M))
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+ 0.08 * cos(4 * M_PI * (double)(i / M));
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}
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/* normalize */
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sum = 0;
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for (i = 0; i <= M; i++)
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sum += taps[i];
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for (i = 0; i <= M; i++)
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taps[i] /= sum;
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/* invert */
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if (invert) {
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for (i = 0; i <= M; i++)
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taps[i] = -taps[i];
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taps[M / 2] += 1.0;
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}
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#ifdef DEBUG_TAPS
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puts("start");
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for (i = 0; i <= M; i++)
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puts(debug_amplitude(taps[i]));
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#endif
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}
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static fir_filter_t *fir_init(double samplerate, double transition_bandwidth)
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{
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fir_filter_t *fir;
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int M;
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/* alloc struct */
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fir = calloc(1, sizeof(*fir));
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if (!fir) {
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fprintf(stderr, "No memory creating FIR filter!\n");
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return NULL;
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}
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/* transition bandwidth */
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M = ceil(1.0 / (transition_bandwidth / samplerate));
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if ((M & 1))
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M++;
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// printf("cutoff=%.4f\n", cutoff / samplerate);
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// printf("tb=%.4f\n", transition_bandwidth / samplerate);
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fir->ntaps = M + 1;
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fir->delay = M / 2;
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/* alloc taps */
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fir->taps = calloc(fir->ntaps, sizeof(*fir->taps));
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if (!fir->taps) {
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fprintf(stderr, "No memory creating FIR filter!\n");
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fir_exit(fir);
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return NULL;
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}
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/* alloc ring buffer */
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fir->buffer = calloc(fir->ntaps, sizeof(*fir->buffer));
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if (!fir->buffer) {
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fprintf(stderr, "No memory creating FIR filter!\n");
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fir_exit(fir);
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return NULL;
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}
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return fir;
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}
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fir_filter_t *fir_lowpass_init(double samplerate, double cutoff, double transition_bandwidth)
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{
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/* calculate kernel */
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fir_filter_t *fir = fir_init(samplerate, transition_bandwidth);
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if (!fir)
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return NULL;
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kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 0);
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return fir;
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}
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fir_filter_t *fir_highpass_init(double samplerate, double cutoff, double transition_bandwidth)
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{
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fir_filter_t *fir = fir_init(samplerate, transition_bandwidth);
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if (!fir)
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return NULL;
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kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 1);
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return fir;
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}
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fir_filter_t *fir_allpass_init(double samplerate, double transition_bandwidth)
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{
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fir_filter_t *fir = fir_init(samplerate, transition_bandwidth);
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if (!fir)
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return NULL;
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fir->taps[(fir->ntaps - 1) / 2] = 1.0;
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return fir;
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}
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fir_filter_t *fir_twopass_init(double samplerate, double cutoff_low, double cutoff_high, double transition_bandwidth)
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{
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int i;
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double sum;
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fir_filter_t *fir = fir_init(samplerate, transition_bandwidth);
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if (!fir)
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return NULL;
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double lp_taps[fir->ntaps], hp_taps[fir->ntaps];
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kernel(lp_taps, fir->ntaps - 1, cutoff_low / samplerate, 0);
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kernel(hp_taps, fir->ntaps - 1, cutoff_high / samplerate, 1);
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sum = 0;
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printf("#warning does not work as expected\n");
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abort();
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for (i = 0; i < fir->ntaps; i++) {
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fir->taps[i] = lp_taps[i] + hp_taps[i];
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sum += fir->taps[i];
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}
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/* hp will die */
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// for (i = 0; i < fir->ntaps; i++)
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// fir->taps[i] /= sum;
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return fir;
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}
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void fir_exit(fir_filter_t *fir)
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{
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if (!fir)
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return;
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free(fir->taps);
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free(fir->buffer);
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free(fir);
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}
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void fir_process(fir_filter_t *fir, sample_t *samples, int num)
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{
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int i, j;
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double y;
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for (i = 0; i < num; i++) {
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/* put sample in ring buffer */
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fir->buffer[fir->buffer_pos] = samples[i];
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if (++fir->buffer_pos == fir->ntaps)
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fir->buffer_pos = 0;
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/* convolve samples */
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y = 0;
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for (j = 0; j < fir->ntaps; j++) {
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2021-01-01 21:11:48 +00:00
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/* convolve sample from ring buffer, starting with oldest */
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2020-02-27 17:47:54 +00:00
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y += fir->buffer[fir->buffer_pos] * fir->taps[j];
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if (++fir->buffer_pos == fir->ntaps)
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fir->buffer_pos = 0;
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}
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samples[i] = y;
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}
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}
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int fir_get_delay(fir_filter_t *fir)
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{
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return fir->delay;
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}
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