Added FIR filter to libfilter
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@ -3,4 +3,6 @@ AM_CPPFLAGS = -Wall -Wextra -g $(all_includes)
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noinst_LIBRARIES = libfilter.a
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libfilter_a_SOURCES = \
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iir_filter.c
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iir_filter.c \
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fir_filter.c
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@ -0,0 +1,197 @@
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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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/* convolve sample from ring buffer, starting with oldes */
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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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@ -0,0 +1,21 @@
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#ifndef _FIR_FILTER_H
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#define _FIR_FILTER_H
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typedef struct fir_filter {
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int ntaps;
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int delay;
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double *taps;
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double *buffer;
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int buffer_pos;
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} fir_filter_t;
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fir_filter_t *fir_lowpass_init(double samplerate, double cutoff, double transition_bandwidth);
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fir_filter_t *fir_highpass_init(double samplerate, double cutoff, double transition_bandwidth);
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fir_filter_t *fir_allpass_init(double samplerate, double transition_bandwidth);
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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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void fir_exit(fir_filter_t *fir);
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void fir_process(fir_filter_t *fir, sample_t *samples, int num);
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int fir_get_delay(fir_filter_t *fir);
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#endif /* _FIR_FILTER_H */
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@ -4,6 +4,7 @@
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#include <string.h>
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#include "../libsample/sample.h"
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#include "../libfilter/iir_filter.h"
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#include "../libfilter/fir_filter.h"
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#include "../libdebug/debug.h"
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#define level2db(level) (20 * log10(level))
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@ -40,6 +41,7 @@ int main(void)
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{
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iir_filter_t filter_low;
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iir_filter_t filter_high;
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fir_filter_t *fir_low/*, *fir_high*/;
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sample_t samples[SAMPLERATE];
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double level;
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int iter = 2;
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@ -102,6 +104,46 @@ int main(void)
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printf("\n");
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}
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double freq = 2000.0;
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double tb = 400.0;
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printf("testing low-pass FIR filter with %.0fHz transition bandwidth\n", tb);
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fir_low = fir_lowpass_init(SAMPLERATE, freq, tb);
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printf("Using %d taps\n", fir_low->ntaps);
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for (i = 0; i < 4001; i += 100) {
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gen_samples(samples, (double)i);
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fir_process(fir_low, samples, SAMPLERATE);
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level = get_level(samples);
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printf("%s%s%4d Hz: %.1f dB", debug_amplitude(level), debug_db(level), i, level2db(level));
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if (i == freq)
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printf(" cutoff\n");
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else
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printf("\n");
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}
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fir_exit(fir_low);
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#if 0
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double freq1 = 1000.0, freq2 = 2000.0;
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tb = 100.0;
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printf("testing two-pass FIR filter\n");
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fir_high = fir_twopass_init(SAMPLERATE, freq1, freq2, tb);
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printf("Using %d taps\n", fir_high->ntaps);
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for (i = 0; i < 4001; i += 100) {
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gen_samples(samples, (double)i);
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fir_process(fir_high, samples, SAMPLERATE);
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level = get_level(samples);
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printf("%s%s%4d Hz: %.1f dB", debug_amplitude(level), debug_db(level), i, level2db(level));
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if (i == freq1 || i == freq2)
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printf(" cutoff\n");
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else
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printf("\n");
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}
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fir_exit(fir_high);
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#endif
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return 0;
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}
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