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Diffstat (limited to 'src/libfilter/fir_filter.c')
| -rw-r--r-- | src/libfilter/fir_filter.c | 197 |
1 files changed, 197 insertions, 0 deletions
diff --git a/src/libfilter/fir_filter.c b/src/libfilter/fir_filter.c new file mode 100644 index 0000000..5df4f40 --- /dev/null +++ b/src/libfilter/fir_filter.c @@ -0,0 +1,197 @@ +/* FIR filter + * + * (C) 2017 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 "../libsample/sample.h" +#include "fir_filter.h" + +//#define DEBUG_TAPS + +static void kernel(double *taps, int M, double cutoff, int invert) +{ + int i; + double sum; + + for (i = 0; i <= M; i++) { + /* gen sinc */ + if (i == M / 2) + taps[i] = 2.0 * M_PI * cutoff; + else + taps[i] = sin(2.0 * M_PI * cutoff * (double)(i - M / 2)) + / (double)(i - M / 2); + /* blackman window */ + taps[i] *= 0.42 - 0.50 * cos(2 * M_PI * (double)(i / M)) + + 0.08 * cos(4 * M_PI * (double)(i / M)); + } + + /* normalize */ + sum = 0; + for (i = 0; i <= M; i++) + sum += taps[i]; + for (i = 0; i <= M; i++) + taps[i] /= sum; + + /* invert */ + if (invert) { + for (i = 0; i <= M; i++) + taps[i] = -taps[i]; + taps[M / 2] += 1.0; + } + +#ifdef DEBUG_TAPS + puts("start"); + for (i = 0; i <= M; i++) + puts(debug_amplitude(taps[i])); +#endif +} + +static fir_filter_t *fir_init(double samplerate, double transition_bandwidth) +{ + fir_filter_t *fir; + int M; + + /* alloc struct */ + fir = calloc(1, sizeof(*fir)); + if (!fir) { + fprintf(stderr, "No memory creating FIR filter!\n"); + return NULL; + } + + /* transition bandwidth */ + M = ceil(1.0 / (transition_bandwidth / samplerate)); + if ((M & 1)) + M++; + +// printf("cutoff=%.4f\n", cutoff / samplerate); +// printf("tb=%.4f\n", transition_bandwidth / samplerate); + fir->ntaps = M + 1; + fir->delay = M / 2; + + /* alloc taps */ + fir->taps = calloc(fir->ntaps, sizeof(*fir->taps)); + if (!fir->taps) { + fprintf(stderr, "No memory creating FIR filter!\n"); + fir_exit(fir); + return NULL; + } + + /* alloc ring buffer */ + fir->buffer = calloc(fir->ntaps, sizeof(*fir->buffer)); + if (!fir->buffer) { + fprintf(stderr, "No memory creating FIR filter!\n"); + fir_exit(fir); + return NULL; + } + + + return fir; +} + +fir_filter_t *fir_lowpass_init(double samplerate, double cutoff, double transition_bandwidth) +{ + /* calculate kernel */ + fir_filter_t *fir = fir_init(samplerate, transition_bandwidth); + if (!fir) + return NULL; + kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 0); + return fir; +} + +fir_filter_t *fir_highpass_init(double samplerate, double cutoff, double transition_bandwidth) +{ + fir_filter_t *fir = fir_init(samplerate, transition_bandwidth); + if (!fir) + return NULL; + kernel(fir->taps, fir->ntaps - 1, cutoff / samplerate, 1); + return fir; +} + +fir_filter_t *fir_allpass_init(double samplerate, double transition_bandwidth) +{ + fir_filter_t *fir = fir_init(samplerate, transition_bandwidth); + if (!fir) + return NULL; + fir->taps[(fir->ntaps - 1) / 2] = 1.0; + return fir; +} + +fir_filter_t *fir_twopass_init(double samplerate, double cutoff_low, double cutoff_high, double transition_bandwidth) +{ + int i; + double sum; + fir_filter_t *fir = fir_init(samplerate, transition_bandwidth); + if (!fir) + return NULL; + double lp_taps[fir->ntaps], hp_taps[fir->ntaps]; + kernel(lp_taps, fir->ntaps - 1, cutoff_low / samplerate, 0); + kernel(hp_taps, fir->ntaps - 1, cutoff_high / samplerate, 1); + sum = 0; + printf("#warning does not work as expected\n"); + abort(); + for (i = 0; i < fir->ntaps; i++) { + fir->taps[i] = lp_taps[i] + hp_taps[i]; + sum += fir->taps[i]; + } + /* hp will die */ +// for (i = 0; i < fir->ntaps; i++) + // fir->taps[i] /= sum; + return fir; +} + +void fir_exit(fir_filter_t *fir) +{ + if (!fir) + return; + free(fir->taps); + free(fir->buffer); + free(fir); +} + +void fir_process(fir_filter_t *fir, sample_t *samples, int num) +{ + int i, j; + double y; + + for (i = 0; i < num; i++) { + /* put sample in ring buffer */ + fir->buffer[fir->buffer_pos] = samples[i]; + if (++fir->buffer_pos == fir->ntaps) + fir->buffer_pos = 0; + + /* convolve samples */ + y = 0; + for (j = 0; j < fir->ntaps; j++) { + /* convolve sample from ring buffer, starting with oldes */ + y += fir->buffer[fir->buffer_pos] * fir->taps[j]; + if (++fir->buffer_pos == fir->ntaps) + fir->buffer_pos = 0; + } + samples[i] = y; + } +} + +int fir_get_delay(fir_filter_t *fir) +{ + return fir->delay; +} + |