**diff options**

Diffstat (limited to 'src/common/iir_filter.c')

-rw-r--r-- | src/common/iir_filter.c | 133 |

1 files changed, 133 insertions, 0 deletions

diff --git a/src/common/iir_filter.c b/src/common/iir_filter.c new file mode 100644 index 0000000..d080728 --- /dev/null +++ b/src/common/iir_filter.c @@ -0,0 +1,133 @@ +/* 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 "sample.h" +#include "iir_filter.h" + +#define PI M_PI + +void iir_lowpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations) +{ + double Fc, Q, K, norm; + + if (iterations > 64) { + fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__); + abort(); + } + + memset(filter, 0, sizeof(*filter)); + filter->iter = iterations; + Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */ + Fc = frequency / (double)samplerate; + K = tan(PI * Fc); + norm = 1 / (1 + K / Q + K * K); + filter->a0 = K * K * norm; + filter->a1 = 2 * filter->a0; + filter->a2 = filter->a0; + filter->b1 = 2 * (K * K - 1) * norm; + filter->b2 = (1 - K / Q + K * K) * norm; +} + +void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations) +{ + double Fc, Q, K, norm; + + memset(filter, 0, sizeof(*filter)); + filter->iter = iterations; + Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */ + Fc = frequency / (double)samplerate; + K = tan(PI * Fc); + norm = 1 / (1 + K / Q + K * K); + filter->a0 = 1 * norm; + filter->a1 = -2 * filter->a0; + filter->a2 = filter->a0; + filter->b1 = 2 * (K * K - 1) * norm; + filter->b2 = (1 - K / Q + K * K) * norm; +} + +void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations) +{ + double Fc, Q, K, norm; + + memset(filter, 0, sizeof(*filter)); + filter->iter = iterations; + Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */ + Fc = frequency / (double)samplerate; + K = tan(PI * Fc); + norm = 1 / (1 + K / Q + K * K); + filter->a0 = K / Q * norm; + filter->a1 = 0; + filter->a2 = -filter->a0; + filter->b1 = 2 * (K * K - 1) * norm; + filter->b2 = (1 - K / Q + K * K) * norm; +} + +void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations) +{ + double Fc, Q, K, norm; + + memset(filter, 0, sizeof(*filter)); + filter->iter = iterations; + Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */ + Fc = frequency / (double)samplerate; + K = tan(PI * Fc); + norm = 1 / (1 + K / Q + K * K); + filter->a0 = (1 + K * K) * norm; + filter->a1 = 2 * (K * K - 1) * norm; + filter->a2 = filter->a0; + filter->b1 = filter->a1; + filter->b2 = (1 - K / Q + K * K) * norm; +} + +void iir_process(iir_filter_t *filter, sample_t *samples, int length) +{ + double a0, a1, a2, b1, b2; + double *z1, *z2; + double in, out; + int iterations = filter->iter; + int i, j; + + /* get states */ + a0 = filter->a0; + a1 = filter->a1; + a2 = filter->a2; + b1 = filter->b1; + b2 = filter->b2; + + z1 = filter->z1; + z2 = filter->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; + } +} + |