initial git import

src/common/filter.c

@@ -0,0 +1,81 @@
+/* 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 "filter.h"
+
+#define PI M_PI
+
+//#define CASCADE
+
+void biquad_init(biquad_low_pass_t *bq, double frequency, int samplerate)
+{
+	double Fc, Q, K, norm;
+
+	memset(bq, 0, sizeof(*bq));
+	Q = sqrt(0.5); /* 0.7071... */
+	Fc = frequency / (double)samplerate;
+	K = tan(PI * Fc);
+	norm = 1 / (1 + K / Q + K * K);
+	bq->a0 = K * K * norm;
+	bq->a1 = 2 * bq->a0;
+	bq->a2 = bq->a0;
+	bq->b1 = 2 * (K * K - 1) * norm;
+	bq->b2 = (1 - K / Q + K * K) * norm;
+}
+
+void biquad_process(biquad_low_pass_t *bq, double *samples, int length, int iterations)
+{
+	double a0, a1, a2, b1, b2;
+	double *z1, *z2;
+	double in, out;
+	int i, j;
+
+	if (iterations > 10) {
+		fprintf(stderr, "%s failed: too many iterations, please fix!\n", __func__);
+		abort();
+	}
+
+	/* get states */
+	a0 = bq->a0;
+	a1 = bq->a1;
+	a2 = bq->a2;
+	b1 = bq->b1;
+	b2 = bq->b2;
+
+	z1 = bq->z1;
+	z2 = bq->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;
+	}
+}
+