added lpf4*

tests/lpf1.c

@@ -0,0 +1,86 @@
+/*
+ *
+ * Copyright 2022 Oleg Borodin  <borodin@unix7.org>
+ *
+ * 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 2 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, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+ * MA 02110-1301, USA.
+ *
+ */
+
+
+#include <stdio.h>
+#include <math.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+
+
+typedef struct {
+    double x0;
+    double x1;
+    double rc;
+} lpf_t;
+
+void lpf_init(lpf_t *lpf, double freq) {
+    lpf->x0 = 0.0;
+    lpf->x1 = 0.0;
+
+    double order = 1.5;
+    double n = 1 / sqrt(pow(2, 1.0 / order) - 1);
+    lpf->rc = 1 / (2 * n * M_PI * freq);
+}
+
+
+double lpf_next(lpf_t *lpf, double x, double dt) {
+
+    double k = dt / (lpf->rc + dt);
+
+    lpf->x1 = lpf->x1 + k * (x - lpf->x1);
+    lpf->x0 = lpf->x0 + k * (lpf->x1 - lpf->x0);
+    return lpf->x0;
+}
+
+int main(int argc, char **argv) {
+
+    double t = 0;
+    double s = 0;
+    double freq1 = 10;
+    double freq2 = 50;
+    double freq3 = 200;
+    double dt = 0.0001;
+    double l = 0.5;
+
+    int fd = open("freq.dat", O_WRONLY | O_TRUNC | O_CREAT, 0666);
+    printf("start\n");
+
+    if (fd < 0) { return 1; };
+
+    lpf_t lpf;
+    lpf_init(&lpf, 30.0);
+
+
+    for (int i = 0; i < (l / dt); i++) {
+        t = i * dt;
+        double s1 = sin(2 * M_PI * freq1 * t);
+        double s2 = cos(2 * M_PI * freq2 * t);
+        double s3 = sin(2 * M_PI * freq3 * t);
+        s = (s1 + s2 + s3) / 3.0;
+        s = lpf_next(&lpf, s, dt);
+        dprintf(fd, "%e\t%e\n", t, s);
+    }
+    fsync(fd);
+    close(fd);
+    return 0;
+}