Restructure: Move scrambler from C-Netz code to 'libscrambler'

src/libscrambler/scrambler.c

@@ -0,0 +1,83 @@
+/* C-Netz audio spectrum inversion (Sprachverschleierung)
+ *
+ * (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 <stdint.h>
+#include <math.h>
+#include "../common/sample.h"
+#include "scrambler.h"
+
+#define PI		M_PI
+
+/* FTZ 171 TR 60 Clause 6.2
+ * Carrier frequency, that is the spectrum that is mirrored */
+#define CARRIER_HZ	3300.0
+#define FILTER_BELOW	300.0
+#define FILTER_TURNS	2
+
+/* FTZ 171 TR 60 Clause 6.3
+ * How much must the carrier frequency be lower than a 1000 HZ tone that passes the inversion.
+ * The filter must be tuned to get that loss. */
+#define TEST_1000HZ_DB	55.0
+
+/* sine wave for carrier to modulate to */
+static double carrier[65536];
+
+void scrambler_init(void)
+{
+	int i;
+
+	for (i = 0; i < 65536; i++) {
+		/* our amplitude must be doubled, since we have one spectrum above and one below carrier */
+		carrier[i] = sin((double)i / 65536.0 * 2 * PI) * 2.0;
+	}
+}
+
+void scrambler_setup(scrambler_t *scrambler, int samplerate)
+{
+	iir_lowpass_init(&scrambler->lp, CARRIER_HZ - FILTER_BELOW, samplerate, FILTER_TURNS);
+	scrambler->carrier_phaseshift65536 = 65536.0 / ((double)samplerate / CARRIER_HZ);
+}
+
+/* Modulate samples to carriere that is twice the mirror frequency.
+ * Then we got spectrum above carrier and mirrored spectrum below carrier.
+ * Afterwards we cut off carrier frequency and frequencies above carrier.
+ */
+void scrambler(scrambler_t *scrambler, sample_t *samples, int length)
+{
+	double phaseshift, phase;
+	int i;
+
+	phaseshift = scrambler->carrier_phaseshift65536;
+	phase = scrambler->carrier_phase65536;
+
+	for (i = 0; i < length; i++) {
+		/* modulate samples to carrier */
+		samples[i] *= carrier[(uint16_t)phase];
+		phase += phaseshift;
+		if (phase >= 65536.0)
+			phase -= 65536.0;
+	}
+
+	scrambler->carrier_phase65536 = phase;
+
+	/* cut off carrier frequency and modulation above carrier frequency */
+	iir_process(&scrambler->lp, samples, length);
+}
+
+