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/* 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);
}
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