84 lines
2.5 KiB
C
84 lines
2.5 KiB
C
/* C-Netz audio spectrum inversion (Sprachverschleierung)
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*
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* (C) 2016 by Andreas Eversberg <jolly@eversberg.eu>
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* All Rights Reserved
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <stdint.h>
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#include <math.h>
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#include "../common/sample.h"
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#include "scrambler.h"
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#define PI M_PI
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/* FTZ 171 TR 60 Clause 6.2
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* Carrier frequency, that is the spectrum that is mirrored */
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#define CARRIER_HZ 3300.0
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#define FILTER_BELOW 300.0
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#define FILTER_TURNS 2
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/* FTZ 171 TR 60 Clause 6.3
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* How much must the carrier frequency be lower than a 1000 HZ tone that passes the inversion.
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* The filter must be tuned to get that loss. */
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#define TEST_1000HZ_DB 55.0
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/* sine wave for carrier to modulate to */
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static double carrier[65536];
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void scrambler_init(void)
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{
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int i;
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for (i = 0; i < 65536; i++) {
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/* our amplitude must be doubled, since we have one spectrum above and one below carrier */
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carrier[i] = sin((double)i / 65536.0 * 2 * PI) * 2.0;
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}
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}
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void scrambler_setup(scrambler_t *scrambler, int samplerate)
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{
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iir_lowpass_init(&scrambler->lp, CARRIER_HZ - FILTER_BELOW, samplerate, FILTER_TURNS);
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scrambler->carrier_phaseshift65536 = 65536.0 / ((double)samplerate / CARRIER_HZ);
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}
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/* Modulate samples to carriere that is twice the mirror frequency.
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* Then we got spectrum above carrier and mirrored spectrum below carrier.
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* Afterwards we cut off carrier frequency and frequencies above carrier.
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*/
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void scrambler(scrambler_t *scrambler, sample_t *samples, int length)
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{
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double phaseshift, phase;
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int i;
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phaseshift = scrambler->carrier_phaseshift65536;
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phase = scrambler->carrier_phase65536;
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for (i = 0; i < length; i++) {
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/* modulate samples to carrier */
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samples[i] *= carrier[(uint16_t)phase];
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phase += phaseshift;
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if (phase >= 65536.0)
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phase -= 65536.0;
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}
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scrambler->carrier_phase65536 = phase;
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/* cut off carrier frequency and modulation above carrier frequency */
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iir_process(&scrambler->lp, samples, length);
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}
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