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Implementation of C-Netz (German mobile telephone system)

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This commit is contained in:
Andreas Eversberg
2016-02-16 18:56:55 +01:00
parent 4b3e3385b5
commit 16acdbf59d
28 changed files with 10428 additions and 12 deletions
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src/cnetz/scrambler.c Normal file
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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 "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 500.0
#define FILTER_TURNS 1
/* 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 dampening. */
#define TEST_1000HZ_DB 55.0
/* sine wave for carrier to modulate to */
static double carrier[256];
void scrambler_init(void)
{
int i;
for (i = 0; i < 256; i++) {
carrier[i] = sin((double)i / 256.0 * 2 * PI);
}
}
void scrambler_setup(scrambler_t *scrambler, int samplerate)
{
biquad_init(&scrambler->bq, CARRIER_HZ - FILTER_BELOW, samplerate);
scrambler->carrier_phaseshift256 = 256.0 / ((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, int16_t *samples, int length)
{
double spl[length];
int32_t sample;
double phaseshift, phase;
int i;
phaseshift = scrambler->carrier_phaseshift256;
phase = scrambler->carrier_phase256;
for (i = 0; i < length; i++) {
/* modulate samples to carrier */
spl[i] = (double)samples[i] / 32768.0 * carrier[((uint8_t)phase) & 0xff];
phase += phaseshift;
if (phase >= 256.0)
phase -= 256.0;
}
scrambler->carrier_phase256 = phase;
/* cut off carrier frequency and modulation above carrier frequency */
biquad_process(&scrambler->bq, spl, length, FILTER_TURNS);
for (i = 0; i < length; i++) {
/* store result */
sample = spl[i] * 2.0 * 32768.0;
if (sample > 32767)
sample = 32767;
else if (sample < -32768)
sample = -32768;
*samples++ = sample;
}
}