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/* AM modulation and de-modulation
*
* (C) 2018 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 <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <math.h>
#include "../libsample/sample.h"
#include "am.h"
static int has_init = 0;
static int fast_math = 0;
static float *sin_tab = NULL, *cos_tab = NULL;
/* global init */
int am_init(int _fast_math)
{
fast_math = _fast_math;
if (fast_math) {
int i;
sin_tab = calloc(65536+16384, sizeof(*sin_tab));
if (!sin_tab) {
fprintf(stderr, "No mem!\n");
return -ENOMEM;
}
cos_tab = sin_tab + 16384;
/* generate sine and cosine */
for (i = 0; i < 65536+16384; i++)
sin_tab[i] = sin(2.0 * M_PI * (double)i / 65536.0);
}
has_init = 1;
return 0;
}
/* global exit */
void am_exit(void)
{
if (sin_tab) {
free(sin_tab);
sin_tab = cos_tab = NULL;
}
has_init = 0;
}
#define CARRIER_FILTER 30.0
/* Amplitude modulation in SDR:
* Just use the base band (audio signal) as real value, and 0.0 as imaginary
* value. The you have two side bands. Be sure to have a DC level, so you
* have a carrier.
*/
int am_mod_init(am_mod_t *mod, double samplerate, double offset, double gain, double bias)
{
memset(mod, 0, sizeof(*mod));
mod->gain = gain;
mod->bias = bias;
if (fast_math)
mod->rot = 65536.0 * offset / samplerate;
else
mod->rot = 2.0 * M_PI * offset / samplerate;
return 0;
}
void am_mod_exit(am_mod_t __attribute__((unused)) *mod)
{
}
void am_modulate_complex(am_mod_t *mod, sample_t *amplitude, uint8_t *power, int num, float *baseband)
{
int s;
double vector;
double rot = mod->rot;
double phase = mod->phase;
double gain = mod->gain;
double bias = mod->bias;
for (s = 0; s < num; s++) {
if (*power++)
vector = *amplitude++ * gain + bias;
else
vector = 0.0;
if (fast_math) {
*baseband++ += cos_tab[(uint16_t)phase] * vector;
*baseband++ += sin_tab[(uint16_t)phase] * vector;
phase += rot;
if (phase < 0.0)
phase += 65536.0;
else if (phase >= 65536.0)
phase -= 65536.0;
} else {
*baseband++ += cos(phase) * vector;
*baseband++ += sin(phase) * vector;
phase += rot;
if (phase < 0.0)
phase += 2.0 * M_PI;
else if (phase >= 2.0 * M_PI)
phase -= 2.0 * M_PI;
}
}
mod->phase = phase;
}
/* init AM demodulator */
int am_demod_init(am_demod_t *demod, double samplerate, double offset, double bandwidth, double gain)
{
memset(demod, 0, sizeof(*demod));
demod->gain = gain;
if (fast_math)
demod->rot = 65536.0 * -offset / samplerate;
else
demod->rot = 2 * M_PI * -offset / samplerate;
/* use fourth order (2 iter) filter, since it is as fast as second order (1 iter) filter */
iir_lowpass_init(&demod->lp[0], bandwidth, samplerate, 2);
iir_lowpass_init(&demod->lp[1], bandwidth, samplerate, 2);
/* filter carrier */
iir_lowpass_init(&demod->lp[2], CARRIER_FILTER, samplerate, 1);
return 0;
}
void am_demod_exit(am_demod_t __attribute__((unused)) *demod)
{
}
/* do amplitude demodulation of baseband and write them to samples */
void am_demodulate_complex(am_demod_t *demod, sample_t *amplitude, int length, float *baseband, sample_t *I, sample_t *Q, sample_t *carrier)
{
int s, ss;
double rot = demod->rot;
double phase = demod->phase;
double gain = demod->gain;
double i, q;
double _sin, _cos;
/* rotate spectrum */
for (s = 0, ss = 0; s < length; s++) {
i = baseband[ss++];
q = baseband[ss++];
phase += rot;
if (fast_math) {
if (phase < 0.0)
phase += 65536.0;
else if (phase >= 65536.0)
phase -= 65536.0;
_sin = sin_tab[(uint16_t)phase];
_cos = cos_tab[(uint16_t)phase];
} else {
if (phase < 0.0)
phase += 2.0 * M_PI;
else if (phase >= 2.0 * M_PI)
phase -= 2.0 * M_PI;
_sin = sin(phase);
_cos = cos(phase);
}
I[s] = i * _cos - q * _sin;
Q[s] = i * _sin + q * _cos;
}
demod->phase = phase;
/* filter bandwidth */
iir_process(&demod->lp[0], I, length);
iir_process(&demod->lp[1], Q, length);
/* demod */
for (s = 0; s < length; s++)
amplitude[s] = carrier[s] = sqrt(I[s] * I[s] + Q[s] * Q[s]);
/* filter carrier */
iir_process(&demod->lp[2], carrier, length);
/* normalize */
for (s = 0; s < length; s++)
amplitude[s] = (amplitude[s] - carrier[s]) / carrier[s] * gain;
}
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