Add libam, a library to do AM modulation and demodulation

pull/1/head
Andreas Eversberg 5 years ago
parent 3fd0707a3b
commit c4d4e7feda
  1. 1
      .gitignore
  2. 1
      configure.ac
  3. 1
      src/Makefile.am
  4. 6
      src/libam/Makefile.am
  5. 133
      src/libam/am.c
  6. 26
      src/libam/am.h

1
.gitignore vendored

@ -44,6 +44,7 @@ src/libmncc/libmncc.a
src/libsound/libsound.a
src/libsdr/libsdr.a
src/libsample/libsample.a
src/libam/libam.a
src/libclipper/libclipper.a
src/anetz/libgermanton.a
src/anetz/anetz

@ -65,6 +65,7 @@ AC_OUTPUT(
src/libscrambler/Makefile
src/libemphasis/Makefile
src/libfsk/Makefile
src/libam/Makefile
src/libfm/Makefile
src/libfilter/Makefile
src/libwave/Makefile

@ -18,6 +18,7 @@ SUBDIRS = \
libscrambler \
libemphasis \
libfsk \
libam \
libfm \
libfilter \
libwave \

@ -0,0 +1,6 @@
AM_CPPFLAGS = -Wall -Wextra -g $(all_includes)
noinst_LIBRARIES = libam.a
libam_a_SOURCES = \
am.c

@ -0,0 +1,133 @@
/* 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 <stdint.h>
#include <string.h>
#include <math.h>
#include "../libsample/sample.h"
#include "am.h"
#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;
mod->phasestep = 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, int num, float *baseband)
{
int s;
double vector;
double phasestep = mod->phasestep;
double phase = mod->phase;
double gain = mod->gain;
double bias = mod->bias;
for (s = 0; s < num; s++) {
vector = *amplitude++ * gain + bias;
*baseband++ = cos(phase) * vector;
*baseband++ = sin(phase) * vector;
phase += phasestep;
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;
demod->phasestep = 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 phasestep = demod->phasestep;
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++];
_sin = sin(phase);
_cos = cos(phase);
phase += phasestep;
if (phase < 0.0)
phase += 2.0 * M_PI;
else if (phase >= 2.0 * M_PI)
phase -= 2.0 * M_PI;
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;
}

@ -0,0 +1,26 @@
#include "../libfilter/iir_filter.h"
typedef struct am_mod {
double phasestep; /* angle to rotate vector per sample */
double phase; /* current phase */
double gain; /* gain to be multiplied to amplitude */
double bias; /* DC offset to add (carrier amplitude) */
} am_mod_t;
int am_mod_init(am_mod_t *mod, double samplerate, double offset, double gain, double bias);
void am_mod_exit(am_mod_t *mod);
void am_modulate_complex(am_mod_t *mod, sample_t *amplitude, int num, float *baseband);
typedef struct am_demod {
double phasestep; /* angle to rotate vector per sample */
double phase; /* current rotation phase (used to shift) */
double last_phase; /* last phase of FM (used to demodulate) */
iir_filter_t lp[3]; /* filters received IQ signal/carrier */
double gain; /* gain to be expected from amplitude */
double bias; /* DC offset to be expected (carrier amplitude) */
} am_demod_t;
int am_demod_init(am_demod_t *demod, double samplerate, double offset, double gain, double bias);
void am_demod_exit(am_demod_t *demod);
void am_demodulate_complex(am_demod_t *demod, sample_t *amplitude, int length, float *baseband, sample_t *I, sample_t *Q, sample_t *carrier);
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