An implementation of Analog cellular networks like A-Netz, B-Netz, C-Netz, NMT, AMPS, TACS, JTACS, Radiocom 2000, IMTS, MPT1327, Eurosignal and more http://osmocom-analog.eversberg.eu/
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osmocom-analog/src/libfsk/fsk.c

288 lines
8.4 KiB

/* FSK audio processing (coherent FSK modem)
*
* (C) 2017 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 "../libdebug/debug.h"
#include "fsk.h"
#define PI M_PI
/*
* fsk = instance of fsk modem
* inst = instance of user
* send_bit() = function to be called whenever a new bit has to be sent
* receive_bit() = function to be called whenever a new bit was received
* samplerate = samplerate
* bitrate = bits per second
* f0, f1 = two frequencies for bit 0 and bit 1
* level = level to modulate the frequencies
* coherent = use coherent modulation (FFSK)
* bitadjust = how much to adjust the sample clock when a bitchange was detected. (0 = nothing, don't use this, 0.5 full adjustment)
*/
int fsk_init(fsk_t *fsk, void *inst, int (*send_bit)(void *inst), void (*receive_bit)(void *inst, int bit, double quality, double level), int samplerate, double bitrate, double f0, double f1, double level, int coherent, double bitadjust)
{
double bandwidth;
int i;
int rc;
PDEBUG(DDSP, DEBUG_DEBUG, "Setup FSK for Transceiver. (F0 = %.1f, F1 = %.1f, peak = %.1f)\n", f0, f1, level);
memset(fsk, 0, sizeof(*fsk));
/* gen sine table with deviation */
fsk->sin_tab = calloc(65536+16384, sizeof(*fsk->sin_tab));
if (!fsk->sin_tab) {
fprintf(stderr, "No mem!\n");
rc = -ENOMEM;
goto error;
}
for (i = 0; i < 65536; i++)
fsk->sin_tab[i] = sin((double)i / 65536.0 * 2.0 * PI) * level;
fsk->inst = inst;
fsk->rx_bit = -1;
fsk->rx_bitadjust = bitadjust;
fsk->receive_bit = receive_bit;
fsk->tx_bit = -1;
fsk->level = level;
fsk->send_bit = send_bit;
fsk->f0_deviation = (f0 - f1) / 2.0;
fsk->f1_deviation = (f1 - f0) / 2.0;
if (f0 < f1) {
fsk->low_bit = 0;
fsk->high_bit = 1;
} else {
fsk->low_bit = 1;
fsk->high_bit = 0;
}
/* calculate bandwidth */
bandwidth = fabs(f0 - f1) * 2.0;
/* init fm demodulator */
rc = fm_demod_init(&fsk->demod, (double)samplerate, (f0 + f1) / 2.0, bandwidth);
if (rc < 0)
goto error;
fsk->bits_per_sample = (double)bitrate / (double)samplerate;
PDEBUG(DDSP, DEBUG_DEBUG, "Bitduration of %.4f bits per sample @ %d.\n", fsk->bits_per_sample, samplerate);
fsk->phaseshift65536[0] = f0 / (double)samplerate * 65536.0;
PDEBUG(DDSP, DEBUG_DEBUG, "phaseshift65536[0] = %.4f\n", fsk->phaseshift65536[0]);
fsk->phaseshift65536[1] = f1 / (double)samplerate * 65536.0;
PDEBUG(DDSP, DEBUG_DEBUG, "phaseshift65536[1] = %.4f\n", fsk->phaseshift65536[1]);
/* use coherent modulation, i.e. each bit has an integer number of
* half waves and starts/ends at zero crossing
*/
if (coherent) {
double waves;
fsk->coherent = 1;
waves = (f0 / bitrate);
if (fabs(round(waves * 2) - (waves * 2)) > 0.001) {
fprintf(stderr, "Failed to set coherent mode, half waves of F0 does not fit exactly into one bit, please fix!\n");
abort();
}
fsk->cycles_per_bit65536[0] = waves * 65536.0;
waves = (f1 / bitrate);
if (fabs(round(waves * 2) - (waves * 2)) > 0.001) {
fprintf(stderr, "Failed to set coherent mode, half waves of F1 does not fit exactly into one bit, please fix!\n");
abort();
}
fsk->cycles_per_bit65536[1] = waves * 65536.0;
}
return 0;
error:
fsk_cleanup(fsk);
return rc;
}
/* Cleanup transceiver instance. */
void fsk_cleanup(fsk_t *fsk)
{
PDEBUG(DDSP, DEBUG_DEBUG, "Cleanup FSK for Transceiver.\n");
if (fsk->sin_tab) {
free(fsk->sin_tab);
fsk->sin_tab = NULL;
}
fm_demod_exit(&fsk->demod);
}
//#define DEBUG_MODULATOR
//#define DEBUG_FILTER
/* Demodulates bits
*
* If bit is received, callback function send_bit() is called.
*
* We sample each bit 0.5 bits after polarity change.
*
* If we have a bit change, adjust sample counter towards one half bit duration.
* We may have noise, so the bit change may be wrong or not at the correct place.
* This can cause bit slips.
* Therefore we change the sample counter only slightly, so bit slips may not
* happen so quickly.
*/
void fsk_receive(fsk_t *fsk, sample_t *sample, int length)
{
sample_t I[length], Q[length], frequency[length], f;
int i;
int bit;
double level, quality;
/* demod samples to offset around center frequency */
fm_demodulate_real(&fsk->demod, frequency, length, sample, I, Q);
for (i = 0; i < length; i++) {
f = frequency[i];
if (f < 0)
bit = fsk->low_bit;
else
bit = fsk->high_bit;
#ifdef DEBUG_FILTER
printf("|%s| %.3f\n", debug_amplitude(f / fabs(fsk->f0_deviation)), f / fabs(fsk->f0_deviation));
#endif
if (fsk->rx_bit != bit) {
#ifdef DEBUG_FILTER
puts("bit change");
#endif
fsk->rx_bit = bit;
if (fsk->rx_bitpos < 0.5) {
fsk->rx_bitpos += fsk->rx_bitadjust;
if (fsk->rx_bitpos > 0.5)
fsk->rx_bitpos = 0.5;
} else
if (fsk->rx_bitpos > 0.5) {
fsk->rx_bitpos -= fsk->rx_bitadjust;
if (fsk->rx_bitpos < 0.5)
fsk->rx_bitpos = 0.5;
}
}
/* if bit counter reaches 1, we subtract 1 and sample the bit */
if (fsk->rx_bitpos >= 1.0) {
/* peak level is the length of I/Q vector
* since we filter out the unwanted modulation product, the vector is only half of length */
level = sqrt(I[i] * I[i] + Q[i] * Q[i]) * 2.0;
/* quality is defined on how accurat the target frequency it hit
* if it is hit close to the center or close to double deviation from center, quality is close to 0 */
if (bit == 0)
quality = 1.0 - fabs((f - fsk->f0_deviation) / fsk->f0_deviation);
else
quality = 1.0 - fabs((f - fsk->f1_deviation) / fsk->f1_deviation);
if (quality < 0)
quality = 0;
#ifdef DEBUG_FILTER
printf("sample (level=%.3f, quality=%.3f)\n", level / fsk->level, quality);
#endif
/* adjust the values, because this is best we can get from fm demodulator */
fsk->receive_bit(fsk->inst, bit, quality / 0.95, level);
fsk->rx_bitpos -= 1.0;
}
fsk->rx_bitpos += fsk->bits_per_sample;
}
}
/* modulate bits
*
* If first/next bit is required, callback function send_bit() is called.
* If there is no (more) data to be transmitted, the callback functions shall
* return -1. In this case, this function stops and returns the number of
* samples that have been rendered so far, if any.
*
* For coherent mode (FSK), we round the phase on every bit change to the
* next zero crossing. This prevents phase shifts due to rounding errors.
*/
int fsk_send(fsk_t *fsk, sample_t *sample, int length, int add)
{
int count = 0;
double phase, phaseshift;
phase = fsk->tx_phase65536;
/* get next bit */
if (fsk->tx_bit < 0) {
next_bit:
fsk->tx_bit = fsk->send_bit(fsk->inst);
#ifdef DEBUG_MODULATOR
printf("bit change to %d\n", fsk->tx_bit);
#endif
if (fsk->tx_bit < 0)
goto done;
/* correct phase when changing bit */
if (fsk->coherent) {
/* round phase to nearest zero crossing */
if (phase > 16384.0 && phase < 49152.0)
phase = 32768.0;
else
phase = 0;
/* set phase according to current position in bit */
phase += fsk->tx_bitpos * fsk->cycles_per_bit65536[fsk->tx_bit & 1];
#ifdef DEBUG_MODULATOR
printf("phase %.3f bitpos=%.6f\n", phase, fsk->tx_bitpos);
#endif
}
}
/* modulate bit */
phaseshift = fsk->phaseshift65536[fsk->tx_bit & 1];
while (count < length && fsk->tx_bitpos < 1.0) {
if (add)
sample[count++] += fsk->sin_tab[(uint16_t)phase];
else
sample[count++] = fsk->sin_tab[(uint16_t)phase];
#ifdef DEBUG_MODULATOR
printf("|%s|\n", debug_amplitude(fsk->sin_tab[(uint16_t)phase] / fsk->level));
#endif
phase += phaseshift;
if (phase >= 65536.0)
phase -= 65536.0;
fsk->tx_bitpos += fsk->bits_per_sample;
}
if (fsk->tx_bitpos >= 1.0) {
fsk->tx_bitpos -= 1.0;
goto next_bit;
}
done:
fsk->tx_phase65536 = phase;
return count;
}
/* reset transmitter state, so we get a clean start */
void fsk_tx_reset(fsk_t *fsk)
{
fsk->tx_phase65536 = 0;
fsk->tx_bitpos = 0;
fsk->tx_bit = -1;
}