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/libsquelch/squelch.c

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5.4 KiB

/* Squelch functions
*
* (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 <string.h>
#include <math.h>
#include "../libdebug/debug.h"
#include "squelch.h"
#define CHAN squelch->chan
/* How does it work:
*
* After init, squelch() is called with the RF level and duration of each chunk.
* Normally quelch() returns SQUELCH_OPEN. If the RF level is below the
* threshold level for multe_time, it returns SQUELCH_MUTE. If the RF level is
* below the threshold level for loss_time, it returns SQUELCH_LOSS, which
* measns that the carrier was loss.
*
* This is done by a counter. Whenever the RF level is below threshold, the mute
* counter is incremented, whenever the RF level is above threshodl, the mute
* counter is decremented. When the mute counter reaches mute_time, the mute
* state is set and the 'mute' condition is returned. When the mute counter
* rechers 0, the mute state is unset and the 'open' condition is returned.
*
* If the mute state is set, the loss counter is incremented. If the mute state
* is not set, the loss counter is reset. When the loss counter reaches
* loss_time, the 'loss' condition is returned.
*/
/* NOTE: SQUELCH must be calibrated !AFTER! DC bias, to get the actual noise floor */
#define SQUELCH_INIT_TIME 0.1 /* wait some time before performing squelch */
#define SQUELCH_AUTO_TIME 0.5 /* duration of squelch quelch calibration */
#define SQUELCH_AUTO_OFFSET 10.0 /* auto calibration: offset above noise floor */
void squelch_init(squelch_t *squelch, int chan, double threshold_db, double mute_time, double loss_time)
{
memset(squelch, 0, sizeof(*squelch));
squelch->chan = chan;
squelch->threshold_db = threshold_db;
/* wait for init condition */
squelch->init_count = 0.0;
/* measure noise floor for auto threshold mode */
if (threshold_db == 0.0) {
/* automatic threshold */
PDEBUG_CHAN(DDSP, DEBUG_INFO, "RF signal squelch: Use automatic threshold\n");
squelch->auto_state = 1;
} else if (!isinf(threshold_db)) {
/* preset threshold */
PDEBUG_CHAN(DDSP, DEBUG_INFO, "RF signal squelch: Use preset threshold of %.1f dB\n", threshold_db);
}
/* squelch is mute on init */
squelch->mute_time = mute_time;
squelch->mute_count = mute_time;
squelch->mute_state = 1;
/* loss condition met on init */
squelch->loss_time = loss_time;
squelch->loss_state = 1;
}
enum squelch_result squelch(squelch_t *squelch, double rf_level_db, double duration)
{
/* squelch disabled */
if (isinf(squelch->threshold_db))
return SQUELCH_OPEN;
/* count until start quelch processing */
squelch->init_count += duration;
if (squelch->init_count < SQUELCH_INIT_TIME)
return SQUELCH_MUTE;
/* measure noise floor and calibrate threashold_db */
if (squelch->auto_state) {
squelch->auto_count += duration;
squelch->auto_level_sum += rf_level_db;
squelch->auto_level_count++;
if (squelch->auto_count >= SQUELCH_AUTO_TIME) {
double noise_db, threshold_db;
noise_db = squelch->auto_level_sum / (double) squelch->auto_level_count;
threshold_db = noise_db + SQUELCH_AUTO_OFFSET;
/* must be 0.1 dB smaller, so we prevent repeated debugging message with similar value */
if (threshold_db < squelch->threshold_db - 0.1) {
squelch->threshold_db = threshold_db;
PDEBUG_CHAN(DDSP, DEBUG_INFO, "RF signal measurement: %.1f dB noise floor, using squelch threshold of %.1f dB\n", noise_db, threshold_db);
}
squelch->auto_count = 0.0;
squelch->auto_level_count = 0;
squelch->auto_level_sum = 0.0;
}
}
/* enough RF level, so we unmute when mute_count reached 0 */
if (rf_level_db >= squelch->threshold_db) {
squelch->mute_count -= duration;
if (squelch->mute_count <= 0.0) {
if (squelch->mute_state) {
PDEBUG_CHAN(DDSP, DEBUG_INFO, "RF signal strong: Unmuting audio (RF %.1f >= %.1f dB)\n", rf_level_db, squelch->threshold_db);
squelch->mute_state = 0;
}
squelch->mute_count = 0.0;
}
} else {
/* RF level too low, so we mute when mute_count reached mute_time */
squelch->mute_count += duration;
if (squelch->mute_count >= squelch->mute_time) {
if (!squelch->mute_state) {
PDEBUG_CHAN(DDSP, DEBUG_INFO, "RF signal weak: Muting audio (RF %.1f < %.1f dB)\n", rf_level_db, squelch->threshold_db);
squelch->mute_state = 1;
}
squelch->mute_count = squelch->mute_time;
}
}
if (squelch->mute_state) {
/* at 'mute' condition, count and check for loss */
squelch->loss_count += duration;
if (squelch->loss_count >= squelch->loss_time) {
if (!squelch->loss_state) {
PDEBUG_CHAN(DDSP, DEBUG_DEBUG, "RF signal loss detected after %.1f seconds\n", squelch->loss_time);
squelch->loss_state = 1;
return SQUELCH_LOSS;
}
}
return SQUELCH_MUTE;
} else {
/* at unmute condition, reset loss counter */
squelch->loss_state = 0;
squelch->loss_count = 0.0;
return SQUELCH_OPEN;
}
}