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demod_poc12.c
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demod_poc12.c
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/*
* demod_poc12.c -- 1200 baud POCSAG demodulator
*
* Copyright (C) 1996
* Thomas Sailer ([email protected], [email protected])
* Copyright (C) 2024
* Marat Fayzullin ([email protected])
*
* POCSAG (Post Office Code Standard Advisory Group)
* Radio Paging Decoder
*
* 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 2 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, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/* ---------------------------------------------------------------------- */
#include "multimon.h"
#include "filter.h"
#include <math.h>
#include <string.h>
#include <stdio.h>
/* ---------------------------------------------------------------------- */
#define FREQ_SAMP 22050
#define BAUD 1200
#define SUBSAMP 2
#define FILTLEN 1
/* ---------------------------------------------------------------------- */
#define SPHASEINC (0x10000u*BAUD*SUBSAMP/FREQ_SAMP)
/* ---------------------------------------------------------------------- */
static void poc12_init(struct demod_state *s)
{
pocsag_init(s);
memset(&s->l1.poc12, 0, sizeof(s->l1.poc12));
}
/* ---------------------------------------------------------------------- */
static void poc12_demod(struct demod_state *s, buffer_t buffer, int length)
{
if (s->l1.poc12.subsamp) {
if (length <= (int)s->l1.poc12.subsamp) {
s->l1.poc12.subsamp -= length;
return;
}
buffer.fbuffer += s->l1.poc12.subsamp;
length -= s->l1.poc12.subsamp;
s->l1.poc12.subsamp = 0;
}
for (; length > 0; length -= SUBSAMP, buffer.fbuffer += SUBSAMP) {
s->l1.poc12.dcd_shreg <<= 1;
s->l1.poc12.dcd_shreg |= ((*buffer.fbuffer) > 0);
verbprintf(10, "%c", '0'+(s->l1.poc12.dcd_shreg & 1));
/*
* check if transition
*/
if ((s->l1.poc12.dcd_shreg ^ (s->l1.poc12.dcd_shreg >> 1)) & 1) {
if (s->l1.poc12.sphase < (0x8000u-(SPHASEINC/2)))
s->l1.poc12.sphase += SPHASEINC/8;
else
s->l1.poc12.sphase -= SPHASEINC/8;
}
s->l1.poc12.sphase += SPHASEINC;
if (s->l1.poc12.sphase >= 0x10000u) {
s->l1.poc12.sphase &= 0xffffu;
pocsag_rxbit(s, s->l1.poc12.dcd_shreg & 1);
}
}
s->l1.poc12.subsamp = -length;
}
static void poc12_deinit(struct demod_state *s)
{
pocsag_deinit(s);
}
/* ---------------------------------------------------------------------- */
const struct demod_param demod_poc12 = {
"POCSAG1200", true, FREQ_SAMP, FILTLEN, poc12_init, poc12_demod, poc12_deinit
};
/* ---------------------------------------------------------------------- */