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demod_clipfsk.c
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demod_clipfsk.c
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/*
* demod_clipfsk.c -- 1200 baud FSK demodulator
*
* Copyright (C) 2007
*
* 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>
/* ---------------------------------------------------------------------- */
/*
* V.23 signaling 1200 baud +/- 1%
* Mark frequency: 2300 Hz +/- 1.5%
* Space frequency: 1100 Hz +/- 1.5%
*/
#define FREQ_MARK 1200
#define FREQ_SPACE 2200
#define FREQ_SAMP 22050
#define BAUD 1200
#define SUBSAMP 2
/* ---------------------------------------------------------------------- */
#define CORRLEN ((int)(FREQ_SAMP/BAUD))
#define SPHASEINC (0x10000u*BAUD*SUBSAMP/FREQ_SAMP)
static float corr_mark_i[CORRLEN];
static float corr_mark_q[CORRLEN];
static float corr_space_i[CORRLEN];
static float corr_space_q[CORRLEN];
/* ---------------------------------------------------------------------- */
static void clipfsk_init(struct demod_state *s)
{
float f;
int i;
clip_init(s);
memset(&s->l1.clipfsk, 0, sizeof(s->l1.clipfsk));
for (f = 0, i = 0; i < CORRLEN; i++) {
corr_mark_i[i] = cos(f);
corr_mark_q[i] = sin(f);
f += 2.0*M_PI*FREQ_MARK/FREQ_SAMP;
}
for (f = 0, i = 0; i < CORRLEN; i++) {
corr_space_i[i] = cos(f);
corr_space_q[i] = sin(f);
f += 2.0*M_PI*FREQ_SPACE/FREQ_SAMP;
}
}
/* ---------------------------------------------------------------------- */
static void clipfsk_demod(struct demod_state *s, buffer_t buffer, int length)
{
float f;
unsigned char curbit;
if (s->l1.clipfsk.subsamp) {
int numfill = SUBSAMP - s->l1.clipfsk.subsamp;
if (length < numfill) {
s->l1.clipfsk.subsamp += length;
return;
}
buffer.fbuffer += numfill;
length -= numfill;
s->l1.clipfsk.subsamp = 0;
}
for (; length >= SUBSAMP; length -= SUBSAMP, buffer.fbuffer += SUBSAMP) {
f = fsqr(mac(buffer.fbuffer, corr_mark_i, CORRLEN)) +
fsqr(mac(buffer.fbuffer, corr_mark_q, CORRLEN)) -
fsqr(mac(buffer.fbuffer, corr_space_i, CORRLEN)) -
fsqr(mac(buffer.fbuffer, corr_space_q, CORRLEN));
s->l1.clipfsk.dcd_shreg <<= 1;
s->l1.clipfsk.dcd_shreg |= (f > 0);
verbprintf(10, "%c", '0'+(s->l1.clipfsk.dcd_shreg & 1));
/*
* check if transition
*/
if ((s->l1.clipfsk.dcd_shreg ^ (s->l1.clipfsk.dcd_shreg >> 1)) & 1) {
if (s->l1.clipfsk.sphase < (0x8000u-(SPHASEINC/2)))
s->l1.clipfsk.sphase += SPHASEINC/8;
else
s->l1.clipfsk.sphase -= SPHASEINC/8;
}
s->l1.clipfsk.sphase += SPHASEINC;
if (s->l1.clipfsk.sphase >= 0x10000u) {
s->l1.clipfsk.sphase &= 0xffffu;
curbit = s->l1.clipfsk.dcd_shreg & 1;
verbprintf(9, " %c ", '0'+curbit);
clip_rxbit(s, curbit);
}
}
s->l1.clipfsk.subsamp = length;
}
/* ---------------------------------------------------------------------- */
const struct demod_param demod_clipfsk = {
"CLIPFSK", true, FREQ_SAMP, CORRLEN, clipfsk_init, clipfsk_demod, NULL
};
/* ---------------------------------------------------------------------- */