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/* spkmodem-recv.c - decode spkmodem signals */
/* SPDX-License-Identifier: GPL-2.0-or-later */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Compilation: gcc -o spkmodem-recv spkmodem-recv */
/* Usage: parec --channels=1 --rate=48000 --format=s16le | ./spkmodem-recv */
#define SAMPLES_PER_TRAME 240
#define FREQ_SEP_MIN 5
#define FREQ_SEP_MAX 15
#define FREQ_DATA_MIN 15
#define FREQ_DATA_THRESHOLD 25
#define FREQ_DATA_MAX 60
#define THRESHOLD 500
#define DEBUG 0
#define FLUSH_TIMEOUT 1
static signed short trame[2 * SAMPLES_PER_TRAME];
static signed short pulse[2 * SAMPLES_PER_TRAME];
static int ringpos = 0;
static int pos, f1, f2;
static int amplitude = 0;
static int lp = 0;
static void
read_sample (void)
{
amplitude -= abs (trame[ringpos]);
f1 -= pulse[ringpos];
f1 += pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
f2 -= pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
fread (trame + ringpos, 1, sizeof (trame[0]), stdin);
amplitude += abs (trame[ringpos]);
if (pos ? (trame[ringpos] < -THRESHOLD)
: (trame[ringpos] > +THRESHOLD))
{
pulse[ringpos] = 1;
pos = !pos;
f2++;
}
else
pulse[ringpos] = 0;
ringpos++;
ringpos %= 2 * SAMPLES_PER_TRAME;
lp++;
}
int
main ()
{
int bitn = 7;
char c = 0;
int i;
int llp = 0;
while (!feof (stdin))
{
if (lp > 3 * SAMPLES_PER_TRAME)
{
bitn = 7;
c = 0;
lp = 0;
llp++;
}
if (llp == FLUSH_TIMEOUT)
fflush (stdout);
if (f2 > FREQ_SEP_MIN && f2 < FREQ_SEP_MAX
&& f1 > FREQ_DATA_MIN && f1 < FREQ_DATA_MAX)
{
#if DEBUG
printf ("%d %d %d @%d\n", f1, f2, FREQ_DATA_THRESHOLD,
ftell (stdin) - sizeof (trame));
#endif
if (f1 < FREQ_DATA_THRESHOLD)
c |= (1 << bitn);
bitn--;
if (bitn < 0)
{
#if DEBUG
printf ("<%c, %x>", c, c);
#else
printf ("%c", c);
#endif
bitn = 7;
c = 0;
}
lp = 0;
llp = 0;
for (i = 0; i < SAMPLES_PER_TRAME; i++)
read_sample ();
continue;
}
read_sample ();
}
return 0;
}
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