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/* spkmodem-recv.c - decode spkmodem signals */
/* SPDX-License-Identifier: GPL-2.0-or-later */
#include <err.h>
#include <errno.h>
#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_FRAME 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
signed short frame[2 * SAMPLES_PER_FRAME];
signed short pulse[2 * SAMPLES_PER_FRAME];
int ringpos = 0;
int pos, f1, f2;
int amplitude = 0;
int lp = 0;
void handle_audio(void);
void fetch_sample(void);
int
main(int argc, char *argv[])
{
(void)argc; (void)argv;
while (!feof(stdin))
handle_audio();
return errno;
}
void
handle_audio(void)
{
static int ascii_bit = 7;
static char ascii = 0;
static int llp = 0;
if (lp > 3 * SAMPLES_PER_FRAME) {
ascii_bit = 7;
ascii = 0;
lp = 0;
llp++;
}
if (llp == FLUSH_TIMEOUT)
if (fflush(stdout) == EOF)
err(errno, NULL);
if (f2 <= FREQ_SEP_MIN || f2 >= FREQ_SEP_MAX
|| f1 <= FREQ_DATA_MIN || f1 >= FREQ_DATA_MAX) {
fetch_sample();
return;
}
#if DEBUG
long stdin_pos = 0;
if ((stdin_pos = ftell(stdin)) == -1)
err(errno, NULL);
printf ("%d %d %d @%ld\n", f1, f2, FREQ_DATA_THRESHOLD,
stdin_pos - sizeof(frame));
#endif
if (f1 < FREQ_DATA_THRESHOLD)
ascii |= (1 << ascii_bit);
ascii_bit--;
if (ascii_bit < 0) {
#if DEBUG
printf("<%c, %x>", ascii, ascii);
#else
printf("%c", ascii);
#endif
ascii_bit = 7;
ascii = 0;
}
lp = 0;
llp = 0;
for (int i = 0; i < SAMPLES_PER_FRAME; i++)
fetch_sample();
}
void
fetch_sample(void)
{
amplitude -= abs(frame[ringpos]);
f1 -= pulse[ringpos];
f1 += pulse[(ringpos + SAMPLES_PER_FRAME) % (2 * SAMPLES_PER_FRAME)];
f2 -= pulse[(ringpos + SAMPLES_PER_FRAME) % (2 * SAMPLES_PER_FRAME)];
if (fread(frame + ringpos, 1, sizeof(frame[0]), stdin)
!= sizeof(frame[0]))
err(errno = ECANCELED, "Could not read frame.");
amplitude += abs(frame[ringpos]);
if (abs(frame[ringpos]) > THRESHOLD) { /* rising/falling edge(pulse) */
pulse[ringpos] = 1;
pos = !pos;
f2++;
} else {
pulse[ringpos] = 0;
}
ringpos++;
ringpos %= 2 * SAMPLES_PER_FRAME;
lp++;
}
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