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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>
#include <unistd.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 ERR() (errno = errno ? errno : ECANCELED)
signed short frame[2 * SAMPLES_PER_FRAME], pulse[2 * SAMPLES_PER_FRAME];
int freq_data, freq_separator, sample_count, ascii_bit = 7;
char ascii = 0;
void handle_audio(void);
void fetch_sample(void);
void read_frame(int ringpos);
int set_ascii_bit(void);
void print_char(void);
int
main(int argc, char *argv[])
{
int c;
#ifdef __OpenBSD__
if (pledge("stdio", NULL) == -1)
err(ERR(), "pledge");
#endif
setvbuf(stdout, NULL, _IONBF, 0);
while (!feof(stdin))
handle_audio();
if (errno)
err(errno, "Unhandled error upon exit. Exit status is errno.");
return 0;
}
void
handle_audio(void)
{
if (sample_count > (3 * SAMPLES_PER_FRAME)) {
ascii_bit = 7;
ascii = sample_count = 0;
}
if ((freq_separator <= FREQ_SEP_MIN) || (freq_separator >= FREQ_SEP_MAX)
|| (freq_data <= FREQ_DATA_MIN) || (freq_data >= FREQ_DATA_MAX)) {
fetch_sample();
return;
}
if (!set_ascii_bit())
print_char();
sample_count = 0;
for (int sample = 0; sample < SAMPLES_PER_FRAME; sample++)
fetch_sample();
}
void
fetch_sample(void)
{
static int ringpos = 0;
freq_data -= pulse[ringpos];
freq_data += pulse[(ringpos + SAMPLES_PER_FRAME)
% (2 * SAMPLES_PER_FRAME)];
freq_separator -= pulse[(ringpos + SAMPLES_PER_FRAME)
% (2 * SAMPLES_PER_FRAME)];
read_frame(ringpos);
if ((pulse[ringpos] = (abs(frame[ringpos]) > THRESHOLD) ? 1 : 0))
++freq_separator;
++ringpos;
ringpos %= 2 * SAMPLES_PER_FRAME;
++sample_count;
}
void
read_frame(int ringpos)
{
if ((fread(frame + ringpos, 1, sizeof(frame[0]), stdin)
!= sizeof(frame[0])) || (ferror(stdin) != 0))
err(ERR(), "Could not read from frame.");
}
int
set_ascii_bit(void)
{
#if DEBUG
long stdin_pos = 0;
if ((stdin_pos = ftell(stdin)) == -1)
err(ERR(), NULL);
printf ("%d %d %d @%ld\n", freq_data, freq_separator,
FREQ_DATA_THRESHOLD, stdin_pos - sizeof(frame));
#endif
if (freq_data < FREQ_DATA_THRESHOLD)
ascii |= (1 << ascii_bit);
return ascii_bit;
}
void
print_char(void)
{
#if DEBUG
printf("<%c, %x>", ascii, ascii);
#else
printf("%c", ascii);
#endif
ascii_bit = 7;
ascii = 0;
}
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