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