util/spkmodem_recv/spkmodem-recv.c

   1 /* spkmodem-recv.c - decode spkmodem signals */
   2 /* SPDX-License-Identifier: GPL-2.0-or-later */
   3
   4 #include <stdio.h>
   5 #include <stdlib.h>
   6 #include <string.h>
   7
   8 /* Compilation:  gcc -o spkmodem-recv spkmodem-recv  */
   9 /* Usage: parec --channels=1 --rate=48000 --format=s16le | ./spkmodem-recv */
  10
  11 #define SAMPLES_PER_TRAME 240
  12 #define FREQ_SEP_MIN 5
  13 #define FREQ_SEP_MAX 15
  14 #define FREQ_DATA_MIN 15
  15 #define FREQ_DATA_THRESHOLD 25
  16 #define FREQ_DATA_MAX 60
  17 #define THRESHOLD 500
  18
  19 #define DEBUG 0
  20 #define FLUSH_TIMEOUT 1
  21
  22 static signed short trame[2 * SAMPLES_PER_TRAME];
  23 static signed short pulse[2 * SAMPLES_PER_TRAME];
  24 static int ringpos = 0;
  25 static int pos, f1, f2;
  26 static int amplitude = 0;
  27 static int lp = 0;
  28
  29 static void
  30 read_sample (void)
  31 {
  32   amplitude -= abs (trame[ringpos]);
  33   f1 -= pulse[ringpos];
  34   f1 += pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
  35   f2 -= pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
  36   fread (trame + ringpos, 1, sizeof (trame[0]), stdin);
  37   amplitude += abs (trame[ringpos]);
  38
  39   if (pos ? (trame[ringpos] < -THRESHOLD)
  40       : (trame[ringpos] > +THRESHOLD))
  41     {
  42       pulse[ringpos] = 1;
  43       pos = !pos;
  44       f2++;
  45     }
  46   else
  47     pulse[ringpos] = 0;
  48   ringpos++;
  49   ringpos %= 2 * SAMPLES_PER_TRAME;
  50   lp++;
  51 }
  52
  53 int
  54 main ()
  55 {
  56   int bitn = 7;
  57   char c = 0;
  58   int i;
  59   int llp = 0;
  60   while (!feof (stdin))
  61     {
  62       if (lp > 3 * SAMPLES_PER_TRAME)
  63         {
  64           bitn = 7;
  65           c = 0;
  66           lp = 0;
  67           llp++;
  68         }
  69       if (llp == FLUSH_TIMEOUT)
  70         fflush (stdout);
  71       if (f2 > FREQ_SEP_MIN && f2 < FREQ_SEP_MAX
  72           && f1 > FREQ_DATA_MIN && f1 < FREQ_DATA_MAX)
  73         {
  74 #if DEBUG
  75           printf ("%d %d %d @%d\n", f1, f2, FREQ_DATA_THRESHOLD,
  76                   ftell (stdin) - sizeof (trame));
  77 #endif
  78           if (f1 < FREQ_DATA_THRESHOLD)
  79             c |= (1 << bitn);
  80           bitn--;
  81           if (bitn < 0)
  82             {
  83 #if DEBUG
  84               printf ("<%c, %x>", c, c);
  85 #else
  86               printf ("%c", c);
  87 #endif
  88               bitn = 7;
  89               c = 0;
  90             }
  91           lp = 0;
  92           llp = 0;
  93           for (i = 0; i < SAMPLES_PER_TRAME; i++)
  94             read_sample ();
  95           continue;
  96         }
  97       read_sample ();
  98     }
  99   return 0;
 100 }