4 files changed, 111 insertions, 0 deletions
diff --git a/util/spkmodem_recv/.gitignore b/util/spkmodem_recv/.gitignore
new file mode 100644
index 00000000..2f5c946c
--- /dev/null
+++ b/util/spkmodem_recv/.gitignore
@@ -0,0 +1 @@
+spkmodem-recv
diff --git a/util/spkmodem_recv/Makefile b/util/spkmodem_recv/Makefile
new file mode 100644
index 00000000..92a3bfe9
--- /dev/null
+++ b/util/spkmodem_recv/Makefile
@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+PREFIX  ?= /usr/local
+INSTALL ?= install
+
+spkmodem-recv:
+	$(CC) -o $@ $@.c
+install: spkmodem-recv
+	$(INSTALL) -d $(DESTDIR)$(PREFIX)/bin/
+	$(INSTALL) $< -t $(DESTDIR)$(PREFIX)/bin/
diff --git a/util/spkmodem_recv/description.md b/util/spkmodem_recv/description.md
new file mode 100644
index 00000000..fe62f169
--- /dev/null
+++ b/util/spkmodem_recv/description.md
@@ -0,0 +1 @@
+Decode spkmodem signals `C`
diff --git a/util/spkmodem_recv/spkmodem-recv.c b/util/spkmodem_recv/spkmodem-recv.c
new file mode 100644
index 00000000..cd7bd483
--- /dev/null
+++ b/util/spkmodem_recv/spkmodem-recv.c
@@ -0,0 +1,100 @@
+/* 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;
+}