diff --git a/util/spkmodem_recv/.gitignore b/util/spkmodem_recv/.gitignore
new file mode 100644
index 0000000000000000000000000000000000000000..2f5c946cc4b2f00e25e0f2282d538d421e279e5f
--- /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 0000000000000000000000000000000000000000..92a3bfe930144461c8de02548efa22950c491ca8
--- /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/annotation b/util/spkmodem_recv/annotation
new file mode 100644
index 0000000000000000000000000000000000000000..eb4dd0533733af5dbe6471b9c058ccb52a7a65ca
--- /dev/null
+++ b/util/spkmodem_recv/annotation
@@ -0,0 +1,119 @@
+/* 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 */
+
+// 1/48000Hz * 240 = 5ms,
+#define SAMPLES_PER_TRAME 240
+// 5 transitions = 2.5 pulses/5ms = 500Hz
+#define FREQ_SEP_MIN 5
+// 15 transitions = 7.5 pulses/5ms = 1500Hz
+#define FREQ_SEP_MAX 15
+// 15 transitions = 7.5 pulses/5ms = 1500Hz
+#define FREQ_DATA_MIN 15
+// 25 transitions = 12.5 pulses/5ms = 2500Hz
+#define FREQ_DATA_THRESHOLD 25
+// 60 transitions = 30 pulses/5ms = 6000Hz
+#define FREQ_DATA_MAX 60
+// Threshold amplitude to register a transition
+#define THRESHOLD 500
+
+#define DEBUG 0
+#define FLUSH_TIMEOUT 1
+
+// Just a ring buffer of the raw samples, not meaningfully
+// used otherwise
+static signed short trame[2 * SAMPLES_PER_TRAME];
+// stores a 1 for each detected edge transition of the audio signal,
+// 0 otherwise
+static signed short pulse[2 * SAMPLES_PER_TRAME];
+// ring buffer index
+static int ringpos = 0;
+// pos is the polarity to look for in the next sample
+// f1 counts the number of transitions in each trame
+// f2
+static int pos, f1, f2;
+// amplitude not meaningfully used
+static int amplitude = 0;
+static int lp = 0;
+
+static void read_sample (void)
+{
+ amplitude -= abs (trame[ringpos]);
+ f1 -= pulse[ringpos];
+ // ringpos + SAMPLES_PER_TRAME will be the results of the
+ // previous trame interval
+ f1 += pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
+ // gradually clear the count of pulses from the previous trame from f1
+ f2 -= pulse[(ringpos + SAMPLES_PER_TRAME) % (2 * SAMPLES_PER_TRAME)];
+ // Read one 16 bit sample to trame index ringpos
+ fread (trame + ringpos, 1, sizeof (trame[0]), stdin);
+ // amplitude will be the difference in magnitude of old value
+ // in trame and new value
+ amplitude += abs (trame[ringpos]);
+
+ // if magnitude of trame[ringpos] greater than threshold, set pulse to 1
+ // found positive peak, now look for negative peak
+ if (pos ? (trame[ringpos] < -THRESHOLD) : (trame[ringpos] > +THRESHOLD)) {
+ pulse[ringpos] = 1;
+ // Invert sign flag
+ pos = !pos;
+ f2++;
+ } else {
+ pulse[ringpos] = 0;
+ }
+ // move buffer index forward
+ ringpos++;
+ ringpos %= 2 * SAMPLES_PER_TRAME;
+ lp++;
+}
+
+int main ()
+{
+ int bitn = 7;
+ char c = 0;
+ int i;
+ int llp = 0;
+ while (!feof (stdin)) {
+ // lp gets reset whenever a valid bit is detected,
+ // reset things if a bit hasn't been read in a while
+ 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;
+}
diff --git a/util/spkmodem_recv/description.md b/util/spkmodem_recv/description.md
new file mode 100644
index 0000000000000000000000000000000000000000..fe62f1691787debcdc3bbf70691fc8101ea00aca
--- /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 0000000000000000000000000000000000000000..cd7bd483d09b3d6be6f9204af1325a4c667bf60a
--- /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;
+}