switch spkmodem_decode back to spkmodem_recvHEADmaster

this version is more tested. i'll merge _decode back
once it's better tested. it contains a lot of invasive
changes, whereas recv is much closer to the original GNU
code that i inherited, that i also know works quite well.

Signed-off-by: Leah Rowe <leah@libreboot.org>

6 files changed, 139 insertions, 757 deletions

diff --git a/util/spkmodem_decode/.gitignore b/util/spkmodem_decode/.gitignore
deleted file mode 100644
index 42814fe4..00000000
--- a/util/spkmodem_decode/.gitignore
+++ /dev/null
@@ -1,2 +0,0 @@
-spkmodem-recv
-spkmodem-decode
diff --git a/util/spkmodem_decode/Makefile b/util/spkmodem_decode/Makefile
deleted file mode 100644
index b00c4f43..00000000
--- a/util/spkmodem_decode/Makefile
+++ /dev/null
@@ -1,30 +0,0 @@
-# SPDX-License-Identifier: MIT
-# SPDX-FileCopyrightText: 2022,2026 Leah Rowe <leah@libreboot.org>
-# SPDX-FileCopyrightText: 2023 Riku Viitanen <riku.viitanen@protonmail.com>
-
-CC?=cc
-CFLAGS?=-Os -Wall -Wextra -Werror -pedantic -std=c90
-DESTDIR?=
-PREFIX?=/usr/local
-INSTALL?=install
-
-PROG=spkmodem-decode
-
-all: $(PROG)
-
-$(PROG): spkmodem-decode.c
-	$(CC) $(CFLAGS) spkmodem-decode.c -o $(PROG)
-
-install: $(PROG)
-	mkdir -p $(DESTDIR)$(PREFIX)/bin/
-	install -c $(PROG) $(DESTDIR)$(PREFIX)/bin/
-
-uninstall:
-	rm -f $(DESTDIR)$(PREFIX)/bin/$(PROG)
-
-clean:
-	rm -f $(PROG)
-
-distclean: clean
-
-.PHONY: all install uninstall clean distclean
diff --git a/util/spkmodem_decode/spkmodem-decode.c b/util/spkmodem_decode/spkmodem-decode.c
deleted file mode 100644
index 3b3b33f8..00000000
--- a/util/spkmodem_decode/spkmodem-decode.c
+++ /dev/null
@@ -1,725 +0,0 @@
-/*
- * SPDX-License-Identifier: GPL-2.0-or-later
- * Copyright (c) 2013 Free Software Foundation, Inc.
- * Copyright (c) 2023, 2026 Leah Rowe <leah@libreboot.org>
- *
- * This program receives text encoded as pulses on the PC speaker,
- * and decodes them via simple FSK (Frequency Shift Keying)
- * demodulation and FIR (Finite Impulse Response) frequency
- * discriminator.
- *
- * It waits for specific tones at specific intervals.
- * It detects tones within the audio stream and reconstructs
- * characters bit-by-bit as the encoded modem signal is received.
- * This is performance-efficient on most CPUs, and has relatively
- * high tolerance for noisy signals (similar to techniques used
- * for data stored on audio cassette tapes).
- *
- * This is a special interface provided by coreboot and GNU GRUB,
- * for computers that lack serial ports (useful for debugging).
- * Note that GRUB and coreboot can both send these signals; this
- * tool merely decodes them. This tool does not *encode*, only
- * decode.
- *
- * Usage example (NOTE: little endian!):
- * parec --channels=1 --rate=48000 --format=s16le | ./spkmodem-decode
- *
- * Originally provided by GNU GRUB, this version is a heavily
- * modified fork that complies with the OpenBSD Kernel Source
- * File Style Guide (KNF) instead of GNU coding standards; it
- * emphasises strict error handling, portability and code
- * quality, as characterised by OpenBSD projects. Several magic
- * numbers have been tidied up, calculated (not hardcoded) and
- * thoroughly explained, unlike in the original version.
- *
- * The original version was essentially a blob, masquerading as
- * source code. This forked source code is therefore the result
- * of extensive reverse engineering (of the GNU source code)!
- * This cleaned up code and extensive commenting will thoroughly
- * explain how the decoding works. This was done as an academic
- * exercise in 2023, continuing in 2026.
- *
- * This fork of spkmodem-recv, called spkmodem-decode, is provided
- * with Libreboot releases:
- * https://libreboot.org/
- *
- * The original GNU version is here, if you're morbidly curious:
- * https://cgit.git.savannah.gnu.org/cgit/grub.git/plain/util/spkmodem-recv.c?id=3dce38eb196f47bdf86ab028de74be40e13f19fd
- *
- * Libreboot's version was renamed to spkmodem-decode on 12 March 2026,
- * since Libreboot's version no longer closely resembles the GNU
- * version at all; ergo, a full rename was in order. GNU's version
- * was called spkmodem-recv.
- */
-
-#define _POSIX_SOURCE
-
-/*
- * For OpenBSD define, to detect version
- * for deciding whether to use pledge(2)
- */
-#ifdef __OpenBSD__
-#include <sys/param.h>
-#endif
-
-#include <errno.h>
-#include <limits.h>
-#include <stdio.h>
-#include <stdarg.h>
-#include <stdlib.h>
-#include <string.h>
-#include <unistd.h>
-
-/*
- * spkmodem is essentially using FSK (Frequency Shift Keying)
- * with two primary tones representing encoded bits,
- * separated by a framing tone.
- * Very cheap on CPU cycles and avoids needing something more
- * expensive like FFT or Goertzel filters, and tolerates
- * weak/noisy signals.
- */
-
-/*
- * Frequency of audio in Hz
- * WARNING: if changing, make sure to adjust
- *     SAMPLES_PER_FRAME accordingly (see maths below)
- */
-#define SAMPLE_RATE 48000
-
-/*
- * One analysis frame spans 5 ms.
- *
- *   frame_time = SAMPLES_PER_FRAME / SAMPLE_RATE
- *
- * With the default sample rate (48 kHz):
- *
- *   frame_time = N / 48000
- *   0.005 s = N / 48000
- *   N = 0.005 × 48000 = 240 samples
- */
-#define SAMPLES_PER_FRAME 240
-
-/*
- * Number of analysis frames per second.
- *
- * Each increment in the frequency counters corresponds
- * roughly to this many Hertz of tone frequency.
- *
- * With the default values:
- *   FRAME_RATE = 48000 / 240 = 200 Hz
- */
-#define FRAME_RATE ((SAMPLE_RATE) / (SAMPLES_PER_FRAME))
-
-/*
- * Two FIR windows are maintained; one for data tone,
- * and one for the separator tone. They are positioned
- * one frame apart in the ring buffer.
- */
-#define MAX_SAMPLES (2 * (SAMPLES_PER_FRAME))
-
-/*
- * Approx byte offset for ring buffer span, just for
- * easier debug output correlating to the audio stream.
- */
-#define SAMPLE_OFFSET ((MAX_SAMPLES) * (sizeof(short)))
-
-/*
- * Expected tone ranges (approximate, derived from spkmodem).
- * These values are intentionally wide because real-world setups
- * often involve microphones, room acoustics, and cheap ADCs.
- */
-#define SEP_TONE_MIN_HZ 1000
-#define SEP_TONE_MAX_HZ 3000
-
-#define SEP_TOLERANCE_PULSES \
-    (((SEP_TONE_MAX_HZ) - (SEP_TONE_MIN_HZ)) / (2 * (FRAME_RATE)))
-
-#define DATA_TONE_MIN_HZ 3000
-#define DATA_TONE_MAX_HZ 12000
-
-/* Mid point used to distinguish the two data tones. */
-#define DATA_TONE_THRESHOLD_HZ 5000
-
-/*
- * Convert tone frequency ranges into pulse counts within the
- * sliding analysis window.
- *
- * pulse_count = tone_frequency / FRAME_RATE
- * where FRAME_RATE = SAMPLE_RATE / SAMPLES_PER_FRAME.
- */
-#define FREQ_SEP_MIN ((SEP_TONE_MIN_HZ) / (FRAME_RATE))
-#define FREQ_SEP_MAX ((SEP_TONE_MAX_HZ) / (FRAME_RATE))
-
-#define FREQ_DATA_MIN ((DATA_TONE_MIN_HZ) / (FRAME_RATE))
-#define FREQ_DATA_MAX ((DATA_TONE_MAX_HZ) / (FRAME_RATE))
-
-#define FREQ_DATA_THRESHOLD ((DATA_TONE_THRESHOLD_HZ) / (FRAME_RATE))
-
-/*
- * These determine how long the program will wait during
- * tone auto-detection, before shifting to defaults.
- * It is done every LEARN_FRAMES number of frames.
- */
-#define LEARN_SECONDS 1
-#define LEARN_FRAMES ((LEARN_SECONDS) * (FRAME_RATE))
-
-/*
- * Sample amplitude threshold used to convert the waveform
- * into a pulse stream. Values near zero are regarded as noise.
- */
-#define THRESHOLD 500
-
-#define READ_BUF 4096
-
-struct decoder_state {
-	unsigned char pulse[MAX_SAMPLES];
-
-	signed short inbuf[READ_BUF];
-	size_t inpos;
-	size_t inlen;
-
-	int ringpos;
-	int sep_pos;
-
-	/*
-	 * Sliding window pulse counters
-	 * used to detect modem tones
-	 */
-	int freq_data;
-	int freq_separator;
-	int sample_count;
-
-	int ascii_bit;
-	unsigned char ascii;
-
-	int debug;
-	int swap_bytes;
-
-	/* dynamic separator calibration */
-	int sep_sum;
-	int sep_samples;
-	int sep_min;
-	int sep_max;
-
-	/* for automatic tone detection */
-	int freq_min;
-	int freq_max;
-	int freq_threshold;
-	int learn_frames;
-
-	/* previous sample used for edge detection */
-	signed short prev_sample;
-};
-
-static const char *argv0;
-
-/*
- * 16-bit little endian words are read
- * continuously. we will swap them, if
- * the host cpu is big endian.
- */
-static int host_is_big_endian(void);
-
-/* main loop */
-static void handle_audio(struct decoder_state *st);
-
-/* separate tone tolerances */
-static void select_separator_tone(struct decoder_state *st);
-static int is_valid_signal(struct decoder_state *st);
-
-/* output to terminal */
-static int set_ascii_bit(struct decoder_state *st);
-static void print_char(struct decoder_state *st);
-static void reset_char(struct decoder_state *st);
-
-/* process samples/frames */
-static void decode_pulse(struct decoder_state *st);
-static signed short read_sample(struct decoder_state *st);
-static void read_words(struct decoder_state *st);
-
-/* continually adjust tone */
-static void detect_tone(struct decoder_state *st);
-static int silent_signal(struct decoder_state *st);
-static void select_low_tone(struct decoder_state *st);
-
-/* debug */
-static void print_stats(struct decoder_state *st);
-
-/* error handling / usage */
-static void err(int errval, const char *msg, ...);
-static void usage(void);
-static const char *progname(void);
-
-/* portability (old systems) */
-int getopt(int, char * const *, const char *);
-extern char *optarg;
-extern int optind;
-extern int opterr;
-extern int optopt;
-
-#ifndef CHAR_BIT
-#define CHAR_BIT 8
-#endif
-
-typedef char static_assert_char_is_8_bits[(CHAR_BIT == 8) ? 1 : -1];
-typedef char static_assert_char_is_1[(sizeof(char) == 1) ? 1 : -1];
-typedef char static_assert_short[(sizeof(short) == 2) ? 1 : -1];
-typedef char static_assert_int_is_4[(sizeof(int) >= 4) ? 1 : -1];
-typedef char static_assert_twos_complement[
-    ((-1 & 3) == 3) ? 1 : -1
-];
-
-int
-main(int argc, char **argv)
-{
-	struct decoder_state st;
-	int c;
-
-	argv0 = argv[0];
-
-#if defined (__OpenBSD__) && defined(OpenBSD)
-#if OpenBSD >= 509
-	if (pledge("stdio", NULL) == -1)
-		err(errno, "pledge");
-#endif
-#endif
-
-	memset(&st, 0, sizeof(st));
-
-	while ((c = getopt(argc, argv, "d")) != -1) {
-		if (c != 'd')
-			usage();
-		st.debug = 1;
-		break;
-	}
-
-	/* fallback in case tone detection fails */
-	st.freq_min = 100000;
-	st.freq_max = 0;
-	st.freq_threshold = FREQ_DATA_THRESHOLD;
-
-	/*
-	 * Used for separator calibration
-	 */
-	st.sep_min = FREQ_SEP_MIN;
-	st.sep_max = FREQ_SEP_MAX;
-
-	st.ascii_bit = 7;
-
-	st.ringpos = 0;
-	st.sep_pos = SAMPLES_PER_FRAME;
-
-	if (host_is_big_endian())
-		st.swap_bytes = 1;
-
-	setvbuf(stdout, NULL, _IONBF, 0);
-
-	for (;;)
-		handle_audio(&st);
-
-	return EXIT_SUCCESS;
-}
-
-static int
-host_is_big_endian(void)
-{
-	unsigned int x = 1;
-	return (*(unsigned char *)&x == 0);
-}
-
-static void
-handle_audio(struct decoder_state *st)
-{
-	int sample;
-
-	/*
-	 * If the modem signal disappears for several (read: 3)
-	 * frames, discard the partially assembled character.
-	 */
-	if (st->sample_count >= (3 * SAMPLES_PER_FRAME) ||
-	    st->freq_separator <= 0)
-		reset_char(st);
-
-	st->sample_count = 0;
-
-	/* process exactly one frame */
-	for (sample = 0; sample < SAMPLES_PER_FRAME; sample++)
-		decode_pulse(st);
-
-	select_separator_tone(st);
-
-	if (set_ascii_bit(st) < 0)
-		print_char(st);
-
-	/* Detect tone per each frame */
-	detect_tone(st);
-}
-
-/*
- * collect separator tone statistics
- * (and auto-adjust tolerances)
- */
-static void
-select_separator_tone(struct decoder_state *st)
-{
-	int avg;
-
-	if (!is_valid_signal(st))
-		return;
-
-	st->sep_sum += st->freq_separator;
-	st->sep_samples++;
-
-	if (st->sep_samples != 50)
-		return;
-
-	avg = st->sep_sum / st->sep_samples;
-
-	st->sep_min = avg - SEP_TOLERANCE_PULSES;
-	st->sep_max = avg + SEP_TOLERANCE_PULSES;
-
-	/* reset calibration accumulators */
-	st->sep_sum = 0;
-	st->sep_samples = 0;
-
-	if (st->debug)
-		printf("separator calibrated: %dHz\n",
-		    avg * FRAME_RATE);
-}
-
-/*
- * Verify that the observed pulse densities fall within the
- * expected ranges for spkmodem tones. This prevents random noise
- * from being misinterpreted as data.
- */
-static int
-is_valid_signal(struct decoder_state *st)
-{
-	if (st->freq_data <= 0)
-		return 0;
-
-	if (st->freq_separator < st->sep_min ||
-	    st->freq_separator > st->sep_max)
-		return 0;
-
-	return 1;
-}
-
-/*
- * Each validated frame contributes one bit of modem data.
- * Bits are accumulated MSB-first into the ASCII byte.
- */
-static int
-set_ascii_bit(struct decoder_state *st)
-{
-	if (st->debug)
-		print_stats(st);
-
-	if (!is_valid_signal(st))
-		return st->ascii_bit;
-
-	if (st->freq_data < st->freq_threshold)
-		st->ascii |= (1 << st->ascii_bit);
-
-	st->ascii_bit--;
-
-	return st->ascii_bit;
-}
-
-static void
-print_char(struct decoder_state *st)
-{
-	if (st->debug)
-		printf("<%c,%x>", st->ascii, st->ascii);
-	else
-		putchar(st->ascii);
-
-	reset_char(st);
-}
-
-static void
-reset_char(struct decoder_state *st)
-{
-	st->ascii = 0;
-	st->ascii_bit = 7;
-}
-
-/*
- * Main demodulation step (moving-sum FIR filter).
- */
-static void
-decode_pulse(struct decoder_state *st)
-{
-	unsigned char old_ring, old_sep;
-	unsigned char new_pulse;
-	signed short sample;
-	int ringpos;
-	int sep_pos;
-	int diff_edge;
-	int diff_amp;
-
-	ringpos = st->ringpos;
-	sep_pos = st->sep_pos;
-
-	/*
-	 * Sliding rectangular FIR (Finite Impulse Response) filter.
-	 *
-	 * After thresholding, the signal becomes a stream of 0/1 pulses.
-	 * The decoder measures pulse density over two windows:
-	 *
-	 * freq_data: pulses in the "data" window
-	 * freq_separator: pulses in the "separator" window
-	 *
-	 * Instead of calculating each window every time (O(N) per frame), we
-	 * update the window sums incrementally:
-	 *
-	 *   sum_new = sum_old - pulse_leaving + pulse_entering
-	 *
-	 * This keeps the filter O(1) per sample instead of O(N).
-	 * The technique is equivalent to a rectangular FIR filter
-	 * implemented as a sliding moving sum.
-	 *
-	 * The two windows are exactly SAMPLES_PER_FRAME apart in the ring
-	 * buffer, so the pulse leaving the data window is simultaneously
-	 * entering the separator window.
-	 */
-	old_ring = st->pulse[ringpos];
-	old_sep  = st->pulse[sep_pos];
-	st->freq_data -= old_ring;
-	st->freq_data += old_sep;
-	st->freq_separator -= old_sep;
-
-	sample = read_sample(st);
-
-	/*
-	 * Avoid startup edge. Since
-	 * it's zero at startup, this
-	 * may wrongly produce a pulse
-	 */
-	if (st->sample_count == 0)
-		st->prev_sample = sample;
-
-	/*
-	 * Detect edges instead of amplitude.
-	 * This is more tolerant of weak microphones
-	 * and speaker distortion..
-	 *
-	 * However, we check both slope edges and
-	 * amplitude, to mitagate noise.
-	 */
-	diff_amp = sample;
-	diff_edge = sample - st->prev_sample;
-	if (diff_edge < 0)
-		diff_edge = -diff_edge;
-	if (diff_amp < 0)
-		diff_amp = -diff_amp;
-	if (diff_edge > THRESHOLD &&
-	    diff_amp > THRESHOLD)
-		new_pulse = 1;
-	else
-		new_pulse = 0;
-	st->prev_sample = sample;
-
-	st->pulse[ringpos] = new_pulse;
-	st->freq_separator += new_pulse;
-
-	/*
-	 * Advance both FIR windows through the ring buffer.
-	 * The separator window always stays one frame ahead
-	 * of the data window.
-	 */
-	if (++ringpos >= MAX_SAMPLES)
-		ringpos = 0;
-	if (++sep_pos >= MAX_SAMPLES)
-		sep_pos = 0;
-
-	st->ringpos = ringpos;
-	st->sep_pos = sep_pos;
-
-	st->sample_count++;
-}
-
-static signed short
-read_sample(struct decoder_state *st)
-{
-	signed short sample;
-	unsigned short u;
-
-	while (st->inpos >= st->inlen)
-		read_words(st);
-
-	sample = st->inbuf[st->inpos++];
-
-	if (st->swap_bytes) {
-		u = (unsigned short)sample;
-		u = (u >> 8) | (u << 8);
-
-		sample = (signed short)u;
-	}
-
-	return sample;
-}
-
-static void
-read_words(struct decoder_state *st)
-{
-	size_t n;
-
-	n = fread(st->inbuf, sizeof(st->inbuf[0]),
-	    READ_BUF, stdin);
-
-	if (n != 0) {
-		st->inpos = 0;
-		st->inlen = n;
-
-		return;
-	}
-
-	if (ferror(stdin))
-		err(errno, "stdin read");
-	if (feof(stdin))
-		exit(EXIT_SUCCESS);
-}
-
-/*
- * Automatically detect spkmodem tone
- */
-static void
-detect_tone(struct decoder_state *st)
-{
-	if (st->learn_frames >= LEARN_FRAMES)
-		return;
-
-	st->learn_frames++;
-
-	if (silent_signal(st))
-		return;
-
-	select_low_tone(st);
-
-	if (st->learn_frames != LEARN_FRAMES)
-		return;
-
-	/*
-	 * If the observed frequencies are too close,
-	 * learning likely failed (only one tone seen).
-	 * Keep the default threshold.
-	 */
-	if (st->freq_max - st->freq_min < 2)
-		return;
-
-	st->freq_threshold =
-	    (st->freq_min + st->freq_max) / 2;
-
-	if (st->debug)
-		printf("auto threshold: %dHz\n",
-		    st->freq_threshold * FRAME_RATE);
-}
-
-/*
- * Ignore silence / near silence.
- * Both FIR windows will be near zero when no signal exists.
- */
-static int
-silent_signal(struct decoder_state *st)
-{
-	return (st->freq_data <= 2 &&
-	    st->freq_separator <= 2);
-}
-
-/*
- * Choose the lowest active tone.
- * Separator frames carry tone in the separator window,
- * data frames carry tone in the data window.
- */
-static void
-select_low_tone(struct decoder_state *st)
-{
-	int f;
-
-	f = st->freq_data;
-
-	if (f <= 0 || (st->freq_separator > 0 &&
-	    st->freq_separator < f))
-		f = st->freq_separator;
-
-	if (f <= 0)
-		return;
-
-	if (f < st->freq_min)
-		st->freq_min = f;
-
-	if (f > st->freq_max)
-		st->freq_max = f;
-}
-
-static void
-print_stats(struct decoder_state *st)
-{
-	long pos;
-
-	int data_hz = st->freq_data * FRAME_RATE;
-	int sep_hz  = st->freq_separator * FRAME_RATE;
-	int sep_hz_min = st->sep_min * FRAME_RATE;
-	int sep_hz_max = st->sep_max * FRAME_RATE;
-
-	if ((pos = ftell(stdin)) == -1) {
-		printf("%d %d %d data=%dHz sep=%dHz(min %dHz %dHz)\n",
-		    st->freq_data,
-		    st->freq_separator,
-		    st->freq_threshold,
-		    data_hz,
-		    sep_hz,
-		    sep_hz_min,
-		    sep_hz_max);
-		return;
-	}
-
-	printf("%d %d %d @%ld data=%dHz sep=%dHz(min %dHz %dHz)\n",
-	    st->freq_data,
-	    st->freq_separator,
-	    st->freq_threshold,
-	    pos - SAMPLE_OFFSET,
-	    data_hz,
-	    sep_hz,
-	    sep_hz_min,
-	    sep_hz_max);
-}
-
-static void
-err(int errval, const char *msg, ...)
-{
-	va_list ap;
-
-	fprintf(stderr, "%s: ", progname());
-
-	va_start(ap, msg);
-	vfprintf(stderr, msg, ap);
-	va_end(ap);
-
-	fprintf(stderr, ": %s\n", strerror(errval));
-	exit(EXIT_FAILURE);
-}
-
-static void
-usage(void)
-{
-	fprintf(stderr, "usage: %s [-d]\n", progname());
-	exit(EXIT_FAILURE);
-}
-
-static const char *
-progname(void)
-{
-	const char *p;
-
-	if (argv0 == NULL || *argv0 == '\0')
-		return "";
-
-	p = strrchr(argv0, '/');
-
-	if (p)
-		return p + 1;
-	else
-		return argv0;
-}
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..3c5dc51f
--- /dev/null
+++ b/util/spkmodem_recv/Makefile
@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+CC?=cc
+CFLAGS?=-Os -Wall -Wextra -Werror -pedantic
+DESTDIR?=
+PREFIX?=/usr/local
+INSTALL?=install
+
+spkmodem-recv:
+	$(CC) $(CFLAGS) -o $@ $@.c
+install: spkmodem-recv
+	$(INSTALL) -d $(DESTDIR)$(PREFIX)/bin/
+	$(INSTALL) $< -t $(DESTDIR)$(PREFIX)/bin/
+clean:
+	rm -f spkmodem-recv
diff --git a/util/spkmodem_recv/spkmodem-recv.c b/util/spkmodem_recv/spkmodem-recv.c
new file mode 100644
index 00000000..4467282d
--- /dev/null
+++ b/util/spkmodem_recv/spkmodem-recv.c
@@ -0,0 +1,124 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* SPDX-FileCopyrightText: 2013 Free Software Foundation, Inc. */
+/* Usage: parec --channels=1 --rate=48000 --format=s16le | ./spkmodem-recv */
+
+/* Forked from coreboot's version, at util/spkmodem_recv/ in coreboot.git,
+ * revision 5c2b5fcf2f9c9259938fd03cfa3ea06b36a007f0 as of 3 January 2022.
+ * This version is heavily modified, re-written based on OpenBSD Kernel Source
+ * File Style Guide (KNF); this change is Copyright 2023 Leah Rowe. */
+
+#include <err.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define SAMPLES_PER_FRAME 240
+#define MAX_SAMPLES (2 * SAMPLES_PER_FRAME)
+#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 ERR() (errno = errno ? errno : ECANCELED)
+#define reset_char() ascii = 0, ascii_bit = 7
+
+signed short frame[MAX_SAMPLES], pulse[MAX_SAMPLES];
+int ringpos, debug, freq_data, freq_separator, sample_count, ascii_bit = 7;
+char ascii = 0;
+
+void handle_audio(void);
+void decode_pulse(void);
+int set_ascii_bit(void);
+void print_char(void);
+void print_stats(void);
+
+int
+main(int argc, char *argv[])
+{
+	int c;
+#ifdef __OpenBSD__
+	if (pledge("stdio", NULL) == -1)
+		err(ERR(), "pledge");
+#endif
+	while ((c = getopt(argc, argv, "d")) != -1)
+		if (!(debug = (c == 'd')))
+			err(errno = EINVAL, NULL);
+	setvbuf(stdout, NULL, _IONBF, 0);
+	while (!feof(stdin))
+		handle_audio();
+	if (errno && debug)
+		err(errno, "Unhandled error, errno %d", errno);
+	return errno;
+}
+
+void
+handle_audio(void)
+{
+	if (sample_count > (3 * SAMPLES_PER_FRAME))
+		sample_count = reset_char();
+	if ((freq_separator <= FREQ_SEP_MIN) || (freq_separator >= FREQ_SEP_MAX)
+	    || (freq_data <= FREQ_DATA_MIN) || (freq_data >= FREQ_DATA_MAX)) {
+		decode_pulse();
+		return;
+	}
+
+	if (set_ascii_bit() < 0)
+		print_char();
+	sample_count = 0;
+	for (int sample = 0; sample < SAMPLES_PER_FRAME; sample++)
+		decode_pulse();
+}
+
+void
+decode_pulse(void)
+{
+	int next_ringpos = (ringpos + SAMPLES_PER_FRAME) % MAX_SAMPLES;
+	freq_data -= pulse[ringpos];
+	freq_data += pulse[next_ringpos];
+	freq_separator -= pulse[next_ringpos];
+
+	fread(frame + ringpos, 1, sizeof(frame[0]), stdin);
+	if (ferror(stdin) != 0)
+		err(ERR(), "Could not read from frame.");
+
+	if ((pulse[ringpos] = (abs(frame[ringpos]) > THRESHOLD) ? 1 : 0))
+		++freq_separator;
+	++ringpos;
+	ringpos %= MAX_SAMPLES;
+	++sample_count;
+}
+
+int
+set_ascii_bit(void)
+{
+	if (debug)
+		print_stats();
+	if (freq_data < FREQ_DATA_THRESHOLD)
+		ascii |= (1 << ascii_bit);
+	--ascii_bit;
+	return ascii_bit;
+}
+
+void
+print_char(void)
+{
+	if (debug)
+		printf("<%c, %x>", ascii, ascii);
+	else
+		printf("%c", ascii);
+	reset_char();
+}
+
+void
+print_stats(void)
+{
+	long stdin_pos;
+	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));
+}