From 76a02622caf8f16979d8bc67890764768f7d06c0 Mon Sep 17 00:00:00 2001
From: Paul Harrison <ch@dorper.me>
Date: Sun, 7 Jun 2026 17:49:16 -0700
Subject: DTMF Encoder

---
 Makefile     |   9 ++++
 dtmfencode.c | 144 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 wav.h        |  18 ++++++++
 3 files changed, 171 insertions(+)
 create mode 100644 Makefile
 create mode 100644 dtmfencode.c
 create mode 100644 wav.h

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..316eca8
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,9 @@
+.PHONY: test
+
+all: dtmfencode test
+
+dtmfencode: dtmfencode.c
+	cc $^ -o $@
+
+test: dtmfencode
+	./dtmfencode test.wav "1234567890*#ABCD" 40000 100
diff --git a/dtmfencode.c b/dtmfencode.c
new file mode 100644
index 0000000..be02660
--- /dev/null
+++ b/dtmfencode.c
@@ -0,0 +1,144 @@
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <math.h>
+#include "wav.h"
+
+void dtmf_freqs(char c, int *a, int *b) {
+  switch (c) {
+  case '1':
+  case '2':
+  case '3':
+  case 'A':
+    *a = 697;
+    break;
+  case '4':
+  case '5':
+  case '6':
+  case 'B':
+    *a = 770;
+    break;
+  case '7':
+  case '8':
+  case '9':
+  case 'C':
+    *a = 852;
+    break;
+  case '*':
+  case '0':
+  case '#':
+  case 'D':
+    *a = 941;
+    break;
+  default:
+    *a = 0;
+  }
+
+  switch (c) {
+  case '1':
+  case '4':
+  case '7':
+  case '*':
+    *b = 1209;
+    break;
+  case '2':
+  case '5':
+  case '8':
+  case '0':
+    *b = 1336;
+    break;
+  case '3':
+  case '6':
+  case '9':
+  case '#':
+    *b = 1477;
+    break;
+  case 'A':
+  case 'B':
+  case 'C':
+  case 'D':
+    *b = 1633;
+    break;
+  default:
+    *b = 0;
+  }
+}
+
+int16_t dtmf_gen(char c, int t, int rate) {
+  int a, b;
+  dtmf_freqs(c, &a, &b);
+  double v = 0.5 * sin(2.0 * M_PI * (double)a * ((double)t / (double)rate));
+  v += 0.5 * sin(2.0 * M_PI * (double)b * ((double)t / (double)rate));
+  return v * 32767.0;
+}
+
+int main(int argc, char *argv[]) {
+  /* usage */
+  if (argc != 5) {
+    fprintf(stderr, "usage: %s <wave file> <dtmf> <width> <spacing>\n", argv[0]);
+    return 2;
+  }
+
+  /* read arguments */
+  char *outfilename = argv[1];
+  char *dtmf = argv[2];
+  int width, spacing;
+  sscanf(argv[3], "%d", &width);
+  sscanf(argv[4], "%d", &spacing);
+
+  /* open output file */
+  FILE *fout = fopen(outfilename, "w");
+  if (fout == NULL) {
+    fprintf(stderr, "could not open '%s' for writing.\n", outfilename);
+    return 1;
+  }
+  
+  /* create header */
+  struct wav_header wav_header;
+  memcpy(wav_header.riff_magic, "RIFF", 4);
+  memcpy(wav_header.wave_magic, "WAVE", 4);
+  memcpy(wav_header.format_magic, "fmt ", 4);
+  memcpy(wav_header.data_magic, "data", 4);
+  wav_header.format_size = 16;
+  wav_header.format_type = 1;
+  wav_header.channels = 1;
+  wav_header.sample_rate = 44100;
+  wav_header.bits_per_sample = 16;
+  wav_header.rate1 = (wav_header.sample_rate * wav_header.bits_per_sample * wav_header.channels) / 8;
+  wav_header.rate2 = (wav_header.bits_per_sample * wav_header.channels) / 8;
+
+  /* go past header */ 
+  fseek(fout, sizeof(wav_header), SEEK_SET);
+
+  /* generate sound */
+  int num_digits = strlen(dtmf);
+  uint32_t sample_count = wav_header.sample_rate * num_digits * (width + spacing);
+
+  for (int digit_index = 0; digit_index < num_digits; digit_index++) {
+    char digit = dtmf[digit_index];
+
+    /* emit dtmf tone */
+    for (int t = 0; t < (float)wav_header.sample_rate * (float)(width) / 1000.0f; t++) {
+      int16_t sample = dtmf_gen(digit, t, wav_header.sample_rate);
+      fwrite(&sample, sizeof(int16_t), 1, fout);
+    }
+
+    /* emit spacing */
+    fseek(fout, (float)wav_header.sample_rate * (float)(spacing) / 1000.0f, SEEK_CUR);
+  }
+
+  /* update header */
+  wav_header.size = ftell(fout) - 8;
+  wav_header.data_size = ftell(fout) - sizeof(wav_header);
+  
+  /* write header */
+  fseek(fout, 0, SEEK_SET);
+  fwrite(&wav_header, sizeof(wav_header), 1, fout);
+
+  /* close file */
+  fclose(fout);
+  
+  return 0;
+}
diff --git a/wav.h b/wav.h
new file mode 100644
index 0000000..d5e26cb
--- /dev/null
+++ b/wav.h
@@ -0,0 +1,18 @@
+#pragma once
+#include <stdint.h>
+
+struct wav_header {
+  char riff_magic[4];
+  uint32_t size;
+  char wave_magic[4];
+  char format_magic[4];
+  uint32_t format_size;
+  uint16_t format_type;
+  uint16_t channels;
+  uint32_t sample_rate;
+  uint32_t rate1;
+  uint16_t rate2;
+  uint16_t bits_per_sample;
+  char data_magic[4];
+  uint32_t data_size;
+};
-- 
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