First PPM experiment

Makefile

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+all: main
+	./main
+main: main.cpp sound_server.h sound_server.cpp Makefile
+	g++ -std=c++11 -o main main.cpp sound_server.cpp `sdl-config --cflags --libs`

main.cpp

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+#include <SDL/SDL.h>
+#include <iostream>
+#include "sound_server.h"
+
+// Naive example of 1-bit audio mixing employing PPM technique
+// Adrian Scripca <benishor@gmail.com>
+
+struct oscillator {
+  void init(uint16_t sr) { samplerate = sr; }
+
+  void set_frequency(uint16_t frequency) {
+    samples_per_cycle = samplerate / frequency;
+    //    phase = 0; // hard sync
+  }
+
+  bool tick() {
+    phase = (phase + 1) % samples_per_cycle;
+    return phase < (samples_per_cycle / 2 * duty_cycle / 100);
+  }
+
+  uint16_t samples_per_cycle;
+  uint16_t duty_cycle = 4;  // percentage
+  uint32_t phase = 0;
+  uint16_t samplerate;
+};
+
+struct synth : sound_producer_1bit {
+  explicit synth(uint16_t samplerate) {
+    for (auto& o : oscs) {
+      o.init(samplerate);
+    }
+  }
+
+  bool tick() override {
+    return oscs[0].tick() | oscs[1].tick() | oscs[2].tick() | oscs[3].tick();
+  }
+
+  void major_chord(float root) {
+    oscs[0].set_frequency(root);
+    oscs[1].set_frequency(root * 5.0f / 4.0f);  // major third
+    oscs[2].set_frequency(root * 3.0f / 2.0f);  // perfect fifth
+    oscs[3].set_frequency(root * 2.0f);         // octave
+  }
+
+  void minor_chord(float root) {
+    oscs[0].set_frequency(root);
+    oscs[1].set_frequency(root * 6.0f / 5.0f);  // minor third
+    oscs[2].set_frequency(root * 3.0f / 2.0f);  // perfect fifth
+    oscs[3].set_frequency(root * 2.0f);         // octave
+  }
+
+  oscillator oscs[4];
+};
+
+int main(int argc, char* argv[]) {
+  std::cout << "1bit PPM technique example" << std::endl;
+  // setup
+  const uint16_t SampleRate = 44100;
+
+  synth instrument{SampleRate};
+  sound_server_1bit sound_server{SampleRate, instrument};
+
+  // fool around. Gmaj, Cmaj, Emaj (la bamba)
+  instrument.major_chord(98);
+  SDL_Delay(1000);
+
+  instrument.major_chord(130.81);
+  SDL_Delay(1000);
+
+  instrument.major_chord(146.83);
+  SDL_Delay(1000);
+
+  return 0;
+}

sound_server.cpp

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+#include "sound_server.h"
+
+#include <SDL/SDL.h>
+
+#include <iostream>
+
+void audio_callback(void* param, uint8_t* stream, int len) {
+  sound_producer_1bit* producer = reinterpret_cast<sound_producer_1bit*>(param);
+  for (int i = 0; i < len; i++) {
+    *stream++ = producer->tick() ? 255 : 0;
+  }
+}
+
+sound_server_1bit::sound_server_1bit(uint16_t samplerate,
+                                     sound_producer_1bit& producer) {
+  if (SDL_Init(SDL_INIT_AUDIO) < 0) {
+    std::cerr << "Init failed" << std::endl;
+    exit(1);
+  }
+
+  SDL_AudioSpec as;
+  as.freq = samplerate;
+  as.format = AUDIO_U8;
+  as.samples = 256;
+  as.callback = audio_callback;
+  as.userdata = reinterpret_cast<void*>(&producer);
+  as.channels = 1;
+
+  if (SDL_OpenAudio(&as, NULL) < 0) {
+    std::cerr << "Unable to open audio" << std::endl;
+    exit(1);
+  }
+
+  SDL_PauseAudio(0);
+}
+
+sound_server_1bit::~sound_server_1bit() { SDL_Quit(); }

sound_server.h

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+#pragma once
+#include <cstdint>
+
+struct sound_producer_1bit {
+  // false = off, true = on
+  virtual bool tick() = 0;
+};
+
+struct sound_server_1bit {
+  sound_server_1bit(uint16_t samplerate, sound_producer_1bit& producer);
+  ~sound_server_1bit();
+};