interscheme/tone.c

// Generate a tone. Generating multiple tones in a row
// will not be extremely accurate, so it's better as just
// a 'bell' or 'notification' tone generator (not music)

// Most of the generic math was taken from an anonymous pastebin post:
// https://pastebin.com/Uq9rcsJz

#include <SDL2/SDL.h>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>

float runUntil = 0.0; // Counts to runFor then exits

// Audio sampling rate. 48k on the Reform.
// TODO: Find a way to get SDL to detect
//       current system's sampling rate.
float sampleRate = 44100.0;

float phase = 0.0;
float frequency = 523.25; // Generate a C tone.

float nextSampleSquare() {
  phase += (1.0 / sampleRate) * frequency;
  if (phase > 1.0) phase -= 1.0;
  return phase > 0.5 ? 1.0 : -1.0;
}

void SDLAudioCallback(void *udata, Uint8 *stream, int len) {
  unsigned int i;
  float *str = (float *) stream;
  for (i = 0; i < len / sizeof(float) / 2; i++) {
    float smp = nextSampleSquare();
    str[i * 2]     = smp;
    str[i * 2 + 1] = smp;

    // Uncommenting this line gives the tone a "rising / sweeping"
    // effect.
    //frequency += frequency * 0.00001;

    
    runUntil += 1.0 / sampleRate;
  }
}

int sdl_generate_tone(float freq, float duration) {
  // 44.1k is constant and cannot be changed.
  // This is because 44.1 is a perfect sinewave and
  // calculating anything higher is redundant anyway.
  frequency = freq; //Set user-defined frequency.
  if (SDL_Init(SDL_INIT_AUDIO) < 0) {
    printf("Can't initialize SDL: \n%s\n", SDL_GetError());
    return 1;
  }
  SDL_AudioSpec audioSpec;
    audioSpec.freq = (int)sampleRate;
    audioSpec.format = AUDIO_F32SYS;
    audioSpec.channels = 2;
    audioSpec. samples = 8192;
    audioSpec.callback = SDLAudioCallback;

  if (SDL_OpenAudio(&audioSpec, 0) < 0) {
    printf("Can't open SDL audio: \n%s\n", SDL_GetError());
    return 1;
  }

  SDL_PauseAudio(0);

  // TODO: This code freezes the program in a busyloop.
  //       Find a better way to do this.
  while (runUntil < duration);
  SDL_CloseAudio();
  runUntil = 0.0;
  return 0;
}