NES noise channel

nes_noise_voice.cpp

class NoiseVoice {
  // Invariant stuff
  static const int fs = 44100; // sample rate
  static const int ntsc = 3579545; // ntsc clock rate
  static const int buffer[32767]; // fill randomly with 0s and 1s
  static const int periods[] = { 4068, 2034, 1016, 762, ... }; // from nesdev wiki
  
  int loop = 93; // or 32767 :)
  float timestep; // can be a fraction less than 1
  float ctr; // accumulates timesteps
  
  // Per-voice stuff
  void noteOn(int noteNum) {
    const int period = periods[noteNum % 16]; // select a period based on MIDI note number
    timestep = ntsc / (fs * 2 * period);
    ctr = 0;
  }

  // Per-voice sample rendering
  int getNextSample() {
    ctr = (ctr + timestep);
    if (ctr >= loop) ctr -= loop;
    return buffer[(int)ctr]; // output will be 0 or 1
  }
}