Question

I’m investigating using the mozilla audio API tone generator:

  function AudioDataDestination(sampleRate, readFn) {
    // Initialize the audio output.
    var audio = new Audio();
    audio.mozSetup(1, sampleRate);

    var currentWritePosition = 0;
    var prebufferSize = sampleRate / 2; // buffer 500ms
    var tail = null, tailPosition;

    // The function called with regular interval to populate 
    // the audio output buffer.
    setInterval(function() {
      var written;
      // Check if some data was not written in previous attempts.
      if(tail) {
        written = audio.mozWriteAudio(tail.subarray(tailPosition));
        currentWritePosition += written;
        tailPosition += written;
        if(tailPosition < tail.length) {
          // Not all the data was written, saving the tail...
          return; // ... and exit the function.
        }
        tail = null;
      }

      // Check if we need add some data to the audio output.
      var currentPosition = audio.mozCurrentSampleOffset();
      var available = currentPosition + prebufferSize - currentWritePosition;
      if(available > 0) {
        // Request some sound data from the callback function.
        var soundData = new Float32Array(available);
        readFn(soundData);

        // Writting the data.
        written = audio.mozWriteAudio(soundData);
        if(written < soundData.length) {
          // Not all the data was written, saving the tail.
          tail = soundData;
          tailPosition = written;
        }
        currentWritePosition += written;
      }
    }, 100);
  }

  // Control and generate the sound.

  var frequency = 0, currentSoundSample;
  var sampleRate = 44100;

  function requestSoundData(soundData) {
    if (!frequency) { 
      return; // no sound selected
    }

    var k = 2* Math.PI * frequency / sampleRate;
    for (var i=0, size=soundData.length; i<size; i++) {
      soundData[i] = Math.sin(k * currentSoundSample++);
    }        
  }

  var audioDestination = new AudioDataDestination(sampleRate, requestSoundData);

  function start() {
    currentSoundSample = 0;
    frequency = parseFloat(document.getElementById("freq").value);
  }

  function stop() {
    frequency = 0;
  }

I’d like this tone to be on for 500ms then off for 500ms then repeat until a stop button is pressed. I thought I could just do this by setting the sample numbers 22,050 to 44,100 to zero. However this method does not seem to work. I think it’s because the function to repopulate the buffer occurs every 100ms, but this is now beyond the limit of my knowledge. Any help would be very much appreciated.

Answer 1

Actually, it does seem to work fine for me. I changed the loop in your requestSoundData function like this:

for (var i=0, size=soundData.length; i<size; i++) {
  if (currentSoundSample % 44100 < 22050)
    soundData[i] = Math.sin(k * currentSoundSample);
  else
    soundData[i] = 0;
  currentSoundSample++;
}

Samples 22,050 to 44,100 are set to zero and that seems to produce exactly the effect you wanted. Here you can try the code: http://jsfiddle.net/95jCt/