This example creates a simple tone generator, and plays the resulting tone. The function requestSoundData() produces the samples to be played. This function is called at a certain interval through the setInterval() function. The function mozWriteAudio() is called to write those samples produced in the audio stream. In order to always have samples to play, a buffer of 500 ms is created. The function mozCurrentSampleOffset() is used to know the position of the samples being played so that we can fill a 500 ms buffer of audio samples.
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Audio Write Example</title>
</head>
<body>
<input type="text" size="4" id="freq" value="440"><label for="hz">Hz</label>
<button onclick="start()">play</button>
<button onclick="stop()">stop</button>
<script type="text/javascript">
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;
// 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);
currentWritePosition += written;
if(written < tail.length) {
// Not all the data was written, saving the tail...
tail = tail.subarray(written);
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(parseFloat(available));
readFn(soundData);
// Writing the data.
written = audio.mozWriteAudio(soundData);
if(written < soundData.length) {
// Not all the data was written, saving the tail.
tail = soundData.slice(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;
}
</script>
</body>
</html>