Audio Data API

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Defining an Enhanced API for Audio (Draft Recommendation)

Note: this API has been deprecated in favor of the Web Audio API chosen by the W3C.


The HTML5 specification introduces the <audio> and <video> media elements, and with them the opportunity to dramatically change the way we integrate media on the web. The current HTML5 media API provides ways to play and get limited information about audio and video, but gives no way to programatically access or create such media. We present a new Mozilla extension to this API, which allows web developers to read and write raw audio data.

Other Contributors
  • Thomas Saunders
  • Ted Mielczarek

Standardization Note

Please note that this document describes a non-standard experimental API. This API is considered deprecated and may not be supported in future releases. The World Wide Web Consortium (W3C) has chartered the Audio Working Group to develop standardized audio API specifications, including Web Audio API. Please refer to the Audio Working Group website for further details.

API Tutorial

This API extends the HTMLMediaElement and HTMLAudioElement (e.g., affecting <video> and <audio>), and implements the following basic API for reading and writing raw audio data:

Reading Audio

Audio data is made available via an event-based API. As the audio is played, and therefore decoded, sample data is passed to content scripts in a framebuffer for processing after becoming available to the audio layer--hence the name, MozAudioAvailable. These samples may or may not have been played yet at the time of the event. The audio samples returned in the event are raw, and have not been adjusted for mute/volume settings on the media element. Playing, pausing, and seeking the audio also affect the streaming of this raw audio data.

Users of this API can register two callbacks on the <audio> or <video> element in order to consume this data:

<audio id="audio" src="song.ogg"></audio>
  var audio = document.getElementById("audio");
  audio.addEventListener('MozAudioAvailable', audioAvailableFunction, false);
  audio.addEventListener('loadedmetadata', loadedMetadataFunction, false);

The loadedmetadata event is a standard part of HTML5. It now indicates that a media element (audio or video) has useful metadata loaded, which can be accessed using three new attributes:

  • mozChannels
  • mozSampleRate
  • mozFrameBufferLength

Prior to the loadedmetadata event, accessing these attributes will cause an exception to be thrown, indicating that they are not known, or there is no audio. These attributes indicate the number of channels, audio sample rate per second, and the default size of the framebuffer that will be used in MozAudioAvailable events. This event is fired once as the media resource is first loaded, and is useful for interpreting or writing the audio data.

The MozAudioAvailable event provides two pieces of data. The first is a framebuffer (i.e., an array) containing decoded audio sample data (i.e., floats). The second is the time for these samples measured from the start in seconds. Web developers consume this event by registering an event listener in script like so:

<audio id="audio" src="song.ogg"></audio>
  var audio = document.getElementById("audio");
  audio.addEventListener('MozAudioAvailable', someFunction, false);

An audio or video element can also be created with script outside the DOM:

var audio = new Audio();
audio.src = "song.ogg";
audio.addEventListener('MozAudioAvailable', someFunction, false);;

The following is an example of how both events might be used:

var channels,

function audioInfo() {
  var audio = document.getElementById('audio');

  // After loadedmetadata event, following media element attributes are known:
  channels          = audio.mozChannels;
  rate              = audio.mozSampleRate;
  frameBufferLength = audio.mozFrameBufferLength;

function audioAvailable(event) {
  var samples = event.frameBuffer;
  var time    = event.time;

  for (var i = 0; i < frameBufferLength; i++) {
    // Do something with the audio data as it is played.
    processSample(samples[i], channels, rate);
Complete Example: Visualizing Audio Spectrum

This example calculates and displays FFT spectrum data for the playing audio:


<!DOCTYPE html>
    <title>JavaScript Spectrum Example</title>
    <audio id="audio-element"
           style="width: 512px;">
    <div><canvas id="fft" width="512" height="200"></canvas></div>

      var canvas = document.getElementById('fft'),
          ctx = canvas.getContext('2d'),

      function loadedMetadata() {
        channels          = audio.mozChannels;
        rate              = audio.mozSampleRate;
        frameBufferLength = audio.mozFrameBufferLength;
        fft = new FFT(frameBufferLength / channels, rate);

      function audioAvailable(event) {
        var fb = event.frameBuffer,
            t  = event.time, /* unused, but it's there */
            signal = new Float32Array(fb.length / channels),

        for (var i = 0, fbl = frameBufferLength / 2; i < fbl; i++ ) {
          // Assuming interlaced stereo channels,
          // need to split and merge into a stero-mix mono signal
          signal[i] = (fb[2*i] + fb[2*i+1]) / 2;


        // Clear the canvas before drawing spectrum
        ctx.clearRect(0,0, canvas.width, canvas.height);

        for (var i = 0; i < fft.spectrum.length; i++ ) {
          // multiply spectrum by a zoom value
          magnitude = fft.spectrum[i] * 4000;

          // Draw rectangle bars for each frequency bin
          ctx.fillRect(i * 4, canvas.height, 3, -magnitude);

      var audio = document.getElementById('audio-element');
      audio.addEventListener('MozAudioAvailable', audioAvailable, false);
      audio.addEventListener('loadedmetadata', loadedMetadata, false);

      // FFT from dsp.js, see below
      var FFT = function(bufferSize, sampleRate) {
        this.bufferSize   = bufferSize;
        this.sampleRate   = sampleRate;
        this.spectrum     = new Float32Array(bufferSize/2);
        this.real         = new Float32Array(bufferSize);
        this.imag         = new Float32Array(bufferSize);
        this.reverseTable = new Uint32Array(bufferSize);
        this.sinTable     = new Float32Array(bufferSize);
        this.cosTable     = new Float32Array(bufferSize);

        var limit = 1,
            bit = bufferSize >> 1;

        while ( limit < bufferSize ) {
          for ( var i = 0; i < limit; i++ ) {
            this.reverseTable[i + limit] = this.reverseTable[i] + bit;

          limit = limit << 1;
          bit = bit >> 1;

        for ( var i = 0; i < bufferSize; i++ ) {
          this.sinTable[i] = Math.sin(-Math.PI/i);
          this.cosTable[i] = Math.cos(-Math.PI/i);

      FFT.prototype.forward = function(buffer) {
        var bufferSize   = this.bufferSize,
            cosTable     = this.cosTable,
            sinTable     = this.sinTable,
            reverseTable = this.reverseTable,
            real         = this.real,
            imag         = this.imag,
            spectrum     = this.spectrum;

        if ( bufferSize !== buffer.length ) {
          throw "Supplied buffer is not the same size as defined FFT. FFT Size: " + bufferSize + " Buffer Size: " + buffer.length;

        for ( var i = 0; i < bufferSize; i++ ) {
          real[i] = buffer[reverseTable[i]];
          imag[i] = 0;

        var halfSize = 1,

        while ( halfSize < bufferSize ) {
          phaseShiftStepReal = cosTable[halfSize];
          phaseShiftStepImag = sinTable[halfSize];
          currentPhaseShiftReal = 1.0;
          currentPhaseShiftImag = 0.0;

          for ( var fftStep = 0; fftStep < halfSize; fftStep++ ) {
            i = fftStep;

            while ( i < bufferSize ) {
              off = i + halfSize;
              tr = (currentPhaseShiftReal * real[off]) - (currentPhaseShiftImag * imag[off]);
              ti = (currentPhaseShiftReal * imag[off]) + (currentPhaseShiftImag * real[off]);

              real[off] = real[i] - tr;
              imag[off] = imag[i] - ti;
              real[i] += tr;
              imag[i] += ti;

              i += halfSize << 1;

            tmpReal = currentPhaseShiftReal;
            currentPhaseShiftReal = (tmpReal * phaseShiftStepReal) - (currentPhaseShiftImag * phaseShiftStepImag);
            currentPhaseShiftImag = (tmpReal * phaseShiftStepImag) + (currentPhaseShiftImag * phaseShiftStepReal);

          halfSize = halfSize << 1;

        i = bufferSize/2;
        while(i--) {
          spectrum[i] = 2 * Math.sqrt(real[i] * real[i] + imag[i] * imag[i]) / bufferSize;
Writing Audio

It is also possible to setup an <audio> element for raw writing from script (i.e., without a src attribute). Content scripts can specify the audio stream's characteristics, then write audio samples using the following methods:

mozSetup(channels, sampleRate)

// Create a new audio element
var audioOutput = new Audio();
// Set up audio element with 2 channel, 44.1KHz audio stream.
audioOutput.mozSetup(2, 44100);


// Write samples using a JS Array
var samples = [0.242, 0.127, 0.0, -0.058, -0.242, ...];
var numberSamplesWritten = audioOutput.mozWriteAudio(samples);

// Write samples using a Typed Array
var samples = new Float32Array([0.242, 0.127, 0.0, -0.058, -0.242, ...]);
var numberSamplesWritten = audioOutput.mozWriteAudio(samples);


// Get current audible position of the underlying audio stream, measured in samples.
var currentSampleOffset = audioOutput.mozCurrentSampleOffset();

Since the MozAudioAvailable event and the mozWriteAudio() method both use Float32Array, it is possible to take the output of one audio stream and pass it directly (or process first and then pass) to a second:

<audio id="a1" 
var a1 = document.getElementById('a1'),
    a2 = new Audio(),
    buffers = [];

function loadedMetadata() {
  // Mute a1 audio.
  a1.volume = 0;
  // Setup a2 to be identical to a1, and play through there.
  a2.mozSetup(a1.mozChannels, a1.mozSampleRate);

function audioAvailable(event) {
  // Write the current framebuffer
  var frameBuffer = event.frameBuffer; // frameBuffer is Float32Array

a1.addEventListener('MozAudioAvailable', audioAvailable, false);
a1.addEventListener('loadedmetadata', loadedMetadata, false);

function writeAudio(audioBuffer) {
  // audioBuffer is Float32Array
  buffers.push({buffer: audioBuffer, position: 0});

  // If there's buffered data, write that
  while(buffers.length > 0) {
    var buffer = buffers[0].buffer;
    var position = buffers[0].position;
    var written = a2.mozWriteAudio(buffer.subarray(position));
    // // If all data wasn't written, keep it in the buffers:
    if(position + written < buffer.length) {
      buffers[0].position = position + written;

Audio data written using the mozWriteAudio() method needs to be written at a regular interval in equal portions, in order to keep a little ahead of the current sample offset (the sample offset that is currently being played by the hardware can be obtained with mozCurrentSampleOffset()), where a little means something on the order of 500ms of samples. For example, if working with 2 channels at 44100 samples per second, a writing interval of 100ms, and a pre-buffer equal to 500ms, one would write an array of (2 * 44100 / 10) = 8820 samples, and a total of (currentSampleOffset + 2 * 44100 / 2).

It's also possible to auto detect the minimal duration of the pre-buffer, such that the sound is played without interruptions, and lag between writing and playback is minimal. To do this start writing the data in small portions and wait for the value returned by mozCurrentSampleOffset() to be more than 0.

var prebufferSize = sampleRate * 0.020; // Initial buffer is 20 ms
var autoLatency = true, started = new Date().valueOf();
// Auto latency detection
if (autoLatency) {
  prebufferSize = Math.floor(sampleRate * (new Date().valueOf() - started) / 1000);
  if (audio.mozCurrentSampleOffset()) { // Play position moved?
    autoLatency = false;
Complete Example: Creating a Web Based Tone Generator

This example creates a simple tone generator, and plays the resulting tone.

<!DOCTYPE html>
    <title>JavaScript Audio Write Example</title>
    <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, 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);

            // 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;

DOM Implementation


Audio data is made available via the following event:

  • Event: AudioAvailableEvent
  • Event handler: onmozaudioavailable

The AudioAvailableEvent is defined as follows:

interface nsIDOMNotifyAudioAvailableEvent : nsIDOMEvent
  // frameBuffer is really a Float32Array
  readonly attribute jsval  frameBuffer;
  readonly attribute float  time;

The frameBuffer attribute contains a typed array (Float32Array) with the raw audio data (32-bit float values) obtained from decoding the audio (e.g., the raw data being sent to the audio hardware vs. encoded audio). This is of the form [channel1, channel2, ..., channelN, channel1, channel2, ..., channelN, ...]. All audio frames are normalized to a length of channels * 1024 by default, but could be any length between 512 and 16384 if the user has set a different length using the mozFrameBufferLength attribute.

The time attribute contains a float representing the time in seconds of the first sample in the frameBuffer array since the start of the audio track.

nsIDOMHTMLMediaElement additions

Audio metadata is made available via three new attributes on the HTMLMediaElement. By default these attributes throw if accessed before the loadedmetadata event occurs. Users who need this info before the audio starts playing should not use autoplay, since the audio might start before a loadmetadata handler has run.

The three new attributes are defined as follows:

  readonly attribute unsigned long mozChannels;
  readonly attribute unsigned long mozSampleRate;
           attribute unsigned long mozFrameBufferLength;

The mozChannels attribute contains the number of channels in the audio resource (e.g., 2). The mozSampleRate attribute contains the number of samples per second that will be played, for example 44100. Both are read-only.

The mozFrameBufferLength attribute indicates the number of samples that will be returned in the framebuffer of each MozAudioAvailable event. This number is a total for all channels, and by default is set to be the number of channels * 1024 (e.g., 2 channels * 1024 samples = 2048 total).

The mozFrameBufferLength attribute can also be set to a new value, if users want lower latency, or larger amounts of data, etc. The size given must be a number between 512 and 16384. Using any other size will result in an exception being thrown. The best time to set a new length is after the loadedmetadata event fires, when the audio info is known, but before the audio has started or MozAudioAvailable events begun firing.

nsIDOMHTMLAudioElement additions

The HTMLAudioElement has also been extended to allow write access. Audio writing is achieved by adding three new methods:

  void mozSetup(in long channels, in long rate);
  unsigned long mozWriteAudio(array); // array is Array() or Float32Array()
  unsigned long long mozCurrentSampleOffset();

The mozSetup() method allows an <audio> element to be setup for writing from script. This method must be called before mozWriteAudio or mozCurrentSampleOffset can be called, since an audio stream has to be created for the media element. It takes two arguments:

  1. channels - the number of audio channels (e.g., 2)
  2. rate - the audio's sample rate (e.g., 44100 samples per second)

The choices made for channel and rate are significant, because they determine the amount of data you must pass to mozWriteAudio(). That is, you must pass an array with enough data for each channel specified in mozSetup().

The mozSetup() method, if called more than once, will recreate a new audio stream (destroying an existing one if present) with each call. Thus it is safe to call this more than once, but unnecessary.

The mozWriteAudio() method can be called after mozSetup(). It allows audio data to be written directly from script. It takes one argument, array. This is a JS Array (i.e., new Array()) or a typed float array (i.e., new Float32Array()) containing the audio data (floats) you wish to write. It must be 0 or N elements in length, where N % channels == 0, otherwise an exception is thrown.

The mozWriteAudio() method returns the number of samples that were just written, which may or may not be the same as the number in array. Only the number of samples that can be written without blocking the audio hardware will be written. It is the responsibility of the caller to deal with any samples that don't get written in the first pass (e.g., buffer and write in the next call).

The mozCurrentSampleOffset() method can be called after mozSetup(). It returns the current position (measured in samples) of the audio stream. This is useful when determining how much data to write with mozWriteAudio().

All of mozWriteAudio(), mozCurrentSampleOffset(), and mozSetup() will throw exceptions if called out of order. mozSetup() will also throw if a src attribute has previously been set on the audio element (i.e., you can't do both at the same time).


Similar to the <canvas> element and its getImageData method, the MozAudioAvailable event's frameBuffer attribute protects against information leakage between origins.

The MozAudioAvailable event's frameBuffer attribute will throw if the origin of audio resource does not match the document's origin. NOTE: this will affect users who have the security.fileuri.strict_origin_policy set, and are working locally with file:/// URIs.

Compatibility with Audio Backends

The current MozAudioAvailable implementation integrates with Mozilla's decoder abstract base classes, and therefore, any audio decoder which uses these base classes automatically dispatches MozAudioAvailable events. At the time of writing, this includes the Ogg, WebM, and Wave decoders.

Additional Resources

A series of blog posts document the evolution and implementation of this API: Another overview by Al MacDonald is available here.

Al has also written 2 very good tutorials and video demos of reading and writing audio with the API.

The BBC Research and Development Blog has also done an excellent overview of the API here.


The work on this API is available in Mozilla bug 490705.

Obtaining Builds

Firefox trunk nightlies include the Audio Data API (starting with 2010-08-26 builds).

JavaScript Audio Libraries

Working Audio Data Demos

NOTE: we recently took down two servers that were hosting many of these demos. We are working to find a new home for them.

A number of working demos have been created, including:

Demos Needing to be Updated to New API

Third Party Discussions

A number of people have written about our work, including: