MediaRecorder polyfill for Opus recording using WebAssembly
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Latest commit 471f7c0 Dec 27, 2018


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opus-media-recorder is a MediaRecorder API polyfill written in ES6 and WebAssembly. It aims a cross-browser Opus codec support with various audio formats such as Ogg and WebM. opus-media-recorder can be used as a ployfill, or it can replace the built-in MediaRecorder since opus-media-recorder supports more MIME types.

opus-media-recorder uses WebAssembly compiled from popular libraries (e.g libopus, libogg, libwebm, and speexdsp) to ensure performance and standards-compliance.

Why opus-media-recorder?

opus-media-recorder Chrome Firefox iOS Edge
audio/ogg O X O X X
audio/webm O O X X X
audio/wav O X X X X

* Both audio/ogg and audio/webm refer containers with Opus audio codec.

Currently the MediaRecorder API suffers from the two problems:

  1. Not all browsers support MediaRecorder.
  2. Even the browsers that provides MediaRecorder don't support the same format.

opus-media-recorder tackles these problems by supporting all major modern browsers (Chrome, Firefox, iOS, and Edge) and by providing various formats.

By taking advantages of WebAssembly and Web Workers, opus-media-recorder tries to have minimum performace panalties of running encoders on a browser.

How to use

opus-media-recorder is compatible with the Mediastream Recording API standard.

Thing to know

  • The page must be served over HTTPS in order to record.
  • Being able to record does not always mean you can play it on browsers:
    • macOS/iOS Safari cannot play Ogg Opus natively yet.
    • Edge requires an extension to play Ogg Opus natively.
    • You can get an Opus decorder to play it. There are Opus decoders available, such as Chris Rudmin's Opus decoder.
    • Otherwise, users can download as a file and play it using apps like VLC.


The MediaRecorder object is available in the global namespace using UMD.

<script src="path/to/MediaRecorder.js"></script>
// Existing MediaRecorder is replaced
var recorder = new MediaRecorder(stream);


For futher usages, see the MDN reference, our example, and other online resources.

window.MediaRecorder = require('opus-media-recorder');

var recorder;

function startRecording () {
  navigator.mediaDevices.getUserMedia({ audio: true }).then((stream) => {
    var option = {
      mimeType: 'audio/ogg' // Choose desired format. Default is audio/ogg
    // Start recording
    recorder = new MediaRecorder(stream, option);
    // Set record to <audio> when recording will be finished
    recorder.addEventListener('dataavailable', (e) => {
      audioElement.src = URL.createObjectURL(;

// Recording should be started in user-initiated event like buttons
recordButton.addEventListener('click', startRecording);

// Stop recording
stopButton.addEventListener('click', () => {
  // Remove “recording” icon from browser tab => i.stop());

Use opus-media-recorder only when a browser doesn't support it

// Check if MediaRecorder available.
if (!window.MediaRecorder) {
  window.MediaRecorder = require('opus-media-recorder');
// Check if a target format (e.g. audio/ogg) is supported.
else if (!window.MediaRecorder.isTypeSupported('audio/ogg;codecs=opus')) {
  window.MediaRecorder = require('opus-media-recorder');

Browser support


  • Chrome >= 58
  • Firefox >= 53
  • Microsoft Edge >= 41
  • Safari (macOS and iOS) >= 11

Browsers with issues:

MIME Type support

  • audio/webm
  • audio/webm; codecs=opus
  • audio/ogg
  • audio/ogg; codecs=opus
  • audio/wav or audio/wave


  • opus-media-recorder throws generic Error objects instead of native DOMException.
  • Because audio/wav is not designed for streaming, when mimeType is audio/wav, each dataavailabe events produces a complete and separated .wav file that cannot be concatenated together unlike Ogg and WebM. Therefore, it is recommended not to use timeslice option when calling start(), unless you know what the implication is.
  • There is no SecurityError case implemented. (WIP)

How to build

  1. To build from the source, you need Emscripten, NPM, Python 2.7 or higher, and basic C program build systems such as GNU Make.

  2. npm install to install JavaScript dependencies.

  3. make all to build. PRODUCTION=1 make all to build for production.

  4. make run to run a test web server locally. Default URL is https://localhost:9000 (It has to be HTTPS).

  5. make clean to clean up build files.