23 Mar 2015

Recording audio in browser

With advancements in HTML5, audio recording in browser is now easier than before. Even though the underlying technologies, Web Audio API + WebRTC, is not fully supported across browsers, it is a better alternative to having users install addons or download extensions to enable audio recording in browser.

The how

In it’s simplest form, getUserMedia is used to capture audio for Web Audio to then record. But of course, you know it’s more complicated than that. Good news is RecorderJs has done all the necessary work.

// include recorder.js script

var recorder;
window.AudioContext = window.AudioContext
                  || window.webkitAudioContext;
navigator.getUserMedia = (navigator.getUserMedia ||
               navigator.webkitGetUserMedia ||
               navigator.mozGetUserMedia ||
var audioContext = new AudioContext;

navigator.getUserMedia({audio: true}, function(mediaStream) {
  var input = audioContext.createMediaStreamSource(mediaStream);
  recorder = new Recorder(input, {
    numChannels: 1
}, function() {
  // No live audio input



The audio is recorded in uncompressed wav format. If you will be uploading the recorded data, the size can be an issue. A one minute, 2 channels recording is around 10mb (one minute mono is around 5mb). What is the possible workaround? Compressing to mp3 - on the client! Don’t fret yet, libmp3lame.js got you covered. There is a good article on implementing libmp3lame.js with Recorderjs here: nusofthq.com/blog/recording-mp3-using-only-html5-and-javascript-recordmp3-js/. Converting to mp3 reduces the size by about 97%. A converted mp3 file of one minute is about 300kb.

There are 2 minor issues with the final working script in that article though. One, it uses an old version of Recorderjs and hard-coded the channel as 1 in several parts of the source. The other issue is with libmp3lame. The converted mp3 file is twice the length of the original recording, with the other half blank. I created a fork that fixed both. I actually didn’t do anything super. I just recreated using a newer version of Recorderjs, commented out a line causing audio feedback and made a tiny tweak in the libmp3lame source as recommended in a comment. You can check out the fork here: github.com/kehers/Recordmp3js



If you want to visualise the audio input live, check out the Microphone plugin of wavesurfer: wavesurfer-js.org/examples/. If you want to delve deeper, check out the MDN Web Audio API docs.

If you are interested in where this experiment leads, follow me on Twitter (@kehers) to follow what I am working on. I promise you will love it :)


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My name is Opeyemi Obembe. I build things for web and mobile and write about my experiments. Follow me on Twitter–@kehers.


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