How to Add Predefined Length to Audio Recorded from Mediarecorder in Chrome

How can I add predefined length to audio recorded from MediaRecorder in Chrome?

This is a chrome bug.

FF does expose the duration of the recorded media, and if you do set the currentTimeof the recorded media to more than its actual duration, then the property is available in chrome...

var recorder,
  chunks = [],
  ctx = new AudioContext(),
  aud = document.getElementById('aud');

function exportAudio() {
  var blob = new Blob(chunks);
  aud.src = URL.createObjectURL(new Blob(chunks));

  aud.onloadedmetadata = function() {
    // it should already be available here
    log.textContent = ' duration: ' + aud.duration;
    // handle chrome's bug
    if (aud.duration === Infinity) {
      // set it to bigger than the actual duration
      aud.currentTime = 1e101;
      aud.ontimeupdate = function() {
        this.ontimeupdate = () => {
          return;
        }
        log.textContent += ' after workaround: ' + aud.duration;
        aud.currentTime = 0;
      }
    }
  }
}

function getData() {
  var request = new XMLHttpRequest();
  request.open('GET', 'https://upload.wikimedia.org/wikipedia/commons/4/4b/011229beowulf_grendel.ogg', true);
  request.responseType = 'arraybuffer';
  request.onload = decodeAudio;
  request.send();
}

function decodeAudio(evt) {
  var audioData = this.response;
  ctx.decodeAudioData(audioData, startRecording);
}

function startRecording(buffer) {

  var source = ctx.createBufferSource();
  source.buffer = buffer;
  var dest = ctx.createMediaStreamDestination();
  source.connect(dest);

  recorder = new MediaRecorder(dest.stream);
  recorder.ondataavailable = saveChunks;
  recorder.onstop = exportAudio;
  source.start(0);
  recorder.start();
  log.innerHTML = 'recording...'
  // record only 5 seconds
  setTimeout(function() {
    recorder.stop();
  }, 5000);
}

function saveChunks(evt) {
  if (evt.data.size > 0) {
    chunks.push(evt.data);
  }

}

// we need user-activation
document.getElementById('button').onclick = function(evt){
  getData();
  this.remove();
}

<button id="button">start</button>
<audio id="aud" controls></audio><span id="log"></span>

Media Recorder Save in WAV format across browsers

I built a package which should do exactly what you want. It's called extendable-media-recorder. It offers the same interface as the native MediaRecorder but allows to "extend" it with custom codecs. The example codec is wav.

Here is how you could use it to record a stream coming from getUserMedia():

import { MediaRecorder, register } from 'extendable-media-recorder';
import { connect } from 'extendable-media-recorder-wav-encoder';

await register(await connect());

const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
const mediaRecoder = new MediaRecorder(stream, { mimeType: 'audio/wav' });

It will use the built-in MediaRecorder to record pcm data in Chrome. In Firefox and Safari the Web Audio API is used to get the pcm data. Once the data has been retrieved it gets send to a worker which decodes it as wav.

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