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

Question

I am in the process of replacing RecordRTC with the built in MediaRecorder for recording audio in Chrome. The recorded audio is then played in the program with audio api. I am having trouble getting the audio.duration property to work. It says

If the video (audio) is streamed and has no predefined length, "Inf" (Infinity) is returned.

With RecordRTC, I had to use ffmpeg_asm.js to convert the audio from wav to ogg. My guess is somewhere in the process RecordRTC sets the predefined audio length. Is there any way to set the predefined length using MediaRecorder?

Answer 1

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, {
    type: 'audio/ogg'
  });
  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);
  }

}



getData();

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

So the advice here would be to star the bug report so that chromium's team takes some time to fix it, even if this workaround can do the trick...

Answer 2

Thanks to @Kaiido for identifying bug and offering the working fix.

I prepared an npm package called get-blob-duration that you can install to get a nice Promise-wrapped function to do the dirty work.

Usage is as follows:

// Returns Promise<Number>
getBlobDuration(blob).then(function(duration) {
  console.log(duration + ' seconds');
});

Or ECMAScript 6:

// yada yada async
const duration = await getBlobDuration(blob)
console.log(duration + ' seconds')

Answer 3

A bug in Chrome, detected in 2016, but still open today (March 2019), is the root cause behind this behavior. Under certain scenarios audioElement.duration will return Infinity.

Chrome Bug information here and here

The following code provides a workaround to avoid the bug.

Usage : Create your audioElement, and call this function a single time, providing a reference of your audioElement. When the returned promise resolves, the audioElement.duration property should contain the right value. ( It also fixes the same problem with videoElements )

/**
 *  calculateMediaDuration() 
 *  Force media element duration calculation. 
 *  Returns a promise, that resolves when duration is calculated
 **/
function calculateMediaDuration(media){
  return new Promise( (resolve,reject)=>{
    media.onloadedmetadata = function(){
      // set the mediaElement.currentTime  to a high value beyond its real duration
      media.currentTime = Number.MAX_SAFE_INTEGER;
      // listen to time position change
      media.ontimeupdate = function(){
        media.ontimeupdate = function(){};
        // setting player currentTime back to 0 can be buggy too, set it first to .1 sec
        media.currentTime = 0.1;
        media.currentTime = 0;
        // media.duration should now have its correct value, return it...
        resolve(media.duration);
      }
    }
  });
}

// USAGE EXAMPLE :  
calculateMediaDuration( yourAudioElement ).then( ()=>{ 
  console.log( yourAudioElement.duration ) 
});