Sound analysis without getUserMedia

Question

I am trying to analyse the audio output from the browser, but I don't want the getUserMedia prompt to appear (which asks for microphone permission). The sound sources are SpeechSynthesis and an Mp3 file. Here's my code:

return navigator.mediaDevices.getUserMedia({
        audio: true
      })
      .then(stream => new Promise(resolve => {
        const track = stream.getAudioTracks()[0];
        this.mediaStream_.addTrack(track);
        this._source = this.audioContext.createMediaStreamSource(this.mediaStream_);
        this._source.connect(this.analyser);
        this.draw(this);

      }));

This code is working fine, but it's asking for permission to use the microphone! I a not interested at all in the microphone I only need to gauge the audio output. If I check all available devices:

navigator.mediaDevices.enumerateDevices()
.then(function(devices) {
  devices.forEach(function(device) {
    console.log(device.kind + ": " + device.label +
            " id = " + device.deviceId);
  });
})

I get a list of available devices in the browser, including 'audiooutput'. So, is there a way to route the audio output in a media stream that can be then used inside 'createMediaStreamSource' function? I have checked all the documentation for the audio API but could not find it. Thanks for anyone that can help!

Answer 1

There are various ways to get a MediaStream which is originating from gUM, but you won't be able to catch all possible audio output...

But, for your mp3 file, if you read it through an MediaElement (<audio> or <video>), and if this file is served without breaking CORS, then you can use MediaElement.captureStream. If you read it from WebAudioAPI, or if you target browsers that don't support captureStream, then you can use AudioContext.createMediaStreamDestination.

For SpeechSynthesis, unfortunately you will need gUM... and a Virtual Audio Device: first you would have to set your default output to the VAB_out, then route your VAB_out to VAB_in and finally grab VAB_in from gUM...

Not an easy nor universally doable task, moreover when IIRC SpeechSynthesis doesn't have any setSinkId method.