In the following simple example, we create a MediaStreamAudioDestinationNode, an OscillatorNode and a MediaRecorder (the example will therefore only work in Firefox and Chrome at this time.) The MediaRecorder is set up to record information from the MediaStreamDestinationNode.
When the button is clicked, the oscillator starts, and the MediaRecorder is started. When the button is stopped, the oscillator and MediaRecorder both stop. Stopping the MediaRecorder causes the dataavailable event to fire, and the event data is pushed into the chunks array. After that, the stop event fires, a new blob is made of type opus â which contains the data in the chunks array, and a new window (tab) is then opened that points to a URL created from the blob.
From here, you can play and save the opus file.
<!doctype html>
<html lang="en-US">
<head>
<meta charset="UTF-8" />
<title>createMediaStreamDestination() demo</title>
</head>
<body>
<h1>createMediaStreamDestination() demo</h1>
<p>Encoding a pure sine wave to an Opus file</p>
<button>Make sine wave</button>
<audio controls></audio>
<script>
const b = document.querySelector("button");
let clicked = false;
const chunks = [];
const ac = new AudioContext();
const osc = ac.createOscillator();
const dest = ac.createMediaStreamDestination();
const mediaRecorder = new MediaRecorder(dest.stream);
osc.connect(dest);
b.addEventListener("click", (e) => {
if (!clicked) {
mediaRecorder.start();
osc.start(0);
e.target.textContent = "Stop recording";
clicked = true;
} else {
mediaRecorder.stop();
osc.stop(0);
e.target.disabled = true;
}
});
mediaRecorder.ondataavailable = (evt) => {
// Push each chunk (blobs) in an array
chunks.push(evt.data);
};
mediaRecorder.onstop = (evt) => {
// Make blob out of our blobs, and open it.
const blob = new Blob(chunks, { type: "audio/ogg; codecs=opus" });
document.querySelector("audio").src = URL.createObjectURL(blob);
};
</script>
</body>
</html>
Note: You can view this example live, or study the source code, on GitHub.
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