I want to encode a video (from a canvas) and add procedural audio to it.
The encoding can be accomplished with MediaRecorder that takes a MediaStream.
For the stream, I want to obtain the video part from a canvas, using the canvas.captureStream() call.
I want to add an audio track to the stream. But instead of microphone input, I want to generate the samples for those on the fly, for simplicity sake, let's assume it writes out a sine-wave.
How can I create a MediaStreamTrack that generates procedural audio?
The Web Audio API has a createMediaStreamDestination() method, which will return a MediaStreamAudioDestinationNode object, on which you'll be able to connect your audio context, and which will give you access to a MediaStream instance fed by the audio context audio output.
document.querySelector("button").onclick = (evt) => {
const duration = 5;
evt.target.remove();
const audioContext = new AudioContext();
const osc = audioContext.createOscillator();
const destNode = audioContext.createMediaStreamDestination();
const { stream } = destNode;
osc.connect(destNode);
osc.connect(audioContext.destination);
osc.start(0);
osc.frequency.value = 80;
osc.frequency.exponentialRampToValueAtTime(440, audioContext.currentTime+10);
osc.stop(duration);
// stream.addTrack(canvasStream.getVideoTracks()[0]);
const recorder = new MediaRecorder(stream);
const chunks = [];
recorder.ondataavailable = ({data}) => chunks.push(data);
recorder.onstop = (evt) => {
const el = new Audio();
const [{ type }] = chunks; // for Safari
el.src = URL.createObjectURL(new Blob(chunks, { type }));
el.controls = true;
document.body.append(el);
};
recorder.start();
setTimeout(() => recorder.stop(), duration * 1000);
console.log(`Started recording, please wait ${duration}s`);
};<button>begin</button>