Web Audio API: Why can you only start sources once?

user_2737696 0
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Suppose you use the Web Audio API to play a pure tone:

ctx = new AudioContext();
src = ctx.createOscillator();
src.frequency = 261.63; //Play middle C
src.connect(ctx.destination);
src.start();

But, later on you decide you want to stop the sound:

src.stop();

From this point on, src is now completely useless; if you try to start it again, you get:

src.start()
VM564:1 Uncaught DOMException: Failed to execute 'start' on 'AudioScheduledSourceNode': cannot call start more than once.
    at <anonymous>:1:5

If you were making, say, a little online keyboard, you're constantly turning notes on and off. It seems really clunky to remove the old object from the audio nodes graph, create a brand new object, and connect() it into the graph, (and then discard the object later) when it would be simpler to just turn it on and off when needed.

Is there some important reason the Web Audio API does things like this? Or is there some cleaner way of restarting an audio source?

web audio javascript
3 Answers

Use connect() and disconnect(). You can then change the values of any AudioNode to change the sound.

(The button is because AudioContext requires a user action to run in Snippet.)

play = () => {
d.addEventListener('mouseover',()=>src.connect(ctx.destination));
d.addEventListener('mouseout',()=>src.disconnect(ctx.destination));
ctx = new AudioContext();
src = ctx.createOscillator();
src.frequency = 261.63; //Play middle C
src.start();
}
div {
height:32px;
width:32px;
background-color:red
}
div:hover {
background-color:green
}
<button onclick='play();this.disabled=true;'>play</button>
<div id='d'></div>

user_43619990

This is exactly how the web audio api works. Sound generator nodes like oscillator nodes and audio buffer source nodes are intended to be used once. Every time you want to play your oscillator, you have to create it and set it up, just like you said. I know it seems like a hassle, but you can abstract it into a play() method that handles those details for you, so you don't have to think about it every time you play an oscillator. Also, don't worry about the performance implications of creating so many nodes. The web audio api is intended to be used this way.

If you just want to make music on the internet, and you're not as interested in learning the ins and outs of the web audio api, you might be interested in using a library I wrote that makes things like this easier: https://github.com/rserota/wad

user_40803060

I am working on a 12 Voice Polyphonic Syntesizer with 2 Osc per Voice.

I now never Stop the Osc's. I disconnect the Osc's. You can do that by setTimeout. For the Time take the longest release Phase (1 of 2) from the amp Enveloop for this Set of Osc's. Subtract the AudioContext.currentTime(), multiply with 1000 (setTimeout works with milisecs, web Audio works with seconds.)

user_122863020
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