Accurately Timing Sounds In-Browser For A Metronome

Question

I want to create a virtual metronome and play a metronome sound with each step. Usual methods such as setTimeout()/setInterval() are not accurate enough. How can I make it work without delay or dropping beats?

Answer 1

Here's a function that beeps every 500ms.

function beep() {
    var snd = new Audio("data:audio/wav;base64,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");  
    snd.play();
}

setInterval(beep, 500)

That's however not very precise. The callback may be fired late (depending on a lot, this can range from a few ms to over 1000ms). To get around this you can use AudioContext. We'll use a simple Oscillator for the Metronomes sound.

You could either schedule multiple notes at once or use setInterval to run a function that schedules the next note that should be played.

function noteDurationToMs (bpm, dur, type) {
    return 60000 * 4 * dur * type / bpm
}

function scheduleNote(ac, time, dur) {
    var osc = ac.createOscillator();
    osc.connect( ac.destination );
    osc.start(time);
    osc.stop(time + dur);
}

const ac = new AudioContext();
let lastNote= ac.currentTime;
const step = noteDurationToMs(120, 1 / 4, 1) / 1000;
const lookAhead = step / 2;

let id, timer = ()=> {
  const diff = ac.currentTime - lastNote;
  if (diff >= lookAhead) {
   const nextNote = lastNote + step;
   scheduleNote(ac, nextNote, 0.025)
   lastNote = nextNote;
  }
}
a.addEventListener('click', () => {
    ac.resume();
    id = setInterval(timer, 15)
})
b.addEventListener('click', () => clearInterval(id))

<button id="a">Start</button>
<button id="b">Stop</button>

Note: for some reason AudioContext doesn't seem to be working in Stack Snippets. Here is a jsfiddle for the first code snippet. Here is one for the second snippet.

function noteDurationToMs (bpm, dur, type) {
    return 60000 * 4 * dur * type / bpm
}

function scheduleNote(ac, time, dur) {
    var osc = ac.createOscillator();
    osc.connect( ac.destination );
    
    
    osc.start(time);
    osc.stop(time + dur);
}

const ac = new AudioContext();
let to, lastNote = 0;

const Metronome = () => {
  const [bpm, setBpm] = React.useState(100);
  const [dur, setDur] = React.useState(1);  
  const [type, setType] = React.useState(1);
    const [run, setRun] = React.useState(false);    
  
  const handleChangeBPM = (e) => {
    setBpm(e.target.value)
  }
  
  const handleChangeDur = (e) => {
    setDur(e.target.value)
  }
  
  const handleChangeType = (e) => {
    setType(e.target.value)
  }
  
  const step = noteDurationToMs(bpm, dur, type) / 1000;
    const lookAhead = step / 2;
  
  const timer = () => {
    const diff = ac.currentTime - lastNote;
    if (diff >= lookAhead) {
     const nextNote = lastNote + step;
     scheduleNote(ac, nextNote, 0.025)
     lastNote = nextNote;
    }
  }
  
  const start = () => {
    ac.resume()
    setRun(true);
  }
  
  const stop = () => {
    clearInterval(to);
    setRun(false);
  }
  
  const toggle = () => {
    run?stop():start();
  }

    React.useEffect(() => {
    if (run) {
        clearInterval(to);
      to = setInterval(timer, step / 4);
    }
  });
  
    return <div>
    <label for="bpm">BPM:</label>

    
      <input id="bpm" type="number" onChange={handleChangeBPM} value={bpm} />
    
    <label for="dur">Duration:</label>
    
    <select id="dur" onChange={handleChangeDur} value={dur}>
      <option value={1}>Whole</option>
      <option value={1 / 2}>Half</option>
      <option value={1 / 4}>Quarter</option>
      <option value={1 / 8}>Eigth </option>
      <option value={1 / 16}>Sixteenth</option>
      <option value={1 / 32}>Thirtysecond</option>
    </select>
    
    <label for="typ">Type:</label>
    <select id="typ" onChange={handleChangeType} value={type}>
      <option value={1}>Regular</option>
      <option value={3 / 2}>Dotted</option>
      <option value={2 / 3}>Triplet</option>
    </select>
    <button onClick={toggle}>{!run?'Start':'Stop'}</button>
    <br />
    <label for="ms">MS:</label>
    <output id="ms">{noteDurationToMs(bpm, dur, type)}</output>
    </div>
}

ReactDOM.render(<Metronome />, document.querySelector("#app"))

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js"></script>
<div id="app"></div>

---

To include your own edit: You are also able to use AudioBufferSourceNode and call .start(), with the when parameter to play an audio exactly at time but you would not be able to run a function with it.

AudioBufferSourceNode.start([when]);