//buil an equalizer with multiple biquad filters

​

var audioCtx = window.AudioContext || window.webkitAudioContext;

​

var canvas;

var audioContext, canvasContext;

var analyser;

var width, height;

​

var dataArray, bufferLength;

​

window.onload = function() {

  audioContext= new audioCtx();

  canvas = document.querySelector("#myCanvas");

  width = canvas.width;

  height = canvas.height;

  canvasContext = canvas.getContext('2d');

  buildAudioGraph();

  requestAnimationFrame(visualize);

};

​

function buildAudioGraph() {

  var mediaElement = document.getElementById('player');

    mediaElement.onplay = (e)=>{audioContext.resume();}

​

      // fix for autoplay policy

  mediaElement.addEventListener('play',() => audioContext.resume());

​

  var sourceNode =   audioContext.createMediaElementSource(mediaElement);

  // Create an analyser node

  analyser = audioContext.createAnalyser();

  // Try changing for lower values: 512, 256, 128, 64...

  analyser.fftSize = 1024;

  bufferLength = analyser.frequencyBinCount;

  dataArray = new Uint8Array(bufferLength);

  sourceNode.connect(analyser);

  analyser.connect(audioContext.destination);

}

​

function visualize() {

  // clear the canvas

  // like this: canvasContext.clearRect(0, 0, width, height);

  // Or use rgba fill to give a slight blur effect

  canvasContext.fillStyle = 'rgba(0, 0, 0, 0.5)';

  canvasContext.fillRect(0, 0, width, height);

  // Get the analyser data

  analyser.getByteTimeDomainData(dataArray);

​

  canvasContext.lineWidth = 2;

  canvasContext.strokeStyle = 'lightBlue';

​

  // all the waveform is in one single path, first let's

  // clear any previous path that could be in the buffer

  canvasContext.beginPath();

  var sliceWidth = width / bufferLength;

  var x = 0;

​

  for(var i = 0; i < bufferLength; i++) {

     var v = dataArray[i] / 255;

     var y = v * height;

​

     if(i === 0) {

        canvasContext.moveTo(x, y);

     } else {

        canvasContext.lineTo(x, y);

     }

​

     x += sliceWidth;

  }

​

  canvasContext.lineTo(canvas.width, canvas.height/2);

  // draw the path at once

  canvasContext.stroke();  

  // call again the visualize function at 60 frames/s

  requestAnimationFrame(visualize);

}

​