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HTML

              
                <div id="container"><canvas id="canvas"></canvas></div>
<audio id="audio" controls crossorigin></audio>
<input id="audioFileInput" type="file" accept="audio/*">
<script id="AudioProvider" type="worklet">
class AudioProvider extends AudioWorkletProcessor {
  constructor() {
    super();
    this.dataArrays = [];
    this.bufferSize = 32768; // can handle more than 32768 samples of PCM data unlike in AnalyserNode.getFloatTimeDomainData, which is capped at 32768 samples
    this.bufferIdx = 0;
    this.currentTimeInSamples = 0;
    this.port.onmessage = (e) => {
      const audioChunks = [],
            retrievalWindowSize = e.data ? Math.min(this.bufferSize, currentFrame - this.currentTimeInSamples) : this.bufferSize,
            timeOffset = this.bufferSize-retrievalWindowSize;
      for (let channelIdx = 0; channelIdx < this.dataArrays.length; channelIdx++) {
        audioChunks[channelIdx] = [];
        for (let i = 0; i < this.dataArrays[channelIdx].length-timeOffset; i++) {
          const data = this.dataArrays[channelIdx][((this.bufferIdx+i+timeOffset) % this.bufferSize + this.bufferSize) % this.bufferSize];
          audioChunks[channelIdx][i] = data !== undefined ? data : 0;
        }
      }
      this.port.postMessage({currentChunk: audioChunks});
      this.currentTimeInSamples = currentFrame;
    };
  }
  
  process(inputs, _, _2) {
    if (inputs[0].length <= 0)
      return true;
    this.dataArrays.length = inputs[0].length;
    for (let i = 0; i < this.dataArrays.length; i++) {
      if (this.dataArrays[i] === undefined)
        this.dataArrays[i] = new Array(this.bufferSize);
      else {
        this.dataArrays[i].length = this.bufferSize;
      }
    }
    
    for (let i = 0; i < inputs[0][0].length; i++) {
      this.bufferIdx = Math.min(this.bufferIdx, this.bufferSize-1);
      for (let channelIdx = 0; channelIdx < inputs[0].length; channelIdx++) {
        this.dataArrays[channelIdx][this.bufferIdx] = inputs[0][channelIdx][i];
      }
      this.bufferIdx = ((this.bufferIdx + 1) % this.bufferSize + this.bufferSize) % this.bufferSize;
    }
    return true;
  }
}

registerProcessor('audio-provider', AudioProvider);
</script>
<script>
class IIRFilter {
  constructor(a0, a1, a2, b1, b2) {
    this.recalcCoeffs(a0, a1, a2, b1, b2);
  }
  
  recalcCoeffs(a0 = 0, a1 = 0, a2 = 0, b1 = 0, b2 = 0) {
    this.a0 = a0;
    this.a1 = a1;
    this.a2 = a2;
    this.b1 = b1;
    this.b2 = b2;
    this.z1 = 0;
    this.z2 = 0;
  }
  
  process(x) {
    const output = x * this.a0 + this.z1;
    this.z1 = x * this.a1 + this.z2 - this.b1 * output;
    this.z2 = x * this.a2 - this.b2 * output;
    return output;
  }
}

class KWeighting {
  constructor(sampleRate) {
    this.highpass = new IIRFilter();
    this.highshelf = new IIRFilter();
    this.recalcCoeffs(sampleRate);
  }
  
  recalcCoeffs(sampleRate = 44100) {
    const hpFreq = 80,
          hpQ = Math.SQRT2/2,
          hsFreq = 1500,
          hsGain = 10 ** (4/20),
          k1 = Math.tan(Math.PI * hpFreq/sampleRate),
          k2 = Math.tan(Math.PI * hsFreq/sampleRate),
          norm1 = 1 / (1 + k1 / hpQ + k1 ** 2),
          norm2 = 1 / (1 + Math.SQRT2 * k2 + k2 ** 2);
    
    // highpass part
    const hpA0 = norm1,
          hpA1 = -hpA0 * 2,
          hpA2 = norm1,
          hpB1 = 2 * (k1 ** 2 - 1) * norm1,
          hpB2 = (1 - k1 / hpQ + k1 ** 2) * norm1,
    // highshelf part
          hsA0 = (hsGain + Math.sqrt(hsGain*2) * k2 + k2 ** 2) * norm2,
          hsA1 = 2 * (k2 ** 2 - hsGain) * norm2,
          hsA2 = (hsGain - Math.sqrt(hsGain*2) * k2 + k2 ** 2) * norm2,
          hsB1 = 2 * (k2 ** 2 - 1) * norm2,
          hsB2 = (1 - Math.SQRT2 * k2 + k2 ** 2) * norm2;
    
    this.highpass.recalcCoeffs(
      hpA0,
      hpA1,
      hpA2,
      hpB1,
      hpB2
    );
    this.highshelf.recalcCoeffs(
      hsA0,
      hsA1,
      hsA2,
      hsB1,
      hsB2
    );
  }
  
  process(x) {
    return this.highshelf.process(this.highpass.process(x));
  }
}
</script>
<script>
  function map(x, min, max, targetMin, targetMax) {
    return (x - min) / (max - min) * (targetMax - targetMin) + targetMin;
  }
  
  function clamp(x, min, max) {
    return Math.min(Math.max(x, min), max);
  }
  
  function idxWrapOver(x, length) {
    return (x % length + length) % length;
  }
  // Hz and FFT bin conversion
function hertzToFFTBin(x, y = 'round', bufferSize = 4096, sampleRate = 44100) {
  const bin = x * bufferSize / sampleRate;
  let func = y;
  
  if (!['floor','ceil','trunc'].includes(func))
    func = 'round'; // always use round if you specify an invalid/undefined value
  
  return Math[func](bin);
}

function fftBinToHertz(x, bufferSize = 4096, sampleRate = 44100) {
  return x * sampleRate / bufferSize;
}
  

// Calculate the FFT
function calcFFT(input) {
  let fft = input.map(x => x);
  let fft2 = input.map(x => x);
  transform(fft, fft2);
  let output = new Array(Math.round(fft.length/2)).fill(0);
  for (let i = 0; i < output.length; i++) {
    output[i] = Math.hypot(fft[i], fft2[i])/(fft.length);
  }
  return output;
}

function calcComplexFFT(input) {
  let fft = input.map(x => x);
  let fft2 = input.map(x => x);
  transform(fft, fft2);
  return input.map((_, i, arr) => {
    return {
      re: fft[i]/(arr.length/2),
      im: fft2[i]/(arr.length/2),
      magnitude: Math.hypot(fft[i], fft2[i])/(arr.length/2),
      phase: Math.atan2(fft2[i], fft[i])
    };
  });
}
  
function calcComplexInputFFT(real, imag) {
  if (real.length !== imag.length)
    return [];
  const fft1 = real.map(x => x),
        fft2 = imag.map(x => x);
  transform(fft1, fft2);
  return real.map((_, i, arr) => {
    return {
      re: fft1[i]/arr.length,
      im: fft2[i]/arr.length,
      magnitude: Math.hypot(fft1[i], fft2[i])/arr.length,
      phase: Math.atan2(fft2[i], fft1[i])
    }
  });
}
  
  /**
 * FFT and convolution (JavaScript)
 * 
 * Copyright (c) 2017 Project Nayuki. (MIT License)
 * https://www.nayuki.io/page/free-small-fft-in-multiple-languages
 */

/* 
 * Computes the discrete Fourier transform (DFT) of the given complex vector, storing the result back into the vector.
 * The vector can have any length. This is a wrapper function.
 */
function transform(real, imag) {
	const n = real.length;
	if (n != imag.length)
		throw "Mismatched lengths";
	if (n <= 0)
		return;
	else if ((2 ** Math.trunc(Math.log2(n))) === n)  // Is power of 2
		transformRadix2(real, imag);
	else  // More complicated algorithm for arbitrary sizes
		transformBluestein(real, imag);
}


/* 
 * Computes the inverse discrete Fourier transform (IDFT) of the given complex vector, storing the result back into the vector.
 * The vector can have any length. This is a wrapper function. This transform does not perform scaling, so the inverse is not a true inverse.
 */
function inverseTransform(real, imag) {
	transform(imag, real);
}


/* 
 * Computes the discrete Fourier transform (DFT) of the given complex vector, storing the result back into the vector.
 * The vector's length must be a power of 2. Uses the Cooley-Tukey decimation-in-time radix-2 algorithm.
 */
function transformRadix2(real, imag) {
	// Length variables
	const n = real.length;
	if (n != imag.length)
		throw "Mismatched lengths";
	if (n <= 1)  // Trivial transform
		return;
	const logN = Math.log2(n);
	if ((2 ** Math.trunc(logN)) !== n)
		throw "Length is not a power of 2";
	
	// Trigonometric tables
	let cosTable = new Array(n / 2);
	let sinTable = new Array(n / 2);
	for (let i = 0; i < n / 2; i++) {
		cosTable[i] = Math.cos(2 * Math.PI * i / n);
		sinTable[i] = Math.sin(2 * Math.PI * i / n);
	}
	
	// Bit-reversed addressing permutation
	for (let i = 0; i < n; i++) {
		let j = reverseBits(i, logN);
		if (j > i) {
			let temp = real[i];
			real[i] = real[j];
			real[j] = temp;
			temp = imag[i];
			imag[i] = imag[j];
			imag[j] = temp;
		}
	}
	
	// Cooley-Tukey decimation-in-time radix-2 FFT
	for (let size = 2; size <= n; size *= 2) {
		let halfsize = size / 2;
		let tablestep = n / size;
		for (let i = 0; i < n; i += size) {
			for (let j = i, k = 0; j < i + halfsize; j++, k += tablestep) {
				const l = j + halfsize;
				const tpre =  real[l] * cosTable[k] + imag[l] * sinTable[k];
				const tpim = -real[l] * sinTable[k] + imag[l] * cosTable[k];
				real[l] = real[j] - tpre;
				imag[l] = imag[j] - tpim;
				real[j] += tpre;
				imag[j] += tpim;
			}
		}
	}
	
	// Returns the integer whose value is the reverse of the lowest 'bits' bits of the integer 'x'.
	function reverseBits(x, bits) {
		let y = 0;
		for (let i = 0; i < bits; i++) {
			y = (y << 1) | (x & 1);
			x >>>= 1;
		}
		return y;
	}
}


/* 
 * Computes the discrete Fourier transform (DFT) of the given complex vector, storing the result back into the vector.
 * The vector can have any length. This requires the convolution function, which in turn requires the radix-2 FFT function.
 * Uses Bluestein's chirp z-transform algorithm.
 */
function transformBluestein(real, imag) {
	// Find a power-of-2 convolution length m such that m >= n * 2 + 1
	const n = real.length;
	if (n != imag.length)
		throw "Mismatched lengths";
	const m = 2 ** Math.trunc(Math.log2(n*2)+1);
	
	// Trignometric tables
	let cosTable = new Array(n);
	let sinTable = new Array(n);
	for (let i = 0; i < n; i++) {
		let j = i * i % (n * 2);  // This is more accurate than j = i * i
		cosTable[i] = Math.cos(Math.PI * j / n);
		sinTable[i] = Math.sin(Math.PI * j / n);
	}
	
	// Temporary vectors and preprocessing
	let areal = newArrayOfZeros(m);
	let aimag = newArrayOfZeros(m);
	for (let i = 0; i < n; i++) {
		areal[i] =  real[i] * cosTable[i] + imag[i] * sinTable[i];
		aimag[i] = -real[i] * sinTable[i] + imag[i] * cosTable[i];
	}
	let breal = newArrayOfZeros(m);
	let bimag = newArrayOfZeros(m);
	breal[0] = cosTable[0];
	bimag[0] = sinTable[0];
	for (let i = 1; i < n; i++) {
		breal[i] = breal[m - i] = cosTable[i];
		bimag[i] = bimag[m - i] = sinTable[i];
	}
	
	// Convolution
	let creal = new Array(m);
	let cimag = new Array(m);
	convolveComplex(areal, aimag, breal, bimag, creal, cimag);
	
	// Postprocessing
	for (let i = 0; i < n; i++) {
		real[i] =  creal[i] * cosTable[i] + cimag[i] * sinTable[i];
		imag[i] = -creal[i] * sinTable[i] + cimag[i] * cosTable[i];
	}
}


/* 
 * Computes the circular convolution of the given real vectors. Each vector's length must be the same.
 */
function convolveReal(x, y, out) {
	const n = x.length;
	if (n != y.length || n != out.length)
		throw "Mismatched lengths";
	convolveComplex(x, newArrayOfZeros(n), y, newArrayOfZeros(n), out, newArrayOfZeros(n));
}


/* 
 * Computes the circular convolution of the given complex vectors. Each vector's length must be the same.
 */
function convolveComplex(xreal, ximag, yreal, yimag, outreal, outimag) {
	const n = xreal.length;
	if (n != ximag.length || n != yreal.length || n != yimag.length
			|| n != outreal.length || n != outimag.length)
		throw "Mismatched lengths";
	
	xreal = xreal.slice();
	ximag = ximag.slice();
	yreal = yreal.slice();
	yimag = yimag.slice();
	transform(xreal, ximag);
	transform(yreal, yimag);
	
	for (let i = 0; i < n; i++) {
		const temp = xreal[i] * yreal[i] - ximag[i] * yimag[i];
		ximag[i] = ximag[i] * yreal[i] + xreal[i] * yimag[i];
		xreal[i] = temp;
	}
	inverseTransform(xreal, ximag);
	
	for (let i = 0; i < n; i++) {  // Scaling (because this FFT implementation omits it)
		outreal[i] = xreal[i] / n;
		outimag[i] = ximag[i] / n;
	}
}


function newArrayOfZeros(n) {
	let result = new Array(n).fill(0);
	return result;
}
</script>
              
            
!

CSS

              
                body {
  margin: 0;
  overflow: hidden;
}

audio {
  display: inline-block;
  width: 100%;
  height: 40px;
}

canvas {
  display: block;
  width: 100%;
}

#container {
  height: calc( 100vh - 40px );
}

#upload {
  display: none;
}
              
            
!

JS

              
                // necessary parts for audio context and audio elements respectively
const audioCtx = new AudioContext();
const audioPlayer = document.getElementById('audio');
const localAudioElement = document.getElementById('audioFileInput');
localAudioElement.addEventListener('change', loadLocalFile);
// canvas is for displaying visuals
const canvas = document.getElementById('canvas'),
      ctx = canvas.getContext('2d'),
      container = document.getElementById('container');
const audioSource = audioCtx.createMediaElementSource(audioPlayer);
const analyser = audioCtx.createAnalyser();
analyser.fftSize = 32768; // maxes out FFT size
const dataArray = new Float32Array(analyser.fftSize);
// variables
const oscilloscopeBuffer = [];
let oscilloscopeIdx = 0;
const delay = audioCtx.createDelay();
audioSource.connect(delay);
delay.connect(audioCtx.destination);
//audioSource.connect(audioCtx.destination);
audioSource.connect(analyser);
let audioProvider,
    currentSampleRate = audioCtx.sampleRate;
const customDSPSource = document.getElementById('AudioProvider'),
      dspSourceBlob = new Blob([customDSPSource.innerText], {type: 'application/javascript'}),
      dspSourceUrl = URL.createObjectURL(dspSourceBlob);

const visualizerSettings = {
  resetAverages: resetSmoothedValues,
  // label part
  showLabels: true,
  showLabelsY: true,
  amplitudeLabelInterval: 6,
  minAmplitudeLabel: -24,
  numDivisions: 10,
  mirrorLabels: true,
  diffLabels: false,
  // triggering part
  outputSize: 1920,
  compensateDC: false,
  polarity: 'up',
  triggeringChannelMode: 'mono',
  triggeringChannelIdx: 0,
  reverseTriggeringIdx: false,
  // other stuffs
  freeze: false,
  darkMode: false,
  useGradient: true,
  autoReset: true,
  resetBoth: resetBoth,
  fftSize: 3840,
  compensateDelay: true
},
      averagingDomains = {
        'Linear': 'linear',
        'Squared (RMS)': 'rms',
        'Logarithmic': 'log'
      },
      loader = {
        url: '',
        load: function() {
          audioPlayer.src = this.url;
          audioPlayer.play();
        },
        loadLocal: function() {
          localAudioElement.click();
        },
        toggleFullscreen: _ => {
          if (document.fullscreenElement === canvas)
            document.exitFullscreen();
          else
            canvas.requestFullscreen();
        }
      };
// dat.GUI for quick customization
let gui = new dat.GUI();
gui.add(loader, 'url').name('URL');
gui.add(loader, 'load').name('Load');
gui.add(loader, 'loadLocal').name('Load from local device');
let settings = gui.addFolder('Visualization settings');
//const amplitudeFolder = settings.addFolder('Amplitude');
settings.add(visualizerSettings, 'fftSize', 32, 65536, 1).name('Input buffer length').onChange(resetBoth);
settings.add(visualizerSettings, 'outputSize', 32, 65536, 1).name('Output buffer length');
// triggering part
const triggerFolder = settings.addFolder('Oscilloscope triggering');
triggerFolder.add(visualizerSettings, 'compensateDC').name('Compensate for DC offset');
triggerFolder.add(visualizerSettings, 'polarity', {
  'Up': 'up',
  'Down': 'down',
  'Both': 'both'}).name('Polarity');
triggerFolder.add(visualizerSettings, 'triggeringChannelMode', {
  'Mono': 'mono',
  'Single channel': 'single',
  'Independent': 'independent'
}).name('Triggering channel mode');
triggerFolder.add(visualizerSettings, 'triggeringChannelIdx', 0, 32, 1).name('Triggering channel index');
triggerFolder.add(visualizerSettings, 'reverseTriggeringIdx').name('Reverse triggering channel index');
// label part
const labelFolder = settings.addFolder('Labels and grids');
labelFolder.add(visualizerSettings, 'showLabels').name('Show horizontal-axis labels');
labelFolder.add(visualizerSettings, 'showLabelsY').name('Show vertical-axis labels');
labelFolder.add(visualizerSettings, 'amplitudeLabelInterval', 0.5, 48).name('dB label interval');
labelFolder.add(visualizerSettings, 'minAmplitudeLabel', -90, 0).name('Minimum dB label display');
labelFolder.add(visualizerSettings, 'numDivisions', 2, 48, 1).name('Number of X-axis label divisions');
labelFolder.add(visualizerSettings, 'mirrorLabels').name('Mirror Y-axis labels');
labelFolder.add(visualizerSettings, 'diffLabels').name('Use difference coloring for labels');
settings.add(visualizerSettings, 'freeze').name('Freeze analyzer');
settings.add(visualizerSettings, 'darkMode').name('Dark mode');
settings.add(visualizerSettings, 'useGradient').name('Use brighter color on dark mode');
settings.add(visualizerSettings, 'autoReset').name('Enable auto-reset');
settings.add(visualizerSettings, 'compensateDelay').name('Compensate for delay');
settings.add(visualizerSettings, 'resetBoth').name('Reset oscilloscope');
gui.add(loader, 'toggleFullscreen').name('Toggle fullscreen mode');

function resetBoth() {
  resetSmoothedValues();
}

function resetSmoothedValues() {
  oscilloscopeBuffer.length = 0;
  oscilloscopeIdx = 0;
}

function resizeCanvas() {
  const scale = devicePixelRatio,
        isFullscreen = document.fullscreenElement === canvas;
  canvas.width = (isFullscreen ? innerWidth : container.clientWidth)*scale;
  canvas.height = (isFullscreen ? innerHeight : container.clientHeight)*scale;
}

addEventListener('click', () => {
  if (audioCtx.state == 'suspended')
    audioCtx.resume();
});
addEventListener('resize', resizeCanvas);
resizeCanvas();

function autoReset() {
  if (visualizerSettings.autoReset && !visualizerSettings.freeze)
    resetBoth();
}
// this below makes it more faithful to how foobar2000 visualizations work
audioPlayer.addEventListener('play', autoReset);
audioPlayer.addEventListener('seeked', autoReset);


function loadLocalFile(event) {
  const file = event.target.files[0],
        reader = new FileReader();
  reader.onload = (e) => {
    audioPlayer.src = e.target.result;
    audioPlayer.play();
  };

  reader.readAsDataURL(file);
}

//visualize();
audioCtx.audioWorklet.addModule(dspSourceUrl).then(() => {
  //let messageCounter = 0;
  audioProvider = new AudioWorkletNode(audioCtx, 'audio-provider');
  audioSource.connect(audioProvider);
  audioProvider.port.postMessage(0);
  audioProvider.port.onmessage = (e) => {
    if (!visualizerSettings.freeze)
      analyzeChunk(e.data.currentChunk);
    audioProvider.port.postMessage(1);
    //if (messageCounter < 1) {
    //  console.log(e.data.currentChunk);
    //}
    //messageCounter++;
  };
  // optional mic input
  /*navigator.mediaDevices.getUserMedia({
    audio: {
      noiseCancellation: false,
      echoCancellation: false,
      autoGainControl: false
    },
    video: false
  }).then((stream) => {
    const audioStream = audioCtx.createMediaStreamSource(stream);
    audioStream.connect(analyser);
    audioStream.connect(audioProvider); // for use with AudioWorklet-based visualizations
  }).catch((err) => {
    console.log(err);
  });*/
  audioProvider.onprocessorerror = (e) => {
    console.log(e.message);
  }
  visualize();
}).catch((e) => {
  console.log(e.message);
});

function analyzeChunk(data) {
  const dataset = [];
  let retrievalLength = 0;
  for (const x of data) {
    retrievalLength = Math.max(retrievalLength, x.length);
  }
  oscilloscopeBuffer.length = data.length;
  for (let i = 0; i < data.length; i++) {
    const length = visualizerSettings.fftSize;
    if (oscilloscopeBuffer[i] === undefined)
      oscilloscopeBuffer[i] = new Array(length);
    else if (oscilloscopeBuffer[i].length !== length)
      oscilloscopeBuffer[i].length = length;
  }
  
  for (let i = 0; i < retrievalLength; i++) {
    for (let channelIdx = 0; channelIdx < data.length; channelIdx++) {
      oscilloscopeBuffer[channelIdx][oscilloscopeIdx] = data[channelIdx][i];
    }
    oscilloscopeIdx = idxWrapOver(oscilloscopeIdx+1, visualizerSettings.fftSize);
  }
  currentSampleRate = audioCtx.sampleRate;
}

function visualize() {
  const actualLength = Math.min(visualizerSettings.fftSize, visualizerSettings.outputSize);
  delay.delayTime.value = (actualLength / audioCtx.sampleRate) * visualizerSettings.compensateDelay / 2;
  /*if (!visualizerSettings.freeze) {
    
  }*/
  const fgColor = visualizerSettings.darkMode ? (visualizerSettings.useGradient ? '#c0c0c0' : '#fff') : '#000',
        bgColor = visualizerSettings.darkMode ? (visualizerSettings.useGradient ? '#202020' : '#000') : '#fff';
  ctx.globalCompositeOperation = 'source-over';
  ctx.fillStyle = bgColor;
  ctx.fillRect(0, 0, canvas.width, canvas.height);
  
  let oscOffset = 0;
  if (oscilloscopeBuffer.length > 0) {
    if (visualizerSettings.triggeringChannelMode === 'single') {
      const i = visualizerSettings.triggeringChannelIdx,
            idx = idxWrapOver(visualizerSettings.reverseTriggeringIdx ? oscilloscopeBuffer.length-i-1 : i, oscilloscopeBuffer.length);
      oscOffset = calcTriggeringOffset(renderOscilloscope(oscilloscopeBuffer, idx));
    }
    else if (visualizerSettings.triggeringChannelMode === 'mono')
      oscOffset = calcTriggeringOffset(getMonoData(oscilloscopeBuffer));
  }
  
  ctx.fillStyle = fgColor;
  ctx.strokeStyle = fgColor;
  for (let i = 0; i < oscilloscopeBuffer.length; i++) {
    const waveData = renderOscilloscope(oscilloscopeBuffer, i);
    if (visualizerSettings.triggeringChannelMode === 'independent')
      oscOffset = calcTriggeringOffset(waveData);
    ctx.beginPath();
    calcOscilloscope(waveData, oscOffset).map((x, idx, arr) => {
      ctx.lineTo(idx*canvas.width/arr.length, (map(x, -1, 1, 1, 0)+i)*canvas.height/oscilloscopeBuffer.length);
    });
    ctx.stroke();
  }
  
  ctx.globalCompositeOperation = visualizerSettings.diffLabels ? 'difference' : 'source-over';
  ctx.fillStyle = visualizerSettings.diffLabels ? '#fff' : fgColor;
  ctx.strokeStyle = visualizerSettings.diffLabels ? '#fff' : fgColor;
  // label part
  const numChannels = oscilloscopeBuffer.length;
  ctx.font = `${Math.trunc(10*devicePixelRatio)}px sans-serif`;
  ctx.textAlign = 'start';
  // time label part
  if (visualizerSettings.showLabels) {
    ctx.globalAlpha = 0.5;
    ctx.setLineDash([]);
    const labelData = [];
    for (let i = 0; i <= visualizerSettings.numDivisions; i++) {
      labelData[i] = i;
    }
    labelData.map(x =>{
      ctx.globalAlpha = 0.5;
      const label = `${Math.round(map(x, 0, visualizerSettings.numDivisions, actualLength, 0))} samples`,
            posX = map(x, 0, visualizerSettings.numDivisions, 0, canvas.width);
      ctx.beginPath();
      ctx.lineTo(posX, canvas.height);
      ctx.lineTo(posX, 0);
      ctx.stroke();
      ctx.globalAlpha = 1;
      if (numChannels > 1) {
        ctx.textBaseline = 'middle';
        for (let i = 0; i < numChannels-1; i++) {
          ctx.fillText(label, posX, (1+i)*canvas.height/numChannels);
        }
      }
      else {
        ctx.textBaseline = 'alphabetic';
        ctx.fillText(label, posX, canvas.height);
      }
    });
    ctx.setLineDash([]);
    ctx.globalAlpha = 1;
    ctx.textAlign = 'start'
    ctx.textBaseline = 'alphabetic';
  }
  // amplitude/dB label part
  if (visualizerSettings.showLabelsY) {
    const dBLabelData = [-Infinity],
          mindB = visualizerSettings.minAmplitudeLabel,
          maxdB = 0,
          minLabelIdx = Math.round(mindB/visualizerSettings.amplitudeLabelInterval),
          maxLabelIdx = Math.round(maxdB/visualizerSettings.amplitudeLabelInterval);
    
    if (isFinite(minLabelIdx) && isFinite(maxLabelIdx)) {
      for (let i = maxLabelIdx; i >= minLabelIdx; i--) {
        dBLabelData.push(i*visualizerSettings.amplitudeLabelInterval);
      }
    }
    
    ctx.globalAlpha = 0.5;
    ctx.setLineDash([]);
    
    dBLabelData.map(x => {
      ctx.globalAlpha = 0.5;
      ctx.textBaseline = 'middle';
      ctx.textAlign = visualizerSettings.mirrorLabels ? 'end' : 'start';
      
      const label = `${x}${isFinite(x) ? '' : ' '}dB`,
            posY = map(10 ** (x/20), -1, 1, canvas.height/numChannels, 0),
            height = canvas.height/numChannels;
      
      for (let i = 0; i < numChannels; i++) {
        const offset = i*height,
              y1 = posY+offset,
              y2 = height-posY+offset;
        ctx.globalAlpha = 0.5;
        ctx.beginPath();
        ctx.lineTo(0, y1);
        ctx.lineTo(canvas.width, y1);
        ctx.stroke();
        // inverted part since waveform has negative polarity, since decibels is 20 * log10(abs(x))
        if (isFinite(x)) {
          ctx.beginPath();
          ctx.lineTo(0, y2);
          ctx.lineTo(canvas.width, y2);
          ctx.stroke();
        }
        ctx.globalAlpha = 1;
        ctx.fillText(label, canvas.width * visualizerSettings.mirrorLabels, y1);
        if (isFinite(x))
          ctx.fillText(label, canvas.width * visualizerSettings.mirrorLabels, y2);
      }
    });
    
    ctx.setLineDash([]);
    ctx.globalAlpha = 1;
    ctx.textAlign = 'start';
    ctx.textBaseline = 'alphabetic';
  }
  
  requestAnimationFrame(visualize);
}
// and here's the additional functions that we can need for this visualization
function renderOscilloscope(data, idx) {
  const dataset = [];
  for (let i = 0; i < data[idx].length; i++) {
    dataset[i] = data[idx][idxWrapOver(i+oscilloscopeIdx-data[idx].length, data[idx].length)];
  }
  return dataset;
}

function getMonoData(data) {
  const dataArray = [];
  for (let i = 0; i < data.length; i++) {
    renderOscilloscope(data, i).map((x, idx) => {
      dataArray[idx] = (dataArray[idx] === undefined ? 0 : dataArray[idx]) + x/data.length;
    });
  }
  return dataArray;
}

// zero-crossing based triggering
function calcTriggeringOffset(waveform, outputSize = visualizerSettings.outputSize, compensateDC = visualizerSettings.compensateDC, polarity = visualizerSettings.polarity) {
  let dc = 0;
  if (compensateDC) {
    waveform.map(x => {
      dc += isFinite(x) ? x : 0; // account for DC offset
    });
    dc /= waveform.length;
  }
  let idx = 0,
      hasNotTriggered = true,
      triggeringEnabled = false;
  for (let i = 1; i < waveform.length-outputSize; i++) {
    triggeringEnabled = true;
    if (
       ((waveform[i-1] >= dc && waveform[i] < dc) && polarity !== 'up') ||
       ((waveform[i-1] < dc && waveform[i] >= dc) && polarity !== 'down')) {
      idx = i;
      hasNotTriggered = false;
    }
  }
  return hasNotTriggered && triggeringEnabled ? waveform.length-outputSize-1 : idx;
}

function calcOscilloscope(data, offset = 0, outputSize = visualizerSettings.outputSize) {
  const dataArray = [];
  for (let i = 0; i < Math.min(data.length, outputSize); i++) {
    dataArray[i] = data[i+offset];
  }
  return dataArray;
}
              
            
!
999px

Console