AnalyserNode.getByteFrequencyData()

The getByteFrequencyData() method of the AnalyserNode interface copies the current frequency data into a Uint8Array (unsigned byte array) passed into it.

The frequency data is composed of integers on a scale from 0 to 255.

Each item in the array represents the decibel value for a specific frequency. The frequencies are spread linearly from 0 to 1/2 of the sample rate. For example, for 48000 sample rate, the last item of the array will represent the decibel value for 24000 Hz.

If the array has fewer elements than the AnalyserNode.frequencyBinCount, excess elements are dropped. If it has more elements than needed, excess elements are ignored.

Syntax

var audioCtx = new AudioContext();
var analyser = audioCtx.createAnalyser();
var dataArray = new Uint8Array(analyser.frequencyBinCount); // Uint8Array should be the same length as the frequencyBinCount

void analyser.getByteFrequencyData(dataArray); // fill the Uint8Array with data returned from getByteFrequencyData()

Parameters

array
The Uint8Array that the frequency domain data will be copied to. For any sample which is silent, the value is -Infinity.
If the array has fewer elements than the AnalyserNode.frequencyBinCount, excess elements are dropped. If it has more elements than needed, excess elements are ignored.

Return value

None.

Example

The following example shows basic usage of an AudioContext to create an AnalyserNode, then requestAnimationFrame and <canvas> to collect frequency data repeatedly and draw a "winamp bargraph style" output of the current audio input. For more examples/information, check out our Voice-change-O-matic demo (see app.js lines 128–205 for relevant code).

var audioCtx = new (window.AudioContext || window.webkitAudioContext)();
var analyser = audioCtx.createAnalyser();

  ...

analyser.fftSize = 256;
var bufferLength = analyser.frequencyBinCount;
console.log(bufferLength);
var dataArray = new Uint8Array(bufferLength);

canvasCtx.clearRect(0, 0, WIDTH, HEIGHT);

function draw() {
  drawVisual = requestAnimationFrame(draw);

  analyser.getByteFrequencyData(dataArray);

  canvasCtx.fillStyle = 'rgb(0, 0, 0)';
  canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);

  var barWidth = (WIDTH / bufferLength) * 2.5;
  var barHeight;
  var x = 0;

  for(var i = 0; i < bufferLength; i++) {
    barHeight = dataArray[i];

    canvasCtx.fillStyle = 'rgb(' + (barHeight+100) + ',50,50)';
    canvasCtx.fillRect(x,HEIGHT-barHeight/2,barWidth,barHeight/2);

    x += barWidth + 1;
  }
};

draw();

Parameters

array
The Uint8Array that the frequency domain data will be copied to.

Specifications

Specification
Web Audio API
# dom-analysernode-getbytefrequencydata

Browser compatibility

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See also