AnalyserNode: fftSize property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2021.

The fftSize property of the AnalyserNode interface is an unsigned long value and represents the window size in samples that is used when performing a Fast Fourier Transform (FFT) to get frequency domain data.

Value

An unsigned integer, representing the window size of the FFT, given in number of samples. A higher value will result in more details in the frequency domain but fewer details in the amplitude domain.

Must be a power of 2 between 2^5 and 2^15, so one of: 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, and 32768. Defaults to 2048.

Exceptions

IndexSizeError DOMException

Thrown if the value set is not a power of 2, or is outside the allowed range.

Examples

The following example shows basic usage of an AudioContext to create an AnalyserNode, then requestAnimationFrame and <canvas> to collect time domain data repeatedly and draw an "oscilloscope style" output of the current audio input. For more complete applied examples/information, check out our Voice-change-O-matic demo (see app.js lines 108–193 for relevant code).

js
const audioCtx = new AudioContext();
const analyser = audioCtx.createAnalyser();

// …

analyser.fftSize = 2048;
const bufferLength = analyser.frequencyBinCount;
const dataArray = new Uint8Array(bufferLength);
analyser.getByteTimeDomainData(dataArray);

// draw an oscilloscope of the current audio source

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

  analyser.getByteTimeDomainData(dataArray);

  canvasCtx.fillStyle = "rgb(200 200 200)";
  canvasCtx.fillRect(0, 0, WIDTH, HEIGHT);

  canvasCtx.lineWidth = 2;
  canvasCtx.strokeStyle = "rgb(0 0 0)";

  canvasCtx.beginPath();

  const sliceWidth = (WIDTH * 1.0) / bufferLength;
  let x = 0;

  for (let i = 0; i < bufferLength; i++) {
    const v = dataArray[i] / 128.0;
    const y = (v * HEIGHT) / 2;

    if (i === 0) {
      canvasCtx.moveTo(x, y);
    } else {
      canvasCtx.lineTo(x, y);
    }

    x += sliceWidth;
  }

  canvasCtx.lineTo(canvas.width, canvas.height / 2);
  canvasCtx.stroke();
}

draw();

Specifications

Specification
Web Audio API
# dom-analysernode-fftsize

Browser compatibility

BCD tables only load in the browser

See also