AnalyserNode.fftSize

AnalyserNode 接口的 fftSize 属性的值是一个无符号长整型的值, 表示(信号)样本的窗口大小。当执行快速傅里叶变换(Fast Fourier Transfor (FFT))时,这些(信号)样本被用来获取频域数据。

fftSize 属性的值必须是从32到32768范围内的2的非零幂; 其默认值为2048.

注意: 如果其值不是2的幂, 或者它在指定范围之外, 则抛出异常INDEX_SIZE_ERR.

语法

var audioCtx = new AudioContext();
var analyser = audioCtx.createAnalyser();
analyser.fftSize = 2048;

一个无符号长整型.

例子

下面的例子展示了 AudioContext 创建一个 AnalyserNode, 然后用 requestAnimationFrame 和 <canvas> 去反复收集当前音频的时域数据, 并绘制为一个示波器风格的输出(频谱).

更多的例子/信息, 查看 Voice-change-O-matic 演示 (相关代码在 app.js 在 128行~205行).

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

  ...

analyser.fftSize = 2048;
var bufferLength = analyser.fftSize;
var 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();

      var sliceWidth = WIDTH * 1.0 / bufferLength;
      var x = 0;

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

        var v = dataArray[i] / 128.0;
        var 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();

规范

Specification Status Comment
Web Audio API
fftSize
Recommendation  

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