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    Displaying a graphic with audio samples

    The following example shows how to take samples from an audio stream and display them behind an image (in this case, the Mozilla logo), giving the impression that the image is built from the samples.

    preview.png

      <!DOCTYPE html>
    <html>
      <head>
        <title>JavaScript Spectrum Example</title>
      </head>
      <body>
        <audio id="audio-element"
               src="revolve.ogg"
               controls="true"
               style="width: 512px;">
        </audio>
        <div><canvas id="fft" width="512" height="200"></canvas></div>
        <img id="mozlogo" style="display:none" src="mozilla2.png"></img>
        <script>
          var canvas = document.getElementById('fft'),
              ctx = canvas.getContext('2d'),
              channels,
              rate,
              frameBufferLength,
              fft;
    
          function loadedMetadata() {
            channels          = audio.mozChannels;
            rate              = audio.mozSampleRate;
            frameBufferLength = audio.mozFrameBufferLength;
             
            fft = new FFT(frameBufferLength / channels, rate);
          }
    
          function audioAvailable(event) {
            var fb = event.frameBuffer,
                t  = event.time, /* unused, but it's there */
                signal = new Float32Array(fb.length / channels),
                magnitude;
    
            for (var i = 0, fbl = frameBufferLength / 2; i < fbl; i++ ) {
              // Assuming interlaced stereo channels,
              // need to split and merge into a stero-mix mono signal
              signal[i] = (fb[2*i] + fb[2*i+1]) / 2;
            }
    
            // Clear the canvas before drawing spectrum
            ctx.fillStyle = "rgb(0,0,0)"; 
            ctx.fillRect (0,0, canvas.width, canvas.height);
            ctx.fillStyle = "rgb(255,255,255)"; 
    
            for (var i = 0; i < signal.length; i++ ) {
              // multiply spectrum by a zoom value
              magnitude = signal[i] * 1000;
    
              // Draw rectangle bars for each frequency bin
              ctx.fillRect(i * 4, canvas.height, 3, -magnitude);
            }
            
            ctx.drawImage(document.getElementById('mozlogo'),0,0, canvas.width, canvas.height);
    
          }
    
          var audio = document.getElementById('audio-element');
          audio.addEventListener('MozAudioAvailable', audioAvailable, false);
          audio.addEventListener('loadedmetadata', loadedMetadata, false);
    
        </script>
      </body>
    </html>
    
    

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    preview.png
    11302 bytes 2010-10-11 10:20:08 sebmozilla

    Document Tags and Contributors

    Contributors to this page: jswisher, sebmozilla
    Last updated by: jswisher,