test_iirFilterNodeGetFrequencyResponse.html

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<head>

  <title>Test IIRFilterNode GetFrequencyResponse</title>

  <script src="/tests/SimpleTest/SimpleTest.js"></script>

  <script type="text/javascript" src="webaudio.js"></script>

  <script type="text/javascript" src="biquad-filters.js"></script>

  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />

</head>

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<pre id="test">

<script class="testbody" type="text/javascript">

SimpleTest.waitForExplicitFinish();

addLoadEvent(function() {

  // Modified from WebKit/LayoutTests/webaudio/iirfilter-getFrequencyResponse.html

  var sampleRate = 48000;

  var testDurationSec = 0.01;

  // Compute a set of linearly spaced frequencies.

  function createFrequencies(nFrequencies, sampleRate)

    var frequencies = new Float32Array(nFrequencies);

    var nyquist = sampleRate / 2;

    var freqDelta = nyquist / nFrequencies;

    for (var k = 0; k < nFrequencies; ++k) {

        frequencies[k] = k * freqDelta;

    return frequencies;

  // Number of frequency samples to take.

  var numberOfFrequencies = 1000;

  var context = new OfflineAudioContext(1, testDurationSec * sampleRate, sampleRate);

  var frequencies = createFrequencies(numberOfFrequencies, context.sampleRate);

  // 1-Pole IIR Filter

  var iir = context.createIIRFilter([1], [1, -0.9]);

  var iirMag = new Float32Array(numberOfFrequencies);

  var iirPhase = new Float32Array(numberOfFrequencies);

  var trueMag = new Float32Array(numberOfFrequencies);

  var truePhase = new Float32Array(numberOfFrequencies);

  // The IIR filter is

  //   H(z) = 1/(1 - 0.9*z^(-1)).

//

  // The frequency response is

  //   H(exp(j*w)) = 1/(1 - 0.9*exp(-j*w)).

//

  // Thus, the magnitude is

  //   |H(exp(j*w))| = 1/sqrt(1.81-1.8*cos(w)).

//

  // The phase is

  //   arg(H(exp(j*w)) = atan(0.9*sin(w)/(.9*cos(w)-1))

  var frequencyScale = Math.PI / (sampleRate / 2);

  for (var k = 0; k < frequencies.length; ++k) {

    var omega = frequencyScale * frequencies[k];

    trueMag[k] = 1/Math.sqrt(1.81-1.8*Math.cos(omega));

    truePhase[k] = Math.atan(0.9 * Math.sin(omega) / (0.9 * Math.cos(omega) - 1));

  iir.getFrequencyResponse(frequencies, iirMag, iirPhase);

  compareChannels(iirMag, trueMag);

  compareChannels(iirPhase, truePhase);

  // Compare IIR and Biquad Filter

  // Create an IIR filter equivalent to the biquad filter. Compute the frequency response for both and verify that they are the same.

  var biquad = context.createBiquadFilter();

  var coef = createFilter(biquad.type,

                          biquad.frequency.value / (context.sampleRate / 2),

                          biquad.Q.value,

                          biquad.gain.value);

  var iir = context.createIIRFilter([coef.b0, coef.b1, coef.b2], [1, coef.a1, coef.a2]);

  var biquadMag = new Float32Array(numberOfFrequencies);

  var biquadPhase = new Float32Array(numberOfFrequencies);

  var iirMag = new Float32Array(numberOfFrequencies);

  var iirPhase = new Float32Array(numberOfFrequencies);

  biquad.getFrequencyResponse(frequencies, biquadMag, biquadPhase);

  iir.getFrequencyResponse(frequencies, iirMag, iirPhase);

  compareChannels(iirMag, biquadMag);

  compareChannels(iirPhase, biquadPhase);

  SimpleTest.finish();

});

</script>

</pre>

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