1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
|
<!DOCTYPE HTML>
<html>
<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>
<body>
<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>
</body>
</html>
|
