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PODiscrete Fourier transform
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  1. PODiscrete Fourier transform
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    copied!<p>I am currently trying to write some fourier transform algorithm. I started with a simple DFT algorithm as described in the mathematical definition:</p> <pre><code>public class DFT { public static Complex[] Transform(Complex[] input) { int N = input.Length; Complex[] output = new Complex[N]; double arg = -2.0 * Math.PI / (double)N; for (int n = 0; n &lt; N; n++) { output[n] = new Complex(); for (int k = 0; k &lt; N; k++) output[n] += input[k] * Complex.Polar(1, arg * (double)n * (double)k); } return output; } } </code></pre> <p>So I tested this algorithm with the following code:</p> <pre><code> private int samplingFrequency = 120; private int numberValues = 240; private void doCalc(object sender, EventArgs e) { Complex[] input = new Complex[numberValues]; Complex[] output = new Complex[numberValues]; double t = 0; double y = 0; for (int i = 0; i &lt; numberValues; i++) { t = (double)i / (double)samplingFrequency; y = Math.Sin(2 * Math.PI * t); input[i] = new Complex(y, 0); } output = DFT.Transform(input); printFunc(input); printAbs(output); } </code></pre> <p>The transformation works fine, but only if numberValues is a multiple number of the samplingFrequency (in this case: 120, 240, 360,...). Thats my result for 240 values:</p> <p><a href="http://s1.directupload.net/images/110928/n3m8hqg6.jpg">http://s1.directupload.net/images/110928/n3m8hqg6.jpg</a></p> <p>The transformation just worked fine.</p> <p>If i am trying to calculate 280 values I get this result:</p> <p><a href="http://s7.directupload.net/images/110928/qizoiqbt.jpg">http://s7.directupload.net/images/110928/qizoiqbt.jpg</a></p> <p>Why I am getting a incorrect result if I change the number of my calculated values? I am not sure if my problem here is a problem with my code or a misunderstanding of the mathematical definition of the DFT. In either way, can anybody help me with my problem? Thanks.</p>
 

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