67 lines
2.0 KiB
JavaScript
67 lines
2.0 KiB
JavaScript
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/*===========================================================================*\
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* Fast Fourier Transform Frequency/Magnitude passes
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*
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* (c) Vail Systems. Joshua Jung and Ben Bryan. 2015
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*
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* This code is not designed to be highly optimized but as an educational
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* tool to understand the Fast Fourier Transform.
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\*===========================================================================*/
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//-------------------------------------------------
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// The following code assumes a complex number is
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// an array: [real, imaginary]
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//-------------------------------------------------
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var complex = require("./complex");
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//-------------------------------------------------
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// By Eulers Formula:
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//
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// e^(i*x) = cos(x) + i*sin(x)
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//
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// and in DFT:
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//
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// x = -2*PI*(k/N)
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//-------------------------------------------------
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var mapExponent = {},
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exponent = function (k, N) {
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var x = -2 * Math.PI * (k / N);
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mapExponent[N] = mapExponent[N] || {};
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mapExponent[N][k] = mapExponent[N][k] || [Math.cos(x), Math.sin(x)]; // [Real, Imaginary]
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return mapExponent[N][k];
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};
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//-------------------------------------------------
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// Calculate FFT Magnitude for complex numbers.
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//-------------------------------------------------
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var fftMag = function (fftBins) {
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var ret = fftBins.map(complex.magnitude);
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return ret.slice(0, ret.length / 2);
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};
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//-------------------------------------------------
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// Calculate Frequency Bins
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//
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// Returns an array of the frequencies (in hertz) of
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// each FFT bin provided, assuming the sampleRate is
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// samples taken per second.
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//-------------------------------------------------
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var fftFreq = function (fftBins, sampleRate) {
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var stepFreq = sampleRate / fftBins.length;
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var ret = fftBins.slice(0, fftBins.length / 2);
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return ret.map(function (__, ix) {
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return ix * stepFreq;
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});
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};
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//-------------------------------------------------
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// Exports
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//-------------------------------------------------
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module.exports = {
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fftMag: fftMag,
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fftFreq: fftFreq,
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exponent: exponent,
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};
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