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# I need help figuring out this FFT method

Kuroyuki
1#
Kuroyuki Published in 2018-02-13 21:49:45Z
 I am trying to get the frequencies from a sound wave using fft. I have found an FFT method, but I am having trouble understanding what it wants me to put in, how to get information out of it, and how to use what I get out of it. I have tried entering the value at index of the buffer as y, with the buffer position as x, but I get weird results. int m seems to be something to do with samples, but I have no clue how to choose what I want to input there. But it effects how much is stored into the x and y double. Luckily dir, is rather easy to understand, it's basically acting like a bool for whether you FFT forward or not. public static void FFT(short dir, int m, double[] x, double[] y) { int n, i, i1, j, k, i2, l, l1, l2; double c1, c2, tx, ty, t1, t2, u1, u2, z; // Calculate the number of points n = 1; for (i = 0; i < m; i++) n *= 2; // Do the bit reversal i2 = n >> 1; j = 0; for (i = 0; i < n - 1; i++) { if (i < j) { tx = x[i]; ty = y[i]; x[i] = x[j]; y[i] = y[j]; x[j] = tx; y[j] = ty; } k = i2; while (k <= j) { j -= k; k >>= 1; } j += k; } // Compute the FFT c1 = -1.0; c2 = 0.0; l2 = 1; for (l = 0; l < m; l++) { l1 = l2; l2 <<= 1; u1 = 1.0; u2 = 0.0; for (j = 0; j < l1; j++) { for (i = j; i < n; i += l2) { i1 = i + l1; t1 = u1 * x[i1] - u2 * y[i1]; t2 = u1 * y[i1] + u2 * x[i1]; x[i1] = x[i] - t1; y[i1] = y[i] - t2; x[i] += t1; y[i] += t2; } z = u1 * c1 - u2 * c2; u2 = u1 * c2 + u2 * c1; u1 = z; } c2 = Math.Sqrt((1.0 - c1) / 2.0); if (dir == 1) c2 = -c2; c1 = Math.Sqrt((1.0 + c1) / 2.0); } // Scaling for forward transform if (dir == 1) { for (i = 0; i < n; i++) { x[i] /= n; y[i] /= n; } } 
Mike Mat
2#
Mike Mat Reply to 2018-02-14 17:44:12Z
 My guess is: m: Log2 of the data length x: real part of input data y: imaginary part of input data (array of 0s if the signal consists of only real numbers)  I have not tested so please let me know if it works.
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