understanding jtransform

Question

I want to do fft of an accelerometer data from android phone.I have chosen jtransform as fft library.Now I need some simple code to understand jtranform to write code to find fft from accelerometer data. Thanks in advance

Answer 1

Since you have the real data, you should pass these values to realForward function as stated here . When you pass the value array to this function it returns the results by overwriting this array with the FFT coefficients.

array = new double[arraySize];
...
DoubleFFT_1D dfft = new DoubleFFT_1D(arraySize);
dfft.realForward(array);

You should also consider the fact that output contains both real and imaginary parts:

if arraySize is even then

 array[2*k] = Re[k], 0 <= k < arraySize/2
 array[2*k+1] = Im[k], 0 < k < arraySize/2
 array[1] = Re[arraySize/2]

if arraySize is odd then

 array[2*k] = Re[k], 0 <= k < (arraySize+1)/2
 array[2*k+1] = Im[k], 0 < k < (arraySize-1)/2
 array[1] = Im[(arraySize-1)/2]