What I want to do is to reconstruct frequencies out of a line of numbers. What I have is an array which I fill with values generated by f(x)= 4 * sin(0,5 * x) + 100 for test purposes. My goal is to get the frequency generated by the sinus-function with just the values in my array.
As far as I understand this is exactly what the fourier transformation does. So I used the code of the Discrete Fourier Transformation Tutorial from here http://docs.opencv.org/2.4/doc/tutorials/core/discrete_fourier_transform/discrete_fourier_transform.html and used the 1D-array as input (instead of a 2D picture). However I didn´t use image padding (line 13 - 16) and the rearrangement of the quadrants (!, line 33 - 52) from the tutorial to keep my code a little bit more simple. For example this is what I get:
Input: 80; 81; 83; 83; 83; 82; 80; 78; 76; 76; 76; 77; 78; 80; 82; 83; 83; 83; 81; 79; 77; 76; 76; 76; 77
Output: 1; 0.01; 0.49; 0.06; 0.08; 0.08; 0.13; 0.07; -3.72529e-09; 0.08; 0.11; 0.09; 0.10; 0.10; 0.09; 0.11; 0.08; 1.15484e-07; 0.07; 0.13; 0.08; 0.08; 0.06; 0.49; 0.01
The first value always seems to be 1 (regardless of what f(x) I use). As far as I know each number in the output represents a specific frequency and the value of the number determines how strong this frequency in the input was.
But how do I know which number in the frequency spectrum output belongs to which frequency? And why do I have often more than one high value in my output (i just use one sinus-function as input, so I exspected just one number in my output to have a high value)?
In other words: What to I have to do to get the searched frequency out of my fourier transformation output?