This is very simplified, but the result of computing the FFT of a signal is an array of numbers . Each element of the array (often referred to as a "bin") corresponds to a small range of frequencies in the input signal. The value in each element allows you to determine the amplitude of the signal corresponding to that frequency.

Subtracting successive voltages only tells you the rate at which the voltage is changing. That doesn't tell you what the frequency is. If it did there would be no need for an FFT.

i am in kind of hurry

You're really going to have to slow down if you want to understand what the FFT does. To start with, to understand the FFT you have to understand the mathematical concept of complex numbers. If you don't know that, you've got a long way to go.

But if you're in a hurry, the executive summary is that what you're doing isn't an FFT.

Pete

Yeah, since i am in a hurry i am not going to use FFT right now i guess... unless my "algorithm" has some kind of fatal weakness....

About complex numbers and math... It is not like i dont know what are complex numbers and stuff... i have been studying in university for 3 ears already(actually this project is my bachelor's work and we are not given much time).

Sorry if i am being persistent by saying same thing all over again, i just think that u did not really understand what i wrote, since my language and explanations are pretty messy.

As i said before substracting is not the keypoint of this algorithm.

I get voltage signal from audio output(example values 252, 50, 100). And now what i do is look in which range of values it goes. For example if i have 5 columns i make ranges like: 0-49, 50-100, 100-150, 150-200, 200-infinity. I have columns array with 5 elements. So if i get 135 i add to columns[2]++. So basically it seems that i do the same thing as it was written in wikipedia? I do the sampling. Then i create an array which tells me how frequently(in the time of sampling) i get the values into those ranges.

Sorry if u did understand it before.

If i am wrong then i do not really understand what it means to have X as frequency. I understand that frequency is basically how many times in a time something happens? As i understand then those Hz are basically saying how often did i get value Y(voltage). If it was not frequent then its somewhere near 0 and if it was frequent it goes somewhere to the right. If that is correct then i do get that, i get how frequently i get values in ranges predefined.