DSP - Fast Fourier Transform


In earlier DFT methods, we have seen that the computational part is too long. We want to reduce that. This can be done through FFT or fast Fourier transform. So, we can say FFT is nothing but computation of discrete Fourier transform in an algorithmic format, where the computational part will be reduced.

The main advantage of having FFT is that through it, we can design the FIR filters. Mathematically, the FFT can be written as follows;

$$x[K] = \displaystyle\sum\limits_{n = 0}^{N-1}x[n]W_N^{nk}$$

Let us take an example to understand it better. We have considered eight points named from $x_0\quad to\quad x_7$. We will choose the even terms in one group and the odd terms in the other. Diagrammatic view of the above said has been shown below −

Fast Fourier Transform

Here, points x0, x2, x4 and x6 have been grouped into one category and similarly, points x1, x3, x5 and x7 has been put into another category. Now, we can further make them in a group of two and can proceed with the computation. Now, let us see how these breaking into further two is helping in computation.

$x[k] = \displaystyle\sum\limits_{r = 0}^{\frac{N}{2}-1}x[2r]W_N^{2rk}+\displaystyle\sum\limits_{r = 0}^{\frac{N}{2}-1}x[2r+1]W_N^{(2r+1)k}$

$= \sum_{r = 0}^{\frac{N}{2}-1}x[2r]W_{N/2}^{rk}+\sum_{r = 0}^{\frac{N}{2}-1}x[2r+1]W_{N/2}^{rk}\times W_N^k$

$= G[k]+H[k]\times W_N^k$

Initially, we took an eight-point sequence, but later we broke that one into two parts G[k] and H[k]. G[k] stands for the even part whereas H[k] stands for the odd part. If we want to realize it through a diagram, then it can be shown as below −

Eight Point H[k]G[k]1 Eight Point H[k]G[k]2

From the above figure, we can see that

$W_8^4 = -1$

$W_8^5 = -W_8^1$

$W_8^6 = -W_8^2$

$W_8^7 = -W_8^3$

Similarly, the final values can be written as follows −

$G[0]-H[0] = x[4]$

$G[1]-W_8^1H[1] = x[5]$

$G[2]-W_8^2H[2] = x[6]$

$G[1]-W_8^3H[3] = x[7]$

The above one is a periodic series. The disadvantage of this system is that K cannot be broken beyond 4 point. Now Let us break down the above into further. We will get the structures something like this



Consider the sequence x[n]={ 2,1,-1,-3,0,1,2,1}. Calculate the FFT.

Solution − The given sequence is x[n]={ 2,1,-1,-3,0,1,2,1}

Arrange the terms as shown below;

FFT Sequence