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Discrete Functions

We now consider discrete functions, as opposed to the continuous functions of the previous lectures. Discrete functions are defined at discrete positions:

f(x) is defined on integer values of x between 0 and N1.

Discrete Fourier Transform (DFT)

The DFT is the discrete analogue of the continuous FT, with a sum replacing the integral:

F(u)=N1x=0f(x)ei2πxNu

The 2πxN term varies in [0,2π) as x varies in [0,N).

When u=1, the sinusoid corresponding to ei2πxNu has a single cycle in [0,N). When u=2, the sinusoid has two cycles in [0,N).

So u corresponds to the wavenumber in the continuous FT. That is, u is the number of cycles in the range [0,N).

Inverse DFT

The inverse DFT is also analagous to its continuous counterpart:

f(x)=1NN1u=0F(u)ei2πuNx


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