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dftmtx

Discrete Fourier transform matrix

Syntax

``a = dftmtx(n)``

Description

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````a = dftmtx(n)` returns an `n`-by-`n` complex discrete Fourier transform matrix.```

Examples

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In practice, it is more efficient to compute the discrete Fourier transform with the FFT than with the DFT matrix. The FFT also uses less memory. The two procedures give the same result.

```x = 1:256; y1 = fft(x); n = length(x); y2 = x*dftmtx(n); norm(y1-y2)```
```ans = 9.3125e-12 ```

Input Arguments

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Discrete Fourier transform length, specified as an integer.

Data Types: `single` | `double`

Output Arguments

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Discrete Fourier transform matrix, returned as a matrix.

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Discrete Fourier Transform Matrix

A discrete Fourier transform matrix is a complex matrix whose matrix product with a vector computes the discrete Fourier transform of the vector. `dftmtx` takes the FFT of the identity matrix to generate the transform matrix.

For a column vector `x`,

`y = dftmtx(n)*x`
is the same as `y = fft(x,n)`. The inverse discrete Fourier transform matrix is
```ainv = conj(dftmtx(n))/n ```