Loren on the Art of MATLAB

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Pedagogical Tool - dftmtx

With that context, I asked how they teach Fourier transforms, especially once they go from the analog domain to the digital. It turns out that most of them were unaware of some of the tools in Signal Processing Toolbox, such as dftmtx.

Here's the help.

h = help('dftmtx');
disp(h(1:744))

 DFTMTX Discrete Fourier transform matrix.
    DFTMTX(N) is the N-by-N complex matrix of values around
    the unit-circle whose inner product with a column vector
    of length N yields the discrete Fourier transform of the
    vector.  If X is a column vector of length N, then
    DFTMTX(N)*X yields the same result as FFT(X); however, 
    FFT(X) is more efficient.
 
    The inverse discrete Fourier transform matrix is
    CONJ(DFTMTX(N))/N.
 
    An interesting example is 
 
      D = dftmtx(4)
 
    which returns
 
      D = [1   1   1   1
           1  -i  -1   i     
           1  -1   1  -1
           1   i  -1  -i]
 
    which illustrates why no multiplications are necessary for
    4-point DFTs.
 
    See also FFT and IFFT.

The help also mentions fft, the discrete Fourier transform, computed with a fast Fourier transform (fft) algorithm. With the information given for dftmtx, let's compare results with those of fft.

More on dftmtx

On further inspection, you may notice that there are comments in the file outlining the calculation. As an aside, the computation is actually done using the function fft in MATLAB.

Here's a brief description of the non-|fft| algorithm. Basically calculate frequencies on the unit circle and multiply by sqrt(-1) (w). Multiply these by integral steps for the number of points in the transform (w*x). And then calculate all the sin and cos terms using exp.

Explicitly, the calculation is:

 f = 2*pi/n;                 % Angular increment.
 w = (0:f:2*pi-f/2).' * i;   % Column.
 x = 0:n-1;                  % Row.
 D = exp(-w*x);              % Exponentiation of outer product.

Compare fft and Discrete Transform Matrix Methods

n = 1024;
X = eye(n);
Xf = fft(X);       % fft result
f = 2*pi/n;
w = (0:f:2*pi-f/2).' * 1i;
x = 0:n-1;
Xd = exp(-w*x);     % matrix representation
norm(Xf-Xd)

ans =
  1.6769e-011

Do You Have Good Links for Fourier Transform Teaching Material?

If you know of web sites with good material for teaching discrete Fourier transforms, would you please post them here? Thanks.

Get the MATLAB code

Published with MATLAB® 7.7