MATLAB: Does the fft automatically use a dft algorithm

dftfft

Hello everybody, Matlab has only the fft command. The fft algorithm is based on a signal of power of 2. If your signal length differs, you can perfomr zero-padding or a cut-off. Well, what is not clear for me: if the signal does not fit the power of 2 condition, does matlab then automatically perform a dft or does it automatically zero-padding/truncating? thanks for your help! Peter

Best Answer

Hi Peter, MATLAB has multiple FFT algorithms. The transform is the discrete Fourier transform (DFT). FFT algorithms are just implementations of the DFT that reduce the computational complexity. You don't want to implement the DFT by a brute force matrix-vector product.

All these algorithms, the classic radix-2 FFT that you refer to, the chirp Z-transform evaluated on the unit circle contour, the Goerzel algorithm are ways of obtaining the DFT.

In particular, the FFTW library implemented in both MATLAB and SIMULINK supports both power of two and non-power of two signal lengths. Please see:

http://www.fftw.org/

So in short: There is no single FFT algorithm. FFT algorithms are implementations of the mathematical transform, the DFT.

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MATLAB: Is it possible to perform FFT on a non-power of 2 length signal in Simulink using DSP System Toolbox

This enhancement has been incorporated in Release 2011b (R2011b). For previous product releases, read below for any possible workarounds:

The ability to perform FFT on a non-power of 2 length signal using a Simulink block is not available in DSP System Toolbox (or Signal Processing Blockset).

To work around this issue, you can use an Embedded MATLAB function block and the eml.extrinsic function to call MATLAB’s FFT function. This will enable you to perform FFT computation on non-power of 2 length signals in Simulink. Below is an example of this procedure for performing an FFT on some signal x.

eml.extrinsic('fft');
y = complex(zeros(size(x),class(x)));
y = fft(x);

The first line tells the Embedded MATLAB compiler to call the FFT function from base MATLAB. The initialization of y just tells the Embedded MATLAB compiler how to dissect the mxArray that MATLAB will return so that it can be stored in an ordinary C array.

However, please note that this approach is not compatible with code generation using Real-Time Workshop or Simulink HDL Coder.

MATLAB: Code for ploting fourier transform spectrum

spectrogram()

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MATLAB: Is it possible to perform FFT on a non-power of 2 length signal in Simulink using DSP System Toolbox

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