DSP Blockset

Magnitude FFT

Compute a nonparametric estimate of the spectrum using the periodogram method.

Library

Estimation / Power Spectrum Estimation

Description

The Magnitude FFT block computes a nonparametric estimate of the spectrum using the periodogram method. For input u, this is equivalent to

y = abs(fft(u,nfft)).^2      % Equivalent MATLAB code

Both an M-by-N frame-based matrix input and an M-by-N sample-based matrix input are treated as M sequential time samples from N independent channels. The block computes a separate estimate for each of the N independent channels and generates an N_fft-by-N matrix output. When Inherit FFT length from input dimensions is selected, N_fft is specified by the frame size of the input, which must be a power of 2. When Inherit FFT length from input dimensions is not selected, N_fft is specified as a power of 2 by the FFT length parameter, and the block zero pads or truncates the input to N_fft before computing the FFT.

Each column of the output matrix contains the estimate of the corresponding input column's power spectral density at N_fft equally spaced frequency points in the range [0,F_s), where F_s is the signal's sample frequency. The output is always sample-based.

Example

The dspsacomp demo compares the periodogram method with several other spectral estimation methods.

Dialog Box

Inherit FFT length from input dimensions

When selected, uses the input frame size as the number of data points, N_fft, on which to perform the FFT.

FFT size

The number of data points on which to perform the FFT, N_fft. If N_fft exceeds the input frame size, the frame is zero-padded as needed. This parameter is enabled when Inherit FFT length from input dimensions is not selected.

References

Oppenheim, A. V. and R. W. Schafer. Discrete-Time Signal Processing. Englewood Cliffs, NJ: Prentice Hall, 1989.

Proakis, J. and D. Manolakis. Digital Signal Processing. 3rd ed. Englewood Cliffs, NJ: Prentice-Hall, 1996.

See Also

Burg Method	DSP Blockset
Short-Time FFT	DSP Blockset
Spectrum Scope	DSP Blockset
Yule-Walker Method	DSP Blockset
pwelch	Signal Processing Toolbox

See Power Spectrum Estimation for related information.

LU Solver

Matrix 1-Norm