Magnitude FFT (DSP Blockset)

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

Library

Estimation / Power Spectrum Estimation
Transforms

Description

The Magnitude FFT block computes a nonparametric estimate of the spectrum using the periodogram method. When the Output parameter is set to Magnitude squared, the block output for an input u is equivalent to

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

When the Output parameter is set to Magnitude, the block output for an input u is equivalent to

y = abs(fft(u,nfft))         % 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.

The block does not accept sample-based 1-by-N row vector inputs.

Example

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

Dialog Box

Output: Determines whether the block computes the magnitude FFT (Magnitude) or magnitude-squared FFT (Magnitude squared) of the input. Tunable.
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.

Supported Data Types

Double-precision floating point
Single-precision floating point

To learn how to convert to the above data types in MATLAB and Simulink, see Supported Data Types and How to Convert to Them.

See Also

Burg Method
DSP Blockset

Short-Time FFT
DSP Blockset

Spectrum Scope
DSP Blockset

Yule-Walker Method
DSP Blockset

pwelch
Signal Processing Toolbox

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. Also see a list of all blocks in the Power Spectrum Estimation library and the Transforms library.

LU Solver Matrix 1-Norm