User`s guide

Magnitude FFT
5-277
5Magnitude FFT
Purpose 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. 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.
Example The dspsacomp demo compares the periodogram method with several other
spectral estimation methods.