Short-Time FFT (DSP Blockset)

Compute a nonparametric estimate of the spectrum using the short-time, fast Fourier transform (ST-FFT) method.

Library

Power Spectrum Estimation, in Estimation

Description

The Short-Time FFT block computes a nonparametric estimate of the spectrum. The block averages the squared magnitude of the FFT computed over windowed sections of the input, and normalizes the spectral average by the square of the sum of the window samples.

The block accepts an M-by-N frame matrix input, where each of the N frames in the matrix contains M sequential time samples from an independent signal. The Number of channels parameter specifies the number of independent channels, N, in the matrix. A value of 1 for Number of channels specifies a single channel (vector) input.

The block computes a separate estimate for each of the N independent channels in the input, generating an N_fft-by-N matrix output, where N_fft is specified as a power of 2 by the FFT Size parameter. A value of -1 for FFT size instructs the block to use the input frame size as the FFT size. Otherwise, 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 Number of spectral averages specifies the number of spectra to average. Setting this parameter to 1 effectively disables averaging.

The Window type, Stopband ripple, Beta, and Window sampling parameters all apply to the specification of the window function; see the reference page for the Window Function block for more details on these four parameters.

Dialog Box

FFT length: The number of data points, N_fft, on which to perform the FFT. If N_fft exceeds the input frame size, the frame is zero-padded as needed.
Number of spectral averages: The number of spectra to average; setting this parameter to 1 effectively disables averaging.
Window type: The type of window to apply. (See the Window Function block reference.)
Stopband ripple: The level (dB) of stopband ripple, R_s, for the Chebyshev window. Disabled for other Window type selections.
Beta: The parameter for the Kaiser window. Disabled for other Window type selections. Increasing Beta widens the mainlobe and decreases the amplitude of the window sidelobes in the window's frequency magnitude response.
Window sampling: The window sampling, symmetric or periodic.
Number of channels: The number of channels (columns) in the input, N.

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.