Overlap-Save FFT Filter (DSP Blockset)

Implement the overlap-save method of frequency-domain filtering.

Library

Filter Realizations, in Filtering

Description

The Overlap-Save FFT Filter block uses an FFT to implement the overlap-save method, a technique that combines successive frequency-domain filtered sections of an input sequence. The overlapping input sections are circularly convolved with the FIR filter coefficients,

which are specified as a vector by the FIR coefficients parameter:

B = [b(1) b(2) ... b(n+1)]

The block's overlap-save operation is equivalent to

y = ifft(fft(u(i:i+(L-1)),nfft) .* fft(B,nfft))

where u is the input and y is the output.

The circular convolution of each section is computed by multiplying the FFT of the input and the filter coefficients, and computing the inverse FFT of the product. For filter length nb (the number of FIR coefficients) and FFT size N_fft, the first nb-1 points of the circular convolution are invalid, and are discarded. The remaining (N_fft-nb+1) points, which are equivalent to the linear convolution, are output in serial fashion by an Unbuffer block.

Dialog Box

FFT size: The size of the FFT, which must be greater than the length of the specified FIR filter.
FIR coefficients: The filter numerator coefficients.

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.