MATLAB SIGNAL PROCESSING BLOCKSET 7 Guia do Utilizador descarregar pdf (Página 492)

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101

Overlap-Save FFT Filter

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The circular convolution of each section is computed by multiplying the FFTs

of the input section and filter coefficients, and computing the inverse FFT of

the product.

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

where nfft is specified by the FFT size parameter as a power-of-two value

greater (typically much greater) than

n+1. Values for FFT size that are not

powers of two are rounded upwards to the nearest power-of-two value to obtain

nfft.

The first

n points of the circular convolution are invalid and are discarded. The

Overlap-Save FFT Filter block outputs the remaining

nfft-n points, which are

equivalent to the linear convolution.

Latency

In single-tasking operation, the Overlap-Save FFT Filter block has a latency of

nfft-n+1 samples. The first nfft-n+1 consecutive outputs from the block are

zero; the first filtered input value appears at the output as sample

nfft-n+2.

In multitasking operation, the Overlap-Save FFT Filter block has a latency of

2*(nfft-n+1) samples. The first 2*(nfft-n+1) consecutive outputs from the

block are zero; the first filtered input value appears at the output as sample

2*(nfft-n)+3.

See “Excess Algorithmic Delay (Tasking Latency)” on page 3-91 and “The

Simulation Parameters Dialog Box” in the Simulink documentation for more

information about block rates and Simulink’s tasking modes.

Dialog Box

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