User`s guide

Overlap-Add FFT Filter
5-332
If either the filter coefficients or the inputs to the block are complex, the
Output parameter should be set to Complex. Otherwise, the default Output
setting,
Real, instructs the block to take only the real part of the solution.
The block’s overlap-add operation is equivalent to
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 block overlaps successive output sections by
n points and sums them.
The first
L samples of each summation are output in sequence. The block
chooses the parameter
L based on the filter order and the FFT size.
L = nfft - n
Latency
In single-tasking operation, the Overlap-Add 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-Add 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.
LL+n
2L 2L+n
3L 3L+n
. . .