Summary of Overlap-Add FFT Processing

Overlap-add FFT processors provide efficient implementations for FIR filters longer than 100 or so taps on single CPUs. Specifically, we ended up with:

$\displaystyle y = \sum_{m=-\infty}^\infty \hbox{\sc Shift}_{mR} \left( \hbox{\sc DFT}_N^{-1} \left\{ H \cdot \hbox{\sc DFT}_N\left[\hbox{\sc Shift}_{-mR}(x)\cdot w \right]\right\}\right)$ (9.24)

where $ \hbox{\sc Shift}()$ is acyclic in this context. Stated as a procedure, we have the following steps in an overlap-add FFT processor:
(1)
Extract the $ m$ th length $ M$ frame of data at time $ mR$ .
(2)
Shift it to the base time interval $ [0,M-1]$ (or $ [-(M-1)/2,(M-1)/2]$ ).
(3)
Optionally apply a length $ M$ analysis window $ w$ (causal or zero phase, as preferred). For simple LTI filtering, the rectangular window is fine.
(4)
Zero-pad the windowed data out to the FFT size $ N$ (a power of 2), such that $ N\geq M+L-1$ , where $ L$ is the FIR filter length.
(5)
Take the $ N$ -point FFT.
(6)
Apply the filter frequency-response $ H=\hbox{\sc FFT}_N(h)$ as a windowing operation in the frequency domain.
(7)
Take the $ N$ -point inverse FFT.
(8)
Shift the origin of the $ N$ -point result out to sample $ mR$ where it belongs.
(9)
Sum into the output buffer containing the results from prior frames (OLA step).
The condition $ N\geq M+L-1$ is necessary to avoid time aliasing, i.e., to implement acyclic convolution using an FFT; this condition is equivalent to a minimum sampling-rate requirement in the frequency domain.

A second condition is that the analysis window be COLA at the hop size used:

$\displaystyle \sum_m w(n-mR) = 1, \, \forall n\in{\bf Z}.$ (9.25)


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Example of Overlap-Add Convolution