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12.2: Fast Convolution by Overlap-Add and Overlap-Save

Last updated

Mar 22, 2021
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- 12.1: Introduction
- 12.3: Block Processing - a Generalization of Overlap Methods

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Fast Convolution by Overlap-Add

In order to use the FFT to convolve (or filter) a long input sequence $x (n) " role="presentation" style="position:relative;" tabindex="0">$

The savings in arithmetic can be considerable when implementing convolution or performing FIR digital filtering. However, there are two penalties. The use of blocks introduces a delay of one block length. None of the first block of output can be calculated until all of the first block of input is available. This is not a problem for “off line" or “batch" processing but can be serious for real-time processing. The second penalty is the memory required to store and process the blocks. The continuing reduction of memory cost often removes this problem.

The efficiency in terms of number of arithmetic operations per output point increases for large blocks because of the $M\log (M)$ requirements of the FFT. However, the blocks become very large ( $L> > M$ ), much of the input block will be the appended zeros and efficiency is lost. For any particular application, taking the particular filter and FFT algorithm being used and the particular hardware being used, a plot of efficiency vs. block length, $x (n) " role="presentation" style="position:relative;" tabindex="0">$

An alternative approach to the Overlap-Add can be developed by starting with segmenting the output rather than the input. If one considers the calculation of a block of output, it is seen that not only the corresponding input block is needed, but part of the preceding input block also needed. Indeed, one can show that a length $M+L-1$ segment of the input is needed for each output block. So, one saves the last part of the preceding block and concatenates it with the current input block, then convolves that with $x (n) " role="presentation" style="position:relative;" tabindex="0">$

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Fast Convolution by Overlap-Add

LL" role="presentation" style="position:relative;" tabindex="0">Fast Convolution by Overlap-SaveLL" role="presentation" style="position:relative;" tabindex="0">

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