# Overlap-Add FFT Filter (Obsolete)

Implement overlap-add method of frequency-domain filtering

## Library

Filtering / Filter Implementations

`dsparch4`

## Description

**Note**

The Overlap-Add FFT Filter block has been replaced with the Frequency-Domain FIR Filter block. Existing instances of the Overlap-Add FFT Filter block continue to run.

The Overlap-Add FFT Filter block uses an FFT to implement the *overlap-add method*, a technique that combines
successive frequency-domain filtered sections of an input sequence.

The block accepts vector or matrix inputs, and treats each column of the input
as an individual channel. The block unbuffers the input data into row
vectors such that the length of the output vector is equal to the number of
channels in the input. The data output rate of the block is
*M* times faster than its data input rate, where
*M* is the length of the columns in the input
(frame-size).

The block breaks the scalar input sequence `u`

, of length
`nu`

, into length-`L`

nonoverlapping data sections,

which it linearly convolves with the filter's FIR coefficients,

$$H(z)=B(z)={b}_{1}+{b}_{2}{z}^{-1}+\dots +{b}_{n+1}{z}^{-n}$$

The numerator coefficients for H(*z*) are specified as a
vector by the **FIR coefficients** parameter. The
coefficient vector, `b = [b(1) b(2) ... b(n+1)]`

, can be
generated by one of the filter design functions in the Signal Processing Toolbox™ product, such as `fir1`

. All filter states
are internally initialized to zero.

When 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 you specify `nfft`

in 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`

.

**Note**

For more information on latency and the Simulink^{®} software tasking modes, see Excess Algorithmic Delay (Tasking Latency)
and Time-Based Scheduling and Code Generation (Simulink Coder).

## Parameters

**FFT size**The size of the FFT, which should be a power-of-two value greater than the length of the specified FIR filter.

**FIR coefficients**The filter numerator coefficients.

**Output**The complexity of the output;

`Real`

or`Complex`

. When the input signal or the filter coefficients are complex, this should be set to`Complex`

.

## 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.

## Supported Data Types

Double-precision floating point

Single-precision floating point

## See Also

Overlap-Save FFT Filter | DSP System Toolbox™ product |

**Introduced before R2006a**