Overlap-Save FFT Filter (Obsolete)
Implement overlap-save method of frequency-domain filtering
Library
Filtering / Filter Implementations
dsparch4
Description
Note
The Overlap-Save FFT Filter block has been replaced with the Frequency-Domain FIR Filter block. Existing instances of the Overlap-Save FFT Filter block continue to run.
The Overlap-Save FFT Filter block uses an FFT to implement the overlap-save 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).
Overlapping sections of input u
are circularly convolved
with the FIR filter coefficients
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 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 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 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
.
Note
For more information on latency and the Simulink® environment 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
orComplex
. When the input signal or the filter coefficients are complex, this should be set toComplex
.
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-Add FFT Filter | DSP System Toolbox |
Version History
Introduced before R2006a