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# fftfilt() - Signal Processing

### Syntax

### Example

### Output / Return Value

### Limitations

### Alternatives / See Also

### Reference

fftfilt filters data using the efficient FFT-based method of overlap-add, a frequency domain filtering technique that works only for FIR filters.y = fftfilt(b,x) filters the data in vector x with the filter described by coefficient vector b. It returns the data vector y. The operation performed by fftfilt is described in the time domain by the difference equation:y(n)=b(1)x(n)+b(2)x(n−1)+⋯+b(nb+1)x(n−nb)An equivalent representation is the Z-transform or frequency domain description:Y(z)=(b(1)+b(2)z−1+⋯+b(nb+1)z−nb)X(z)By default, fftfilt chooses an FFT length and a data block length that guarantee efficient execution time.If x is a matrix, fftfilt filters its columns. If b is a matrix, fftfilt applies the filter in each column of b to the signal vector x. If b and x are both matrices with the same number of columns, the ith column of b is used to filter the ith column of x.y = fftfilt(b,x,n) uses n to determine the length of the FFT. See Algorithms for information.y = fftfilt(d,x) filters the data in vector x with a digitalFilter object, d. Use designfilt to generate d based on frequency-response specifications.y = fftfilt(d,x,n) uses n to determine the length of the FFT.y = fftfilt(gpuArrayb,gpuArrayX,n) filters the data in the gpuArray object, gpuArrayX, with the FIR filter coefficients in the gpuArray, gpuArrayb. See Establish Arrays on a GPU for details on gpuArray objects. Using fftfilt with gpuArray objects requires Parallel Computing Toolbox™ software and a CUDA-enabled NVIDIA GPU with compute capability 1.3 or above. See http://www.mathworks.com/products/parallel-computing/requirements.html for details. The filtered data, y, is a gpuArray object. See Overlap-Add Filtering on the GPU for example of overlap-add filtering on the GPU.fftfilt works for both real and complex inputs.Comparison to filter functionWhen the input signal is relatively large, it is advantageous to use fftfilt instead of filter, which performs N multiplications for each sample in x, where N is the filter length. fftfilt performs 2 FFT operations — the FFT of the signal block of length L plus the inverse FT of the product of the FFTs — at the cost of½Llog2Lwhere L is the block length. It then performs L point-wise multiplications for a total cost ofL + Llog2L = L(1 + log2L)multiplications. The cost ratio is thereforeL(1+log2L) / (NL) = (1 + log2L)/Nwhich is approximately log2L / N.Therefore, fftfilt becomes advantageous when log2Lis less than N.

y = fftfilt(b,x)y = fftfilt(b,x,n)y = fftfilt(d,x)y = fftfilt(d,x,n)y = fftfilt(gpuArrayb,gpuArrayX,n)

y = ifft(fft(x(i:i+L-1),nfft).*fft(b,nfft));