I'm trying to do bandpass filtering of a EEG signal samples at 250Hz and benchmarking the following 4 methods of FIR filtering for different filter orders. The length of the signal is 15000 samples.
- Time Domain Approach (convolution)
- Direct frequency domain approach
For follow the procedure listed in the following Wiki pages for overlap-add and overlap-save http://en.wikipedia.org/wiki/Overlap%E2%80%93add_method http://en.wikipedia.org/wiki/Overlap%E2%80%93save_method
For frequency domain approach I do FFT for the entire length of the signal, multiply with the frequency response of filter H and then IFFT the result.
For overlap and add, I chose M(overlap) to be 1+length_of_response and L to be twice of M.
Although one would expect Overlap-add to outperform Direct Frequency domain approach, this is not what I see in my benchmark results(see attached figure).
Please help me out in understanding where I might be going wrong.