How to calculate passband and stopband coefficients for signal filtering?
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Maria Amr on 3 Nov 2020
Commented: Star Strider on 14 Jan 2021
Hello Star Strider,
I have a signal, that is a well log Acoustic Impedance log and I need to apply a Antialias filter in coarser samoling that means a bandpass filter.
Sample frequency is 6562 per kilometer (28000 sample/4.27km). I do need to identify cutt off frequencies correctly. I applied your passband code into my dataset but my proffessors says: apply an anti-alias filter that has a flat spectrum over the band of interest. Would you please direct me how I can identify cutoff frequencies? Appreciated!
Star Strider on 3 Nov 2020
If you are concerned with an anti-aliasing filter for the time-domain signal, you need to go back and design a hardware Bessel (besself) lowpass anti-aliasing filter with a cutoff frequency that is equal to the Nyquist frequency (half the sampling frequency), then do the sampling. Frequencies higher than the Nyquist frequency are already aliased in your signal and now cannot be removed.
The frequency of ‘6562 per kilometer’ is obviously a spatial frequency, and I assume normal signal processing approaches would apply, so use the lowpass function, or design your own elliptic filter (ellip and related functions), each with a cutoff frequency of 3218 samples/km, as the anti-aliasing filter. Use filtfilt to do the actual filtering.
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Maria Amr on 4 Nov 2020
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