How to calculate passband and stopband coefficients for signal filtering?

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Maria Amr 2020-11-3

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https://ww2.mathworks.cn/matlabcentral/answers/635084-how-to-calculate-passband-and-stopband-coefficients-for-signal-filtering

评论： Star Strider 2021-1-14

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!

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Star Strider 2020-11-3

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https://ww2.mathworks.cn/matlabcentral/answers/635084-how-to-calculate-passband-and-stopband-coefficients-for-signal-filtering#answer_532819

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.

Then do whatever signal processing you want on the remaining signal. Use a highpass or bandpass filter to eliminate baseline offset and baseline drift. Using elliptic filters to do these are also straightforward to design in MATLAB.