how to use fir1 function?

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nadav potasman 2017-1-15

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https://ww2.mathworks.cn/matlabcentral/answers/320483-how-to-use-fir1-function

评论： Star Strider 2017-1-15

In fir1 one argument is Wn. I can't understand how its possible to create filter without specify the real frequency? For example, if I try to run low-pass fir filter with Wn=0.5 on two signals, one with Fs=1000 and one with Fs=10,000 how the filter can "know" what frequency is Wn=0.5 to perfom the low-pass filtering on each signal? thank you

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Answer 1

Star Strider 2017-1-15

4
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https://ww2.mathworks.cn/matlabcentral/answers/320483-how-to-use-fir1-function#answer_250654

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The ‘Wn’ argument expresses the normalised frequencies for the passbands and stopbands. That means that for a discrete filter, these are already on the interval (0,pi), where pi is defined as the Nyquist frequency (half the sampling frequency). So that information is inherent in the ‘Wn’ argument, even if not explicitly stated.

If you use a window function (such as I did here in a code snippet from another Answer):

fcuts = [0.08  0.19];                                           % Frequency Vector
mags =   [1 0];                                                 % Magnitude (Defines Passbands & Stopbands)
devs = [0.01  0.05];                                            % Allowable Deviations
[n,Wn,beta,ftype] = kaiserord(fcuts,mags,devs,Fs);              % Kaiser Window
n = n + rem(n,2);
hh = fir1(n,Wn,ftype,kaiser(n+1,beta),'scale');                 % Design FIR Lowpass Filter

the window function incorporates the sampling information (in ‘Fs’, the last argument to kaiserord here) and then presents fir1 with the normalised passbands and stopbands.