Basic fir1 question-what is N?

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Louise Wilson 2019-7-23

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回答： Star Strider 2019-7-23

HI everyone, I am working through code from a colleague (retired) and slowly interpreting what everything means as I learn matlab.

One of the lines is:

    hp = fir1(128, 0.10, 'high'); %create hp filt to be applied later
                                  %designs N'th order highpass filter
                                  %0.10 (Wn) is cut-off frequency
                                  
                                  

...which I understand is creating a filter that I will later multiply my data array with. I am wondering where the 128 comes from? I see the filter has N+1 values (129) but wondering where the reasoning is for using 128 in first place. Or how do you decide from the start what to put N as?

Thanks!

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Answer 1

Star Strider 2019-7-23

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https://ww2.mathworks.cn/matlabcentral/answers/472934-basic-fir1-question-what-is-n#answer_384286

The order (the ‘n’ parameter in fir1) for FIR filters is actually somewhat arbitrary. It has to be long enough to provide a decent filter response. (I use kaiserord to design FIR filters, since it arrives at the optimal value for this and several other parameters on its own.)