What type of filter is thisr?

Hello,

I have found following piece of code:

N            = length(SomeArray);
Mean_len     = 4;
Offset_calc  = ones(length(I_filt),1);

i=1;
while (i < N)
    if (i < Mean_len/2+1)
        Offset_calc(i,1) = SomeArray(i,1)-mean(SomeArray(1:Mean_len));
    elseif(i < N-Mean_len/2)
        Offset_calc(i,1) = SomeArray(i,1)-mean(SomeArray(i-Mean_len/2:i+Mean_len/2));
    else
        Offset_calc(i,1) = SomeArray(i,1)-mean(SomeArray(N-Mean_len:N));
    end
    i=i+1;
end

It's simple to understand. From some array a mean value is subtracted. The length of the mean value is set . It's not an Avarage Filter since you divide by the mean value in it, but here you subtract it. What does it do? Where do you use this type of filter?.... if it's even a filter.

Regards, Den

Answer 1

[Jan]

Here the moving average around each point is subtracted. Therefore it is a high pass filter: high frequencies, which change the signal inside the Mean_len window rapidly are kept, slow changes and the DC offset are removed. It is equivalent to:

B = ones(Mean_len, 1) / Mean_len;
Y = X - filter(B, 1, X);

[Jan]

[Moved from asnwer to comment section:]

DenLi wrote: first of all, thank you for your answer!

Hmm now that you say it, it does make sense that high frequencies are kept, but ...still wasn't obvious to me at all. So since it's a high-pass filter, how do I figure out the cut-off frequency? It obviously depends on the length of the Moving Average ... but what is the relationship?

Thanks in advance, Den

[Jan]

@Den: I don't know. But it seems to be a good question for an internet search engine: "cut off frequency moving mean filter". I find:

• https://www.dsprelated.com/showthread/comp.dsp/155807-1.php
• http://ptolemy.eecs.berkeley.edu/eecs20/week12/freqResponseRA.html
• https://www.mathworks.com/examples/matlab-dsp-system/mw/dsp-ex04244460-frequency-response-of-moving-average-filter-and-fir-filter