How to write my own filter function with initial condition similar to Filter.m?

I have read the documents on how MATLAB implements Filter(b,a,y,z0). To fully understand the concept and learn more about Signal Processing, I tried to implement it by myself, but the answer is different from MATLAB filter.m

Could anyone let me know what I am missingg?! :-(

    function y =myfilter(num, den, x, zi)

    N = length(x);

    % assuming order of Num and Den are the same.
    ord = (length(num)-1)/2;

    y = zeros(N,1);
    for i=1:N
        c = 0;
        for j=1:2*ord
            if i-j < 1
                c = c+1;              
                xo(j) = zi(c);
                yo(j) = 0;
            else
                xo(j) = x(i-j);
                yo(j) = y(i-j);
            end
        end
        y(i) = num(1)*x(i) + sum(num(2:end).*xo') - sum(den(2:end).*yo');
    end

Answer 1

[Hossein]

Ok, I searched online and found out that I was wrong on the concept :) Here' the correct formulation of 1D filtering with delay initial values: SciPY Filtering Documentation, and it outputs the same result as MATLAB filter() does.

[Mariasole Laureti]

Hi, I have the same problem, can I ask you if you can show me your code? I would be very grateful. Thank you very much

Answer 2

[ssp]

Hi! Greetings!

I was also trying to write my filter function. However y=filter(b,a,x) seems easy to implement, but I am stuck at implementing y=filter(b,a,x,zi). What would be the new sequence of the following sequence after applying initial condition zi? y(n) = b(1)*x(n) + b(2)*x(n-1) + ... + b(nb+1)*x(n-nb) - a(2)*y(n-1) - ... - a(na+1)*y(n-na)

Thanks! SSP

[Jan]

See this full implementation for filter(b,a,X,z): https://www.mathworks.com/matlabcentral/answers/9900-use-filter-constants-to-hard-code-filter#answer_13623

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