How to implement a generic 'filter' function using direct form I?

I need a code for the filter function in direct form I, like as I implemented a filter function in direct form II transposed below:

function[y] = filtro(a,b,x)
% Generic filter function in direct form II transposed

    N = size(b,2) - 1;% filter order 
    y = zeros(size(x)); % Initializes y with zeros

    %% FOR THE CASE FIR
    if not(size(a,2) == size(b,2))
        a(1) = 1;
        a = zeros(size(b));
    end
    %% Recursive implementation of equations to differences
    aux = zeros(1,N); 
    for r = 1 :size(x,2)      
        y(r) = b(1)*x(r) + aux(1);
        for n = 2 : N   
            aux(n-1) = b(n)*x(r) + aux(n);
            aux(n-1) = aux(n-1)- a(n)*y(r);
        end
        aux(N) = b(N+1)*x(r) - a(N+1)*y(r);
    end
end % END OF FUNCTION

Thanks!

[Walter Roberson]

What is the difference between this question and the other one you asked about these filters?

[vinicius lanziotti]

The difference is that if I use the command "edit(which('dfilt.df1'))" the function has only 2 inputs (num, den), and use others parameters already defined in Matlab, like (dfilt).

% Using edit(which('dfilt.df1'))
function Hd = df1(num,den)
Hd = dfilt.df1;
Hd.ncoeffs = [1 1];
Hd.FilterStructure = 'Direct-Form I'; 
Hd.tapIndex = [0 0];
if nargin>=1
  Hd.Numerator = num;
end
if nargin>=2
  Hd.Denominator = den;
end

But I need of 3 inputs, like the filter function. But this function was made using direct-form II transposed, and I want the direct-form I from function "filter". And no one parameters already defined in Matlab.

Answer 1

[Walter Roberson]

filterdf1 = @(a,b,x) filter( dfilt.df1(a,b), x);

This is a three-argument filter function that uses direct type 1 form.