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

### vinicius lanziotti (view profile)

on 14 Jun 2019
Latest activity Answered by Walter Roberson

on 14 Jun 2019

### Walter Roberson (view profile)

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

### Walter Roberson (view profile)

on 14 Jun 2019
What is the difference between this question and the other one you asked about these filters?
vinicius lanziotti

### vinicius lanziotti (view profile)

on 14 Jun 2019
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.

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