What is the MATLAB function for FILTER in direct-form I?

I need of the code of this function, if it exists.

I just know about the function 'filter', but this function was implemented in direct-form II transposed and I need of the 'filter' that was implemented in direct-form I.

Thanks.

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/signal/ref/dfilt.df1.html

edit(which('dfilt.df1'))

Will show you the exact code that is used.