Re: Filter Implementation

[Doug Stewart]

On Mon, Jul 3, 2017 at 8:05 PM, shall689 <address@hidden> wrote:

Thanks Doug,


Your work is getting me going in the right direction.  I don't know why I
was ever used excel in the first place.

You used a(1) (=1) for the y(n-1) term.   I was thinking that a(2)
(=-0.985033202259162) should be used for that term, i.e. y(n) = b(1)*x(n) +
b(2)*x(n-1) + a(2)*y(n-1).  Is that correct?

If a(2) is used for the y(n-1) term, the plot look like my excel plot, so
there is something I am misunderstanding about implementing the filter
equation.

Also, the filtered output has an offset if a(1) is used with y(n-1).

Stephen

you have

y                   b0 *Z +b1

----      =    -----------------------

x                   a0*Z  - a1

y                   b0 *Z +b1                  z^-1

----      =    -----------------------    *    -------

x                   a0*Z  - a1                  z^-1

y                   b0  +b1* z^-1

----      =    -----------------------

x                   a0  - a1 *z^-1

z^-1  means 1 time ago

y*(a0  - a1 *z^-1)= x*( b0  +b1* z^-1)

a0*y=  x*( b0  +b1* z^-1)   + y*a1*z^-1

since octave cant index at 0 we rename them and get my program.

--

DAS