[sdiy] 2164 Korgasmatron VCF

[cheater cheater]

Gotcha. Very well explained, thanks. I still don't understand why the
extra "s" effects in this exact sort of change, but I don't understand
transfer functions well enough yet :)

D.

On Wed, Sep 1, 2010 at 20:04, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> Oh.. so you used HP and BP in parallel?
>
> No!  That transfer function is simply what you get from the MS-20 filter in
> HP mode, because of the way it is organized.
>
>> I thought you wanted HP and LP for notch response.
>
> You do.  The "notch" mode of the MS-20 is not really a notch, but just a
> flat response with a 6db dip at the cutoff.  Again, this is a manifestation
> of the transfer function.
>
> Here (again) is the transfer function for an MS-20 filter in HP mode:
>
>     s^2 + s
> ----------------
> s^2 + (2-G)s + 1
>
> Here is the transfer function for an MS-20 filter in LP mode:
>
>       -1
> ----------------
> s^2 + (2-G)s + 1
>
> Add them together and you get the "notch" response:
>
>  s^2 + s - 1
> ----------------
> s^2 + (2-G)s + 1
>
> It's that "s" in the numerator which prevents this from being a true notch.
>
>