[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

> 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.