[sdiy] 'State-Variable' vs. 'Multi-Mode' filters - differences?

[David G Dixon]

> With regards to filter types - are the terms 'State-Variable' 
> and 'Multi'Mode' essentially interchangeable or do they refer 
> to two different filter types?
> 
> If they refer to two different filter types - what are the 
> differences?

A state-variable filter is a certain configuration of two filter stages and
a summing amp which can be tapped to give LP, BP, and HP responses from
three different points in the circuit.  It is one form of multimode filter,
but one of the few which has been specifically designed to be one.

One can derive multiple filter modes from just about any filter if one knows
how.  For example, one can tap the outputs of a 24-dB cascaded-stage filter
and sum them in various ratios to derive just about any filter mode
imaginable.  I recently took a slightly different approach with my dual
12-dB Korgasmatron filter, and used multiple input points (into the first
state, the second stage, the resonance feedback loop, and the output amp) to
derive six different modes.  The beauty of this scheme is that different
input signals can have different filter responses through the same filter at
the same time.  This approach is possible with other filters as well.