The Z-Plane Filter - E-Mu ProteusX Operation Manual

9 - Synthesizer Basics
Dynamic Filters

The Z-Plane Filter

A Z-plane filter is a filter which can change its function over time. In a Z-plane filter, we
start with two complex filter types and interpolate between them using a single
parameter. Refer to the diagram below.
Morph
B Filter
A Filter
Morph
Frequency
The Z-plane filter has the unique ability to change its function over time.
Filters A and B represent two different complex filters or "frames". By changing a single
parameter, the Morph, many complex filter parameters can now be changed simulta-
neously. Following along the Morph axis you can see that the filter response smoothly
interpolates between the two filters. This is the essence of the Z-plane filter. Through the
use of interpolation, many complex parameters are condensed down into one
manageable entity.
Consider, as an example, the human vocal tract, which is a type of complex filter or
resonator. There are dozens of different muscles controlling the shape of the vocal tract.
When speaking, however, we don't think of the muscles, we just remember how it feels
to form the vowels. A vowel is really a configuration of many muscles, but we consider it
a single object. In changing from one vowel to another, we don't need to consider the
frequencies of the resonant peaks! You remember the shape of your mouth for each
sound and interpolate between them. This is similar to what the Z-plane filter does.
This Z-plane filter sweep can be controlled by an envelope generator, an LFO,
modulation wheels or pedals, keyboard velocity, key pressure, etc. In fact, any of the
modulation sources can control the Z-plane filter.
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