in de EOS 4.0.pdf staat het op pagina 273
Parametric Filters
A more complex type of filter is called a parametric filter or Swept EQ. A
parametric filter allows control over three basic parameters of the filter. The
three parameters are: Frequency, Bandwidth, and Gain. The Frequency
parameter allows you to select a range of frequencies to be boosted or cut,
the Bandwidth parameter allows you to select the width of the range, and
the Gain parameter either boosts or cuts the frequencies within the selected
band by a specified amount. Frequencies not included in the selected band
are left unaltered. This is different from a band pass filter which attenuates
(reduces) frequencies outside the selected band.
Another parameter sometimes used on a parametric filter is Shelving.
Shelving simply widens the passband so that it extends to the limit of the
upper or lower frequency range.
The parametric filter is quite flexible. Any range of frequencies can be either
amplified or attenuated. Several parametric sections are often cascaded in
order to create complex filter response curves.
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 on the following page.
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 simultaneously. 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 interpo-
lation, 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 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.
