E-Mu XL-7 Command Station Owner's Manual page 204

Programming Basics
Dynamic Filters
Another characteristic of a filter is the number of poles it contains. Tradi-
tional synthesizer filters were usually either 2-pole or 4-pole filters. The XL-
7 has selectable 2, 4, and 6-pole low-pass filters. The number of poles in a
filter describes the steepness of its slope. The more poles, the steeper the
filter's slope and the stronger the filtering action. The tone controls on your
home stereo are probably one-pole or two-pole filters. Parametric equalizers
are usually either two-pole or three-pole filters.
In terms of vintage synthesizers, Moog and ARP synthesizers used 4-pole
filters, Oberheim and E-mu synthesizers were famous for their 2-pole filter
sound.
6-pole 4-pole 2-pole
Lowpass Lowpass Lowpass
Frequency
Using a filter, we have a way to control the harmonic content of a sound.
As it turns out, even a simple low-pass filter can simulate the response of
many natural sounds.
For example, when a piano string is struck by its hammer, there are initially
a lot of high frequencies present. If the same note is played softer, there are
fewer high frequencies generated by the string. We can simulate this effect
by routing keyboard velocity to control the low-pass filter. The result is
expressive, natural control over the sound.
If you use an envelope generator to control the cutoff frequency of a filter,
the frequency content can be varied dynamically over the course of the
note. This adds animation to the sound as well as simulates the response of
many natural instruments.
192 E-MU Systems