Table of Contents
Advertisement
A Notch Filter is just the opposite of a bandpass filter and is used to eliminate a narrow
band of frequencies.
Another control found on traditional filters is called Q or resonance. A lowpass filter
with a high Q would emphasize the frequencies around the cutoff frequency. The
following chart shows how different amounts of Q affect the low pass filter response. In
terms of sound, frequencies around the cutoff tend to "ring" with high Q settings. If the
filter is slowly swept back and forth, with a high Q, various overtones are "picked out"
of the sound and amplified as the resonant peak sweeps over them. Bells and gongs are
real world examples of sounds which have a high Q.
Low Q
Med Q
Frequency
Another characteristic of a filter is the number of poles it contains. Traditional synthe-
sizer filters were usually either 2-pole or 4-pole filters. The Proteus has a selectable 2, 4
or 6-pole low pass filter. 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
Lowpass
Frequency
Using a filter, we now have a way to control the harmonic content of a sampled 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 will be fewer of the 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.
E-MU Systems
High Q
4-pole
2-pole
Lowpass
Lowpass
9 - Synthesizer Basics
Dynamic Filters
155
Advertisement
Table of Contents
