E-Mu Planet Phatt Operation Manual page 52

PROGRAMMING BASICS
••• The initial filter Fc and
all Fc modulators ADD
algebraically to determine
the actual Fc. If you are not
getting sound, adjust the
initial Fc or reduce the
amount of modulation.
Careful adjustment of all the
filter parameters is the secret
to getting great sounds.
Most of the instruments on Planet Phatt are complex waves contain-
ing many sine waves of various amplitudes and frequencies. A filter is
a device which allows us to remove certain components of a sound
depending on its frequency. For example, a Low Pass Filter, one of
the Z-plane filter types in Planet Phatt, lets the low frequencies pass
and removes only the high frequencies.
100
80
60
40
20
The point at which the frequencies begin to be cut is called the Cutoff
Frequency (or Fc for short). A filter that let only the high frequencies
pass would be called a High Pass filter. Using a filter, we now have a
way to control the harmonic content of a sampled sound. As it turns
out, a 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 the velocity of the
keyboard to control the amount of high frequencies that the low pass
filter lets through. The result is expressive, natural control over the
sound.
The auxiliary envelope generator is commonly used to control the
cutoff frequency of the Z-plane filter. This allows the frequency
content to be varied dynamically over the course of the note.
Dynamic filtering coupled with all the different instruments available,
makes for almost endless possibilities in the final sound. Any modula-
tion source can be used to modulate the filter.
44 Planet Phatt Operation Manual
Cutoff Frequency
Output of Filter
40 80 160 360 720 1440 2880
Frequency
...