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4.11 The Filter Module 39
The cutoff frequency and resonance of the filters can be modulated with different modulation
sources. The modulation sources include the keyboard pitch signal (Kbd) and the output of the
Filter Envenvelope generator (Env) and LFO modules. Modulation signals with a positive value
will increase the cutoff frequency and Q-factor of the filters while a negative value will decrease
them. Note that the filter parameters can further be modulated by the velocity signal from the
keyboard through the use of the velocity modulation parameter of the Filter Env modules.
The modulation knob associated with the cutoff frequency can be adjusted to its center position
by clicking on the green LED located above the knob. In this position, the cutoff frequency will
exactly follow the pitch of the note played on the keyboard in the case of the Kbd modulation, it
will vary exactly one octave higher and lower in the case of the LFO modulation and will vary one
octave higher or lower depending on the position of the inv switch with the Env modulation. Now
let’s have a closer look at the different filter types available.
4.11.1 Resonant Low-Pass Filter
Amplitude (dB)
Frequency (Hz)
Q=10
Q=4
Q=2
Q=1
Cutoff
Frequency
0dB
−3dB
Slope (dB/Oct)
Figure 19: Frequency response of the low-pass filter.
A low-pass filter is used to remove the higher spectral components of the signal while leaving
the lower components unchanged. The frequency at which attenuation begins to take effect is
called the cutoff frequency. In a resonant filter, frequencies located around the cutoff frequency
can also be emphasized by an amount called the quality factor or Q-factor of the filter as illustrated
in Figure 19. The higher the Q-factor, the louder and sharper the response of the filter around
the cut-off frequency. When the Q-factor is set to 1 (Q knob fully turned to the left), there is no
emphasis around the cutoff frequency and the attenuation is -3dB at the cutoff frequency. The
attenuation for frequencies located above the cut-off frequency depends on the order of the filter
which is determined by the Order menu, a slope of -12dB/Oct corresponding to a second order
filter and a slope of -24dB/Oct to a fourth order filter.
39


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