Filter and Low Pass Gate Basics:
There are many different types of filters. The most commonly used filters in synthesizers are
Low Pass, High Pass and Band Pass. In musical terms, a filter removes harmonic content
from the signal making it sound different. A Low Pass filter lets frequencies beLOW the
cut-off frequency pass through while all others are weakened by attenuation. The result is a
duller sound as the cut-off frequency is lowered, and brighter as the cut-off frequency is
raised. A High-Pass filter lets frequencies HIGHer than the cut-off frequency pass through
while all others are weakened. This results in the sound becoming thinner as the Cut-Off
Frequency is increased, and fuller as the Cut-off is lowered. Although these changes start to
occur at the cut-off frequency, it is not a severe operation! There is a transition from amplifi-
cation to complete attenuation of the frequency beyond the cut-off point. This transition is
called a "Slope." The Slope represents the steepness of the frequency response curve at the
filter's cut-off frequency. In the case of filters for musical synthesis applications, slopes of 12
and 24db/ Octave are most common. For reference, the classic Moog filter is 24db/ Octave.
In synthesizer filters there is typically a feedback path that allows the Q-Factor of the circuit
to be adjusted. Increasing the gain of this feedback path results in ringing oscillations at the
cut-off frequency known as Resonance. Resonance is what makes a synthesizer filter growl!
The Low Pass Gate is a specialized type of synthesizer filter developed by Don Buchla
where the amplitude of the signal is not only frequency dependent, but also under voltage
control. It is in essence a Voltage Controlled Filter and Amplifier, a VCFA. Traditionally the
Low Pass Gate has had a very mild filter slope of 6db/ Octave. This mild filtering combined
with the use of a Vactrol based gain cell that turns off completely allows for the control of
both amplitude and frequency domains simultaneously.
The MMG has both Low Pass and High Pass filter circuits. There is a normalization between
the FREQuency control signal and the MODE CV input that allows for a quasi-band-pass
behavior to be programmed as well (with nothing patched to MODE CV IN, set MODE to LP,
and MODE CV Attenuator to full CW).
Like most synthesizer filters the MMG has Resonance that is controlled by the Q-Factor
circuit. The MMG Q control is capable of being more aggressive than the typical resonance
control and will distort at higher settings.
The MMG inplements the same Vactrol based gain cell used in the original Low Pass Gate
circuits, and thus there is a range of operation where the amplitude is both frequency depen-
dent and under voltage control. However, the MMG has a steeper filter slope of 12db/
Octave, so it will not sound the same as the typical Low Pass Gate (such as the Make Noise
Optomix).
(12 pages)
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