Moog Music Minimoog Voyager XL - Manual (Page 19)

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Voyager XL User’s Manual - The Components
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Voyager XL User’s Manual - The Components
C – The Filter Section
Filters are used to adjust the tone color of an audio signal. Filters modify sounds by rejecting some
frequencies while allowing others to pass through. To understand the operation of filters and how they
process sound, there are a few important terms to know.
In the original Minimoog, the Resonance control was called ‘Emphasis’. Many of the
current Minimoog emulations (both hardware and software) use the term ‘Emphasis’
instead of ‘Resonance’ in the filter section to preserve the authentic vibe of the
original hardware.
The first term is ‘Cutoff Frequency’. The cutoff frequency is the point at which an audio
signal’s frequencies begin to be rejected. Then there are the different types of filters;
some of the most common and most musically useful filter types are ‘lowpass’, ‘highpass’,
and ‘bandpass’. A lowpass filter behaves as its name indicates; it passes all frequencies
below the cutoff frequency and rejects frequencies above the cutoff. A highpass filter
does the opposite. It passes all frequencies above the cutoff point and rejects the
frequencies below the cutoff. A bandpass filter does a bit of both, since it is created
by combining lowpass and highpass filters. In the case of a bandpass filter, the lowpass
section defines the maximum frequency that will pass through, while the highpass
section defines the minimum frequency that will pass through. What’s left is a band of
frequencies that will pass through the filters unaffected, hence the name, bandpass.
Another key term is the ‘Cutoff Slope’. The cutoff slope determines the amount of
attenuation that occurs above the cutoff frequency. The cutoff slope is specified in
decibels per octave (commonly written as ‘dB/oct’). The electrical design of a filter
determines the cutoff slope. You may have heard the term ‘pole’ as it refers to filters.
A pole is simply a design aspect of a filter, and each pole in a filter typically adds 6dB
to the cutoff slope. This means that a one-pole filter has a cutoff slope of 6db/oct, a
2-pole filter has a 12dB/oct cutoff slope, etc. The classic Moog filter – the sound that
started it all – is a 4-pole, 24dB/oct lowpass filter.
Another important filter term is ‘Resonance’. Resonance refers to a peak that appears at the cutoff
frequency. In synthesizers, this resonant peak is usually an adjustable parameter (called ‘ Resonance’ ) that
is part of the filter controls. High resonance values emphasize the overtones that fall within the frequency
range of the resonant peak, giving the processed sound a character that
can be described as vocal, quacky or zappy, depending on the source
sound. When the resonance is turned up past about 8 on the dial, the
filter begins to self-oscillate at the cutoff frequency, producing a sine
wave tone. The Keyboard Control Amount control sets how much the
filters’ cutoff frequencies track the keyboard note that is played. As you
play higher on the keyboard, the cutoff frequency goes higher, too.