Moog Music Voyager Old School - Manual (Page 16)

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Voyager OS User’s Manual - The Components
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Voyager OS 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 fi lters 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 fi lter section to preserve the authentic
vibe of the original hardware.
The fi rst 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 fi lters;
some of the most common and most musically useful fi lter types are ‘lowpass’,
‘highpass’, and ‘bandpass’. A lowpass fi lter behaves as its name indicates; it passes all
frequencies below the cutoff frequency and rejects frequencies above the cutoff. A
highpass fi lter does the opposite. It passes all frequencies above the cutoff point and
rejects the frequencies below the cutoff. A bandpass fi lter does a bit of both, since it
is created by combining lowpass and highpass fi lters. In the case of a bandpass fi lter,
the lowpass section defi nes the maximum frequency that will pass through, while the
highpass section defi nes the minimum frequency that will pass through. What’s left
is a band of frequencies that will pass through the fi lters unaffected, hence the name,
bandpass.
Another key fi lter term is the ‘Cutoff Slope’. The cutoff slope determines the amount
of attenuation that occurs above the cutoff frequency. The cutoff slope is specifi ed
in decibels per octave (commonly written as ‘dB/oct’). The electrical design of a fi lter
determines the cutoff slope. You may have heard the term ‘pole’ as it refers to fi lters. A
pole is simply a design aspect of a fi lter, and each pole in a fi lter adds 6dB to the cutoff
slope. This means that a one-pole fi lter has a cutoff slope of 6db/oct, a 2-pole fi lter has
a 12dB/oct cutoff slope, etc. The classic Moog fi lter – the sound that started it all – is a
dB/Oct lowpass fi lter.
The last fi lter term to consider 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 fi lter controls. When the resonant peaks of the lowpass fi lters pass through the overtones of
the sound being fi ltered, those overtones are reinforced. This gives the
fi lter a character that can sound vocal, quacky, or zappy, depending on
how it’s used. When the resonance is turned up past about 8 on the
dial, the fi lter begins to self-oscillate at the cutoff frequency, producing a
sine wave tone. The Keyboard Control Amount control sets how much
the fi lters’ cutoff frequencies track the keyboard note that is played. As
you play higher on the keyboard, the cutoff frequency goes higher, too.