Moog-music Slim Phatty User Manual download pdf (Page 12)

Page 14

Slim Phatty User’s Manual - The Components

Page 15

Slim Phatty User’s Manual - The Components

B. The Filter Section

Filters are used for adjusting the timbre of an audio signal. Filters modify sound by

attenuating some frequencies while allowing others to pass through unaffected. An

important term to understand regarding lters is “Cutoff Frequency”. This is the

point at which frequencies begin to be rejected. The SP features a lowpass lter,

which behaves as its name indicates: it allows low frequencies to pass and rejects

high frequencies.

Another important aspect of lters is the cutoff slope. The cutoff slope denes

how well the lter rejects signals above the cutoff frequency. The cutoff slope is

measured in decibels per octave (dB/Octave), and is always specied as a multiple

of six. A 6dB/Octave slope–the simplest possible lter design–exhibits a gentle

roll-off with a minimum rejection of frequencies above the cutoff. By comparison, a

12dB/Octave slope is twice as steep, and rejects frequencies above the cutoff twice

as much. An 18dB/Octave slope is steeper still, with a corresponding frequency

rejection. Finally, a 24dB/Octave slope provides the steepest rejection of frequen-

cies above the cutoff point. The classic Moog ‘ladder’ lter is a 24dB/Octave

lowpass lter.

The Moog lter also features a parameter called Resonance. This

parameter adds a resonant peak at the cutoff frequency. When the

resonant peak passes through the overtones of the sound being

ltered, those overtones are reinforced. This gives the lter a charac-

ter that can sound nasal, buzzy, raspy, etc., depending on how it’s used.

When the resonance is turned up past about 3 o’clock on the dial, the

lter begins to self-oscillate at the cutoff frequency, producing a sine

wave tone.

Other lter section controls include Keyboard Amount (KB), Envelope

Generator Amount (EG) and Overload (O.L)

The KB parameter allows you to set the degree to which the lter cutoff frequency tracks the note that

is played. For example, when KB is set to it’s highest level (100%), the lter cutoff will rise by an octave

each time you play an octave higher on the keyboard. This setting allows you to maintain a consistent lter

tone regardless of whether you play low or high on the keyboard. When KB is set to zero, the lter cutoff

remains at the value determined by the analog edit control, regardless of where you play on the keyboard.

This can make the sound less bright as you play higher up on the keyboard. The KB parameter includes the

keyboard GLIDE control signal, which allows the lter cutoff to follow the glide of the notes being played

(when GLIDE is switched ON). Using the KB control, you can adjust the amount of Glide that will affect the

lter cutoff.

The EG parameter allows you to set the degree to which the Filter EG affects the lter cutoff frequency.

The EG parameter is bi-polar, meaning the Filter EG can affect the lter cutoff either in a positive or negative

manner. A positive EG amount will raise the cutoff frequency, while a negative EG amount will lower it.

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Moog-music Slim Phatty User Manual Page 12

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