How Filters Are Designed - Alesis ANDROMEDA A6 Reference Manual

16-voice real analog synthesizer
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Chapter 5: Oscillators and Filters
H
F
A
D
OW
ILTERS
RE
ESIGNED
As important as the amount of audio frequencies that pass through the filter is the
type of filtering that is employed. The basic filtering process depends, therefore, on
the mode of the filter – whether it's low pass, high pass, band pass or notch filtering.
Let's take a moment to describe these modes.
Filter Modes
A filter determines what harmonics pass through it by its mode. The four filter
describe how harmonics pass (the keyword here is pass)
modes of the
FILTER 1
through the filter:
Low Pass
As its name implies, low frequency harmonics (including the fundamental wave) are
passed through the filter. This means that high frequencies are filtered out.
FILTER 1
provides low-pass filtering in addition to high-pass, band-pass and notch filtering.
is low-pass only.
FILTER 2
CUTOFF
FREQUENCY
FREQUENCY
HARMONIC
SPECTRUM
1 kHz
2 kHz
5 kHz
10 kHz
20 kHz
High Pass
High pass filtering is the opposite of low pass: high frequency harmonics are passed
through the filter. This means that low frequencies, including the fundamental
wave, are filtered out (if the frequency of the filter is above the fundamental). In
essence, a high pass filter just passes some of the harmonics.
CUTOFF
FREQUENCY
FREQUENCY
HARMONIC
SPECTRUM
1 kHz
2 kHz
5 kHz
10 kHz
20 kHz
A
A6 R
M
117
NDROMEDA
EFERENCE
ANUAL

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