Doepfer A-121-12dB User Manual
doepfer
System A - 100
1. Introduction
VCF 2
A-121
Audio Level
FCV 2
QCV 2
Audio
FCV 1
FCV 2
QCV 1
QCV 2
A-121
In
MULTIMODE
FILTER
Freq.
Res.
Low Band High Notch
Module A-121 (VCF 2) is a voltage-controlled multimode filter with a cut-off slope of -12 dB / octave.
Four simultaneous outputs are available, each with
different characteristics: low-pass, band-pass, high-
pass and notch (or band reject).
The cut-off frequency determines the point at which
the respective filter effects appear. The frequency can
be adjusted manually, or by voltage control (Filter
modulation, for instance by an LFO or ADSR). Two
CV inputs are available, whose control voltages are
summed.
Resonance (Emphasis or Q ) can be adjusted
manually, or by voltage control (voltage-controlled
resonance / VCQ), right up to self-oscillation, in which
case it will behave like a sine wave oscillator.
1
A-121
VCF 2
System A - 100
doepfer
2. VCF 2 - Overview
MULTIMODE FILTER
Audio In
➊
➋
➌
➍
➎
Notch
➏
High
➐
Band
➑
Low
➒
FCV 1
FCV 2
QCV 1
QCV 2
VCF 2
0
0
0
0
0
10
Freq.
10
FCV 2
10
10
QCV 2
10
Audio
Level
Res.
➀
➁
➂
➃
➄
Controls:
1 Audio Level : Input signal attenuator
2 Freq. : Cut-off frequency control
3 FCV 2 : Attenuator for filter CV §
4 Res. : Resonance control
5 QCV 2 : Attenuator for resonance CV %
In / Outputs:
! Audio In : Audio input to the filter
" FCV 1 : Cut-off frequency CV input
§ FCV 2 : ditto, level controlled by 3
$ QCV 1 : Resonance CV input
% QCV 2 : ditto, level controlled by 5
& Low : Low-pass filter output
/ Band : Band-pass filter output
( High : High-pass filter output
) Notch : Notch filter output
2
doepfer
System A - 100
VCF 2
A-121
3. Basics
Low Pass
The most common type of filter in analogue sound
production is the low-pass, which filters out the higher
parts of the sound spectrum, and lets the lower fre-
High Pass
f
c
C
f
c
de-
Freq.
quencies pass unchanged. Cut-off frequency f
termines the frequency at which this occurs (see Fig.
1).
Out
Out
Low Pass
f
c
Band Pass Notch
f
c
Out
Freq.
Out
Freq. Freq.
High Pass
The high-pass filter is a precise mirror-image of the
low-pass filter: while it lets frequencies that are higher
than the cut-off frequency f
through, it attenuates
C
frequencies below the cut-off point (see Fig. 1).
Band Pass
In a band-pass filter, both ends of the frequency
spectrum are attenuated (see Fig. 1), and the cut-off
frequency f
becomes the mid frequency. It gives
C
you the ability to highlight a particular frequency band.
Notch
A notch filter is the opposite of a band-pass filter,
letting through the upper and lower end of the frequency spectrum, but rejecting a band in the middle
(see Fig. 1). If the mid-frequency f
LFO, the result sounds very similar to phasing.
is modulated by an
C
Fig. 1: Typical response curves of the four filters.
3
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