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doepfer
System A - 100
VCF 4 A-123
1. Introduction
Level
FCV 2
QCV
Audio
In
FCV 1
FCV 2
QCV
Audio
Out
A-123
VCF 4
Frequency
Resonance
Module A-123 (VCF 4) is a voltage-controlled high-
pass filter
sound spectrum, and lets higher frequencies pass
through.
The
filtering takes effect. You can control this manually, or
by voltage control (
an LFO). Two CV inputs are available.
The cut-off slope is -24 dB/octave. The circuitry uses
a Curtis CEM 3320 chip.
Voltage controlled resonance: on the VCF 4, resonance can be controlled not just manually, but by
voltages as well, right up to self-oscillation. In this
case, the filter behaves like a sine wave oscillator.
, which filters out the lower parts of the
cut-off frequency
determines the point at which
filter modulation
, for instance by
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A-123
VCF 4
System A - 100
doepfer
2. VCF 4 - Overview
A-123
24 dB High Pass
Audio In
0
FCV 1
0
FCV 2
0
QCV
0
Audio Out
0
VCF 4
Lev.
10
Frq.
10
FCV
10
QCV
10
Res.
10
➀
➁
➂
➃
➄
Controls:
Lev. : Attenuator for audio input
1
2 Frq. : Cut-off frequency control
: Attenuator for filter CV at input
FCV
3
QCV : Attenuator for resonance CV at input
4
$
: Control for setting the filter’s reso-
Res.
5
nance (emphasis)
!
§
In / Outputs:
Audio In : Input to the filter
!
" FCV 1 : Input for voltage control of the filter
cut-off frequency (1 V /octave)
§ FCV 2 : ditto, level controlled by 3
: Input for voltage control of the filter’s
QCV
$
resonance; level controlled by
% Audio Out : Output from the filter
4
2
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doepfer
System A - 100
VCF 4 A-123
3. Controls
1 Lev.
Use this attenuator to control the amount of signal
entering the filter input $.
H
2 Freq.
With this control you adjust the Cut-Off Frequency fc,
below which the filter attenuates all frequencies.
At zero, the filter is fully open. The more you turn the
filter up, the more the low frequencies are filtered (see
Fig. 1): the sound becomes thinner and less solid, until
at 10 the filter is completely shut, and blocks off all
frequencies below 15kHz. Frequencies above 15kHz
are still let through, but are generally outside normal
adult hearing range.
3 FCV
For voltage control or modulation of the cut-off frequency using CV input § (see Fig. 1), use attenuator
to control the level of voltage control.
3
If the filter’s output sounds distorted, turn this
control down, unless you deliberately want
the sound as a special effect.
CV
OutOut
f
c
Fig. 1: White noise put through a high pass filter
f
c
Freq.Freq.
4 QCV
Attenuator 4 gives you control over the level of voltage control
Res.
5
With this control you adjust the filter’s resonance (or
‘emphasis’) - the parameter which emphasises the
frequencies around the cut-off point f
strengthens or emphasises the band of frequencies
around the filter’s cut-off point.
applied to resonance.
(see Fig. 2). It
C
3
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A-123
VCF 4
System A - 100
doepfer
At close to maximum resonance, the filter starts to
self-oscillate, and behaves like a sine wave oscilla-
tor. Thanks to this effect, you can use the filter as an
independent tone source.
Resonanz
0 db
f
c
Fig. 2: How resonance affects the behaviour of a
high pass filter.
Frequenz
4. In / Outputs
! Audio In
This is the filter’s audio input socket, where you
patch in the output from any sound source.
" FCV 1
Socket FCV 1 is a voltage control input for the filter.
It works on the 1V / octave rule, like the VCOs.
If you connect the output of a modulation source (eg
LFO, ADSR) to this input, the cut-off frequency of the
filter will be modulated by its voltage: ie, the sound
color changes according to the voltage put out by the
modulator.
P If you use this VCF as a sine wave oscillator,
connect a pitch control voltage to this input.
Do the same if you want the filter’s cut-off
frequency to track exactly with the pitch of a
note.
FCV 2
§
Socket § is also a voltage-control input for the filter.
Unlike on socket ", though, you can adjust the level of
voltage by using the attenuator 3, and thus control the
intensity of modulation effect on the filter.
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doepfer
System A - 100
VCF 4 A-123
$ QCV
This socket is the voltage control input for the filter’s
resonance.
If you patch a modulation source (eg LFO, ADSR) to
this input, the resonance of the filter will be modulated
by it: increases in voltage will increase the amplitude
of the frequencies around the filter cut-off point.
% Audio Out
Filter output % sends out the filtered audio signal.
5. User examples
The filter’s cut-off frequency can be modulated in
various ways:
• VCF - LFO
Modulation of the cut-off frequency produces cyclical changes of the sound spectrum. At low
frequencies (c. 1 - 5 Hz), you get a "Wah-Wah"-
. Modulation in the audio range produces
effect
interesting sounds; the same principles apply here
as with frequency modulation of the A-110 VCO
(see chapter 6).
• VCF - ADSR
Modulation by an envelope results in gradual
change of the
include filter sweeps, which slowly sweep through
the audio spectrum, picking out different harmonics
in turn.
• VCF - Keyboard CV
This modulation produces pitch-related filter opening.
sound spectrum
. Typical uses
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A-123
VCF 4
System A - 100
doepfer
24 dB notch filter with voltage control of
middle frequency and bandwidth
The patch in Fig. 3 shows a 24 dB notch filter with
voltage-controlled middle frequency and bandwidth.
Freq.
Audio
In
FCV
BCV
Fig. 3: 24 dB notch filter with voltage control of
Ö
A-122
CV 1 CV 2
A-175
f
L
A-138
A-123
CV 1 CV 2
Ö
Freq.
middle frequency and bandwidth.
Freq.
Ö
Audio
Out
Freq.
f
M
f
H
For this patch, set both f
(L ow Pass A-122) and f
L
(H igh Pass A-123) to roughly equal cut-off points
(judging by ear).
Use a control voltage "FCV" to alter the middle frequency f
of the notch filter: fM = (fL + fH ) / 2.
M
Use a control voltage "BCV" to alter the bandwidth,
which is determined by how far apart the two filters’
cut-off frequencies are. Using the A-175 Voltage
Inverter, these frequencies
and
f
L
are altered sym-
f
H
metrically around the middle frequency (see Fig. 4).
FCV
BCV
Ö
f
M
Freq.
Fig. 4: Effect of FCV and BCV on the notch filter’s
response.
H
Ö
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System A - 100
VCF 4 A-123
H
Fig. 5: Effect of FCV and BCV on the band pass
To create a
voltage-controlled middle frequency and
bandwidth, put both filters in series (see the
A-122 user examples). For voltage control of
middle frequency and bandwidth, follow the
relevant notch filter instructions. (see Fig. 5).
filter response.
24 dB bandpass filter
FCV
BCV
f
M
Freq.
with
Ö
7
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A-123
VCF 4
6. Patch-Sheet
System A - 100
doepfer
The following diagrams of the module can help
you recall your own
Patches
. They’re designed so
that a complete 19” rack of modules will fit onto an
A4 sheet of paper.
Photocopy this page, and cut out the pictures of
this and your other modules. You can then stick
them onto another piece of paper, and create a
diagram of your own system.
Make multiple copies of your composite diagram,
and use them for remembering good patches and
set-ups.
P • Draw in patchleads with colored
pens.
• Draw or write control settings in the
little white circles.
A-123
24 dB High Pass
Audio In
FCV 1
FCV 2
QCV
Audio Out
VCF 4
0
0
0
0
0
10
10
10
10
10
Lev.
Frq.
FCV
QCV
Res.
A-123
24 dB High Pass
Audio In
FCV 1
FCV 2
QCV
Audio Out
VCF 4
0
0
0
0
0
10
10
10
10
10
Lev.
Frq.
FCV
QCV
Res.
A-123
24 dB High Pass
Audio In
FCV 1
FCV 2
QCV
Audio Out
VCF 4
0
0
0
0
0
Lev.
10
Frq.
10
FCV
10
QCV
10
Res.
10
8
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