Doepfer – Doepfer A-100(~ 40 MB) User Manual

Page 181

DOEPFER

System A-100

Synthesizer Voice A-111-5

7 VCF Frequency
8 VCF Tracking Switch
9a VCF exponential FM Source Switch
9b VCF exponential FM Level
10 VCF linear FM Level

This group of elements is responsible for the frequency of
the Voltage Controlled Filter (VCF). A 24dB Low Pass Filter
with adjustable resonance is used in the module. In addition
the filter features linear frequency modulation. A detailed
description of different filter types can be found in the user
manuals of the A-100 filter modules (e.g. A-120 or A-121).

Frequency control 7 adjusts the frequency of the filter
manually over ~ 12 octaves ( ~ 5Hz ... 20 kHz).

Switch 8 can be used to add the control voltage applied to
the VCO also to the VCF. This function is called filter
tracking because the VCF tracks to the VCO:

upper position: half tracking
center position: no tracking
lower position:

full tracking

If no tracking is selected the sound character becomes
darker as the VCO frequency increases as the filter does
not follow to the frequency of the VCO. If full tracking is
selected the VCF follows to the VCO, i.e. if e.g. the VCO
goes up one octave even the VCF frequency is increased by

one octave. This position of the switch is also chosen if the
filter is played as sine wave oscillator (see below). If half
tracking is selected the filter frequency follows partly the
VCO frequency (if the VCO goes up two octaves the filter
frequency is increased by one octave). This position is
chosen if an intermediate behaviour between full and no
tracking is desired.

Switch 9a and control 9b are responsible for the
exponential frequency modulation (XM) of the VCF. Switch

9a is used to select the XM source:

upper position: LFO2 = XM source
center position: no internal XM
lower position:

ADSR = XM source

Control 9b is used to adjust the intensity of the internal
exponential frequency modulation. A potentiometer with an
exponential scale is used ! This may lead to an unexpected
behaviour if the ADSR is chosen as XM source because
usually linear potentiometers are used here (the influence of
the ADSR to the filter frequency comes "later" than usually).
But we decided to use a potentiometer with exponential
scale here to enable a finer resolution in the lower range –
especially for more sensitive control of FM effects in the
audio range.