Doepfer Dark Energy I (device no longer available) - Manual (Page 14)

Thecutoff frequency of alowpass filter de-
pending on the control voltage
The resonance characteristics of a lowpass
filter
14
Dark EnerGy DOEPFER Functionality
3.2.2. VCF
Filters are a very importand tool in every analogue synthesizer. Filters modify sounds by rejecting some
frequencies while allowing others to pass through in order to manipulate the harmonic content of the sound.
There are different types of filters; the most common and most musically useful filter type is the 24 dB ‘low-
pass’, which is used in Dark Energy. According to this termination, a lowpass filter passes all frequencies
below the so called „cutoff frequency“ and rejects frequencies above the cutoff. This point can be controlled
by a knob or modulated by a control voltage in order to achieve dynamic timbres.
Another importand filter term is ‘resonance’. The resonance function emphases the overtones (or harmo-
nics) close to the cutoff frequency. The sound becomes more and more „sharper“. At a certain level, the
filter starts to self oscillate and generates a waveform close to a sine wave. Dark Energy‘s VCF can also
be driven into self oscillation in oder to use it as an additional signal source to provide certain, very popular
sound effects.
The VCF provides the following controls:
• Manual frequency knob (Freq.)
This knob manually controls the cutoff frequency of the VCF. In other words you
„open“ and „close“ the pass-through „window“ of the filter. At fully clockwise position,
the filter is completely opened and the entire frequency range of the incomming signal
is passed through. At fully counterclockwise position, the filter is closed so far, that all
audible frequencies are rejected and no output signal is audible. The entire frequency
range of Dark Energy‘s filter covers approx. 12 octaves. The more this knob is turned
into clockwise direction, the smaller is the effect of the envelope (ADSR) to the sound.
• XM knob
In order to achieve dynamic and periodic changes in timbre, the cutoff frequency
can be modulated via control voltages. The XM knob sets the modulation depth. Set
fully counterclockwise, no modulation appears, while fully clockwise position means
maximum modulation depth. The modulation depth is added to the setting of the
manual frequency knob. Since the sum of both cannot exceed the maximum value of
the cutoff parameter range, the setting of the „Freq.“ knob influences the modulation
depth: The higher it‘s setting, the lower the effect of the XM knob to the sound. The
XM function uses exponential frequency modulation.
In some cases, the FM has to be controlled in a very fine way, especially when the fil-
ter is self oscillating (resp. working as a sinewave oscillator) and modulated by LFO2
in the audiorange. In order to optimize the parameter scaling, the XM knob works in
an exponential way. Thus, audible modulation might need settings above „5“, espe-
cially when using the ADSR as a modulation source.
fc
CV
t u O t u O
. q e r F . q e r F
fc
Resonance changes the filter characteristics
fc
Resonance
Frequency
0 db
Attenuation