Doepfer Dark Energy I (device no longer available) - Manual (Page 28)

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Dark EnerGy DOEPFER Basics of Soundgeneration
frequency. For the given example this would result in: 1000 Hz, 1400 Hz, 11800 Hz, 200 Hz, etc...
Strictly speaking this only applies to pure sine wave frequeny modulation, i.e. if both, VCO and LFO,
are sine wave oscillators. With other wave forms additional (non-)harmonics will be added. Unhar-
monic sounds, similar to frequency modulation, are also produced by pulse width modulation with
high frequencies. In practice this priciple is used for creating extreme, unharmonic sounds. The de-
gree of being unharmonic depends on the strength (amplitude) of LFO modulation as well as on the
frequency ratio of VCO and LFO. “Practice is better than theory” applies here. You should try various
settings for LFO frequency and LFO amplitude. Most probably the resulting wave forms are not suited
for sweet, “beautiful” sounds but can be used for extreme, “vigorous” sounds instead.
Filters
With subtractive sound synthesis the further processing of the tonal “raw material” (VCO signal) will
be done by a voltage controlled filter (VCF) first which is usually followed by a voltage controlled
amplifier (VCA).
There are different types or characteristics of filters. The
basic types are lowpass, bandpass and highpass while the
types notch and allpass are less common. The filter types
differ in how they feed the frequency through resp. which
frequency ranges are rejected. A lowpass passes all fre-
quencies that are below the cutoff frequency and cuts off all
frequencies above. The highpass works just the opposite
way since it passes all frequencies that are above the cutoff
frequency and cuts off all frequencies below. A bandpass
passes all frequencies within a certain range (band) and
rejects all frequencies outside this range. A notch cuts out a
certain frequency range (inverse function of the bandpass).
An allpass passes all frequencies but it performs a phaseshift dependant on the frequency. For musi-
cal purposes the lowpass is by far the most efficient filtertype, thus it is used in Dark Energy.
Next to the filtertype, another important parameter of a filter is slope which is measured in dB/Oc-
tave. It describes how steep the transition from passing to recection occurs. The “ideal” filter would
have an infinite slope, i.e. the transition occurs immediately (e.g. 999 Hz would pass totally while
1000 Hz would be supressed totally). In reality this transition is not a jump but a gliding, depending
on the filter’s slope. For musical purposes, a filter with a high value would have a slope of 24 dB/Oc-
tave but 12 dB/Octave are also useful for special applications (usually for high pass, band pass and
notch). Dark Energy uses a 24 dB/Octace filter, since it worked out to be the most musical useful and
efficient.
A further filter parameter is resonance (or emphasis). If the
filter provides an adjustable resonance, frequencies close
to the cutoff point can be emphasized. The filter becomes
more of a bandpass with an underlaid low pass. For musi-
cal applications it is very important that the filter resonance
is able to be adjusted. VCFs without adjustable resonance
are seldomly used. Overtones close to the cutoff point are
emphasized the bigger the resonance is set (see fig. XX).
This results in the typical resonance and tweet sounds of
an analog filter. With many filters the resonance can be set
to such a high degree that self oscillation of the filter occurs
(even without any input). This allows you to use the filter as
a sine wave oscillator instead of the VCO for special pur-
poses. You guessed it – also Dark Energy’s filter is capable
to self oscillate and thus to create a lot of typical sounds.
fc
CV
t u O t u O
. q e r F . q e r F
fc
Resonance changes the filter characteristics
f
c
Resonance
Frequency
0 db
Attenuation
Charakteristics of a lowpass filter