Dark energy doepfer, 27 basics of soundgeneration – Doepfer Dark Energy I (device no longer available) User Manual

Page 27

Basics of Soundgeneration

Dark EnerGy

DOEPFER

The symmetrical

pulsewave (or “squarewave”, pulsewidth =

50 %) contains odd harmonics only (see Fig.). An unsymmetrical

pulsewave (often simply called “pulsewave”) contains all harmon-

ics with their amplitudes being dependant on the pulsewidth.

The more the pulsewidth differs from the symmetrical 50 %, the

stronger the higher harmonics emboss the sound, i.e. its timbre

becomes more “nasal”.

The

width of a pulsewave can be modulated by a low frequency

oscillator (LFO) or, less common, by an envelope generator. Then

the overtone spectrum of the pulsewave continuously changes.

The resulting sound is similar to a beat wave which is the result

of two, nearly equally tuned oscillators. The modulation frequency

must be very low (approx. 1 Hz or lower) since the oscillator

seems detuned otherwise.

The

triangle and sine waveforms have only a weakly pronounced

harmonic structure resp. no harmonics at all (sine). The triangle

waveform contains only the odd harmonics like the sawtooth but

their amplitudes decreases by the power of two while with the

sawtooth they are decreasing linearely in their numerical order

and therefore much slower.

Modulation

The amount of overtones of the base material is decisive for the possibilities in sound shaping with the sub-

sequent voltage-controlled filter (VCF) since subtractive sound synthesis with a VCF (see below) only al-

lows to cut-out or amplify overtones that exist. Therefore, the sine and triangle wave only play a minor part

here. Under the condition that a VCF works as sine wave oscillator when resonance is set to maximum, it

can be used in this way if a sine wave oscillator is required for a special sound.

If a low frequency oscillator is available, it can be used for modulating

either the VCO pitch (frequency modulation = FM, also called “vibrato”)

or the pulse width (pulse width modulation = PWM). Simple LFOs provide

frequencies in the range of 0.1 Hz to 10 Hz while better ones have a sig-

nificantly wider range (0.01 Hz to 5 kHz) with switchable frequency bands

for better adjustment.

Modulation frequencies within the audio range should be dealt with in more detail since their result are very

special sounds. If a VCO is modulated with a frequency close to its own, a completely new sound charac-

teristic is established. In this case the VCO also produces non-harmonics, i.e. tones whose frequencies are

non-integer multiples of the VCO frequency. This acoustical characteristic is the basis for the so-called FM

Synthesis which is used with several digital synthesizers. If e.g. a VCO, oscillating at 1 kHz (1000 Hz), is

modulated with a frequency of 400 Hz, so-called “side bands” around the the VCO frequency are created

which are grouped symmetrically around the VCO frequency as integer multiples of the modulation

100%

Harmonics

100%

Harmonics