Doepfer A-130/A-131 VCA (linear / logarithmic) - Manual (Page 6)

A-130 / A-131 VCA System A - 100 doepfer
6
Amplitude modulation
In Fig. 5, an LFO is modulating an A-130 linear VCA
(with Gain > 0), so that the amplification changes
cyclically with the LFO’s voltage. (Amplitude modu-
lation / AM.)
With an LFO frequency in the sub-audio range (1 Hz to
around 15 Hz) the result is Tremolo (see Fig. 5).
With a modulation frequency in the audio range, side-
bands occur like those produced by FM (Frequency
Modulation), and interesting timbres emerge.
Fig. 5: VCA amplitude modulation with an LFO
Modulation depth is adjusted with control 2.
Fig. 6 shows a way of voltage-controlling this modula-
tion depth using another VCA. In this example, the
VCAs have the following functions:
•
VCA 1 (A-130): AM control
•
VCA 2 (A-131): total volume control
•
VCA 3 (A-130): modulation depth control
The voltage control input A to the modulation depth VCA
can come from an ADSR, MIDI controller, etc..
Fig. 6: AM with voltage-controlled modulation depth
LFO
VCO
Au di o
In 1
C V 2
C V 2
Ga in
A-130
VCA-LIN.
In 1
Aud io Out
Out
Gain > 0
VCO
LFO
A
CV
CV
VCA 1
VCA 3
VCA 2