analogue solutions
Leipzig
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14
VCA
The Voltage Controlled Amplifi er (VCA) is the circuit that is used
to change the output volume. Normally an envelope signal would be
used to do this, so the sound starts loud then gradually fades away.
The fi lter audio output is hard-wired to the VCA signal input.
Mode rotary switch control
The VCA can be controlled in 4 modes;
EG1
When EG1 is selected, the envelope signal of EG1 is used to
modulate the VCA level. Use it if you require the VCA level to change
over time.
EG2
When EG2 is selected, the envelope signal of EG2 is used to
modulate the VCA level. Use it if you require the VCA level to change over time.
GATE
When GATE is selected, the MIDI converter's Gate signal is used to modulate the
VCA. With this, the VCA level is either off or full on. The audio envelope would be like an
organ's, with no attack or decay time.
THRU
When THRU (on or bypass) is selected, the VCA is left permanently on at full level.
Use this setting if you wish to use the Filter as an effects processor, to process external
audio fed into Semblance.
VOLUME
This sets the output volume of synth, i.e. the output level from the VCA that is output
from the SIGNAL output socket.
LFO
This is in the MODULATION panel (see later). The Low Frequency Oscillator (LFO)
is basically identical to a VCO; it is another oscillator, except that it produces periodic
wave forms of low frequency, typically sub-audio. These slow cyclic waveforms are used
for modulating other circuit parameters, for example, for sweeping the fi lter cut-off up and
down, either slowly for a nice sweep, or faster for a 'wah-wah' type effect. It could be used
to modulate the VCO pitch for vibrato.
The LFO is found in the MODULATION panel (see diagram).
LFO RATE
This sets the frequency (speed) of the LFO.
LFO SPEED LED indicator
This wil turn on each time the LFO signal is at a positive voltage, indicating the speed
of the LFO waveform.
