Operation Manual

6.97 VCO (Voltage Controlled Oscillator) 156
Tuning the Output Pitch
The coarse and fine knobs and the range switch are used to tune
the output frequency (or pitch) to the desired level. The variations in output pitch caused by changes
in the modulation signals are relative to this level. When the two knobs are in their center position
(green LED on for the coarse knob), the range switch is set to 8 and there is no modulation signal,
the playing frequency has a value of 261.6 Hz, which corresponds to the C3 key on a piano (middle
C). The range switch transposes the pitch one or two octaves up or down. The reading on the
counter gives the frequency of the output signal, in Hertz, when there is no modulation signal.
Pitch Variation
The amount of variation of the playing frequency obtained with the modulation inputs depends on
the adjustment of the mod1 and mod2 gain knobs. The total modulation signal is the sum of the two
inputs each multiplied by the gain corresponding to its respective mod knob. When the knobs are in
the center position (green LEDs on), the gain equals 1 and the pitch variation is 1 Volt/octave. This
position is used to play an equal temperament scale when connecting the output of a Keyboard
module to a modulation input of a VCO. The frequency variation with the modulation signal can
be increased or decreased by turning the modulation knobs clockwise or anti-clockwise.
Waveform
The wavetype switch switches between the four well-known waveforms: noise, sawtooth, pulse
and sine. When choosing the pulse wave, the width of the pulses is adjusted with the PWM knob.
When the PWM knob is adjusted in the right position the waveform is square and only includes
odd harmonics. The pulse width can be modulated by an external signal through the pulse width
modulation input. The PWM knob is used to control the amount of modulation applied by this
third input signal. When this knob is in the left position, there is no modulation applied to the pulse
width while when it is in the right position, the amplitude of the modulation is almost equal to the
width of the pulse. Figure 98 shows the result of the modulation of the pulse width by a sine wave
+1V
-1V
amp
amp
time
time
Figure 98: Sine Wave Modulation of Pulse Width.