of Microprocessors

RATIO
CONTROL
VOLTAGE-CONTROL METHODS
veD I OUT
SINE-
EXP I [XP 2 LIN
•
TO FURTHER
PROCESSING
99
MAIN
FREQUENCY
CONTROL
VOLTAGE
TRIGGER
GATE
....
..
veo 2 ~~ir- VCA r-
EXP
•
ENVELOPE
GEN
EXP
Fig. ~9.
Patch for dynamic depth FM
Patch for Dynamic Depth FM
Figure 3-9 shows a possible patch for dynamic depth FM. VCO 1 is the
primary VCO) which provides the modulated signal. veo 2 supplies the
modulating signal. veo 1 is modulated by veo 2 through an auxiliary
linear control input. This is necessary for the preceding analysis to be valid.
Note that the main frequency control voltage is connected to both veos.
Since these control inputs are exponential, the "ratio" voltage level determines
the ratio between the modulated signal's cerreer frequency and the
modulating frequency. If, for example, it was set to 2 V, then veo 1
(unmodulated) would track veo 2) but two octaves higher. The modulation
depth is set by a VCA controlled by an envelope generator.
Thus) if this setup were connected to a keyboard) for example, the
keyboard output voltage would control the frequency of both oscillators) but
veo 1 would always be four times higher in frequency than veo 2. The
apparent pitch of the generated tones would be equal to the frequency of
veo 2. The trigger and gate signals from the keyboard would be connected
ro the envelope generator, which, in turn, produces the control voltage that
determines the modulation index, and thus the spectrum of the rone. For
producing actual notes, the output of veo 1 would at the very least pass
through another VeA controlled by the same or another envelope generator
in order to apply an amplitude envelope.
Changing the ratio voltage also affects the modulation index and, if not
set for a simple ratio) results in inharmonic spectra. This voltage could even
be set so that the modulated signal's center frequency is less than the
modulating frequency. One point should be kept in mind as the parameters
in the system are varied. With the linear input to VCO 1, it is possible to
specify a negative instantaneous frequency. The most popular veo designs