of Microprocessors

188 MUSICAL ApPLICATIONS OF MICROPROCESSORS
center frequency is varied. For musical purposes, this is generally more
desirable than 'constant deviation. If the linear control input becomes sufficiently
negative, the reference current may be completely shut off, giving
zero output current and thus zero frequency.
Another possibility for a linear input is a separate linear voltage-tocurrent
converter whose output current is summed with the exponential
current via a direct connection. This gives constant deviation FM, which also
has some uses. For maximum usefulness, the linear current converter should
be able to sink current for a positive input and source current for a negative
input. A circuit configuration having these characteristics is shown in Fig.
6--6B. The type 3080 operational transconductance amplifier gives an output
current that is proportional to the difference in voltage at the plus and minus
inputs. This current is positive if the plus input is more positive and negative
if it is more negative. The sensitivity and range of the circuit are adjusted by
changing the 3080's bias current. Although the actual control current can
become positive (sourcing) with this circuit, the following current-controlled
oscillator would probably stall if that were to happen.
The 3080 is described more fully in the next section on voltagecontrolled
amplifiers.
CONTROL
VOLTAGE
INPUTS
(r.- .J....""""W.......
SENSITIVITY
470 K
OCTAVES/VOLT
TUNE
TO
500 • 0 EXPONENTIAL
CONVERTER
+15 V o---JW\r-O -15 V
BASIS
FREQUENCY
TUNE
Fig. &-7. Input voltage processor
Input Processor
The exponential converter transistors are driven by the input-processor
block. Typically, several control voltages are summed together to form a
composite control voltage with two or three of the control inputs coming
directly from the front panel of the module. The sensitivity of one or more of
these front-panel inputs is usually adjusted by front-panel controls. Another
one or two voltages come from additional panel controls used to adjust the
tuning of the module.
The ideal circuit configuration for combining these together with complete
isolation is the inverting op-amp summer as shown in Fig. 6-7. In this
example, inputs A and B would have a fixed sensitivity, usually of 1 octavelV