of Microprocessors

218 MUSICAL ApPLICATIONS OF MICROPROCESSORS
0 100 kil 2 kil
CONTROL
2
0 100 kil
CONTROL
I
160 kil
VOLTS PER
o DOUBLE
INITIAL
0
-IOref
oCONTROL
50 kil CURRENT
I Mil
10kil
Fig. 6-28. Q .control for VCF in Fig. 6-27
Figure 6-28 shows a suitable Q control circuit. Note that it is similar
to the VCO frequency control circuit except that a pnp matched pair is
needed to satisfy the control current needs of the 3080. The reversal in
transistor polariry also reverses the control sense as required for Q control.
Quad Voltage-Controlled Integrator IC
Besides use in the state-variable filter, voltage-controlled integrators
can actually be used in any kind of active filter circuit, even the simple
one-pole low-pass discussed earlier. Since any filter function that can be done
with resistors, capacitors, and inductors can also be done with combinations
of one- and two-pole RC active filters, it follows that voltage-controlled
integrators cart be used to make any of those filters voltage controlled. As a
result, a quad voltage-controlled integrator IC has been developed, also by
Solid State Music, for use in voltage-controlled filters.
Figure 6-29 shows a block diagram of the type 2040 voltage-controlled
filter Ie. Basically, four transconductance gain cells driving four high-
5 MATCHED TRANSISTORS
TRANSCONDUCTANCE
GAIN CELL
IN I CAP lOUT I IN 2 CAP 2 OUT 2 IN 3 CAP 3 OUT 3
Fig. 6-29. Block diagram of SSM 2040 quad voltage-controlled integrator IC