key is pressed on the keyboard (assuming that the test programs have
been loaded.) The “Q CAL” trimmer is used to set all VCFs so that they
oscillate at the same resonance setting. When set properly, all of the
VCFs should burst into oscillation at the same point on the front panel Q
control pot.
4.4 VCO
Multiplexing
System
The CD4052 U19 is used to multiplex four VCO1s and four VCO2s to
two independent lines called VCO1 MUX and VCO2 MUX. These signals
serve two distinct purposes.
First, when the CPU is autotuning the VCOs, this multiplexer allows it to
route one of the VCOs to the DAC card for frequency measurement.
Secondly, it allows the modulation system to use the VCOs as
modulation sources. The scanning clocks for the multiplexer, labeled
VMA1 and VMA2 are derived from the CPU during autotune and from the
modulation system during normal operation. This clock selection is
determined by the CPU on the DIGIMOD card. The LF347 buffer
amplifiers that precede the CD4052 inputs prevent the scanning clocks
from leaking into the VCF audio inputs.
4.5 DC
Voltages
The card uses +/- 15 Volts to drive the SSM ICs and associated audio
opamps. The LM325 ICs are driven from +8V and -7V to limit their range
to within the specifications of the SSM 2044 control inputs. The CD4052s
used in the modulation demultiplexing are also biased with these
voltages as their outputs must be bipolar while their maximum bias must
be less than 16V. The F
c
INITIAL trimmers are biased using a cascade
transistor pair to derive a buffered reference for the trimmers as well as
the -7V card voltage. The +8V is derived from a 78L08 voltage regulator
driven from the +15V line.
The 15V modulation scan clocks and VCF bypass control signal are
converted to 8V by the CD4050 U5 buffer to be compatible with the bias
on the CD4052 demultiplexers and the MC14551 switch.
4.6
Card Pin Signals
The signals coming into and out of the card are as follows:
4.6.1 Inputs
•
Modulation multiplexed inputs for F
c
, Q, VCO1 and VCO2 coming
from ANAMOD 1.
•
Power supply /- 15V and Analog Ground
