of Microprocessors

196 MUSICAL ApPLICATIONS OF MICROPROCESSORS
reduced to the same order of magnitude as other circuit drifts in the typical
studio environment.
Two other minor errors are corrected by modifying the reference current
regulator for the exponential converter. One of these is due to the finite
"bulk" resistance in the converter transistors. The other is due to finite
discharge time in the sawtooth oscillator. Both effects cause high frequencies
to be lower than they should be. The magnitude of both errors is directly
proportional to the magnitude of the control current from the converter. The
addition of Oland R5 couples a voltage that is directly proportional to the
control current back into the control input summer. This voltage is developed
across the 10K protective resistor in series with the reference current
regulator. The diode cancels a o.6-v offset that exists at low values of control
current. In use, R5 is adjusted for optimum high-frequency tracking.
The sawtooth oscillator proper uses a high-speed, very-Iow-inputcurrent
op-amp as the integrator. The odd power supply hookup for A3 is
necessary because it cannot stand total supply voltages beyond 15 V. Note
that the type 311 comparator has an open-collector output so that when its
pullup resistor is tied to ground its output voltage swings between -15 V
and a V, the range needed by Q4.
Figure 6-11 shows the waveform standardizers and shapers with all
adjustments and parts values. The sawtooth output is taken from the second
inverter rather than the first shown earlier so that a positive going ramp is
produced. The negative ramp at the output of A4 could also be brought out
if desired. The glitch in the triangle wave mentioned earlier is largely cancelled
by injecting an opposite polarity pulse derived from the rapid retrace of
the negative ramp. The rectangular-wave amplitude is standardized by the
saturating complementary emitter follower, Q5 and Q6. Using this circuit,
voltage levels of the rectangle will equal the ± 10-V system reference voltages
to within a couple of millivolts.
Adjustment
Adjustment of the circuit is straightforward and need not require a lot
of test equipment, although a frequency counter, accurate digital voltmeter,
and oscilloscope are' helpful. For use in a precalibrated computer-controlled
system, the adjustments should be made to the values listed in Table 6-1.
These values offer the most logical control relationship and widest possible
range in a precalibrated system. Ideally, they should be set as accurately as
possible and then rechecked every few months of operation. This will allow
programming of the system to proceed with minimal concern over analog
errors in the system. Also shown in the calibration table are performance
parameters obtained from a breadboard of the circuit. As can be seen, the
performance is excellent in nearly all respects and certainly much better than
could be obtained just a few years ago at 10 times the cost.