of Microprocessors

VOLTAGE-CONTROL METHODS 83
high ones. Thus, the vibrato amplitude would have to be made proportional
to frequency through the use of a voltage-controlled amplifier.
Although exponential control of frequency at the rate of 1 V/octave is
an industry-wide standard, the best veo modules do have a linear control
voltage input available. Its primary use is in using rapid and deep frequency
modulation as a method of altering the spectrum of the oscillator's output
waveform without incurring an undesirable average frequency shift due to the
modulation.
Amplitide Relation
Amplitude is the other parameter of primary importance that needs a
control voltage relationship established. Whereas the audible frequency
range is a 1,000: 1 ratio, the audible amplitude range is 1,000 times greater
yet or 10 6 : 1. This 120-dB range might be a nice goal for live listening in a
soundproof room but certainly could not be utilized in a normal environment.
Furthermore, recording equipment is limited to a 60-dB to 70-dB
range at best. Thus, a 10,000: 1 or SO-dB range on voltage-controlled
amplifiers would be a reasonable requirement.
The accuracy requirement for amplitude control is not nearly as stringent
as frequency control accuracy. A 10% error, which corresponds to about
0.8 dB, would be barely audible. However, if the voltage-controlled
amplifier were being used in a control voltage path processing frequencyrelated
control voltages, considerably better accuracy would be desirable.
Without going through the linear versus exponential argument again, it is
clear that the control voltage relationship should be exponential.
A common setup would be such that 8 V would specify unity (0 dB)
gain and lesser voltages would decrease gain at the rate of 10 dB/volt.
Typically, as the control voltage approaches zero and the gain is decreasing
toward - 80dB, a squelch circuit comes into play, which actually cuts the
signal completely off Control voltages greater than + 8 V can provide some
positive gain in the circuit. With such a setup, the gain expression would be
AV = 10(V-8)/2, where AV is the voltage gain and V is the control voltage in
volts. Like the oscillator, an added linear mode control input is helpful for
special applications using amplitude modulation for timbre modification.
Most parameters used in other modules are related to either frequency
or amplitude and thus usually have an exponential relationship. However,
any time that a variable is required to cover a wide range, the exponential
relationship is useful.
Standard Voltage Levels
Signal levels in a voltage-controlled synthesizer, both audio and control,
are generally kept at a constant amplitude except when amplitude