of Microprocessors

VOLTAGE-CONTROL METHODS 81
understanding of the parameters of sound has been developed, most patches
are obvious. Note that more modules may actually be tied up in processing
control voltages than audio signals!
Signal Levels in the Synthesizer
So far the value of voltage conrrol has been established and the desirability
of an accurate relationship between control voltage magnitude and
sound parameter magnitude has been mentioned. However, now the exact
form of this relationship needs to be established. Although there is considerable
agreement in the industry on the control relations to be discussed, there
are a few manufacturers of smaller systems that do things differently.
Frequency-Control Relation
Of all the parameters of sound, frequency is probably the most important.
The ear is considerably more sensitive to small changes in frequency
than any other parameter. Also, music theory is significantly more concerned
with intervals, chords, and other pitch-related topics than it is with ocher
areas. Thus, the choice of a relationship between a frequency-control voltage
and the resulting output frequency should be chosen carefully.
The range of human hearing can be considered to be 10 octaves, which
is a 2 10 or 1,024: 1 or 20 Hz to 20 kHz range. Within this range, a relative
error of 1% is a minimum goal with considerably less being desirable in the
middle four octaves. A relative error of 1% means that an intended frequency
of 20 Hz cannot actually be less than 20 - 20/100 = 19.8 Hz or greater
than 20.2 Hz. Likewise, any frequency between 19.8 kHz and 20.2 kHz
would be acceptable for an intended value of 20 kHz. Note that the absolute
magnitude of error at the high end is a whopping 200 Hz, while at the low
end it is only 0.2 Hz. Expressed as a full-scale accuracy, a specification
method almost universally used by the measuring-instrument industry, the
requirement would be 0.2 Hz/20 kHz = 0.001%. Laboratory voltagemeasuring
instruments with this kind of accuracy are almost nonexistent,
exceedingly expensive, and fragile, yet this and more is being asked of a
voltage-controlled oscillator module for a synthesizer!
The most obvious relationship between control voltage and frequency is
a linear one, that is, a direct relationship between voltage and frequency. For
the sake of argument, let us assume the relationship F= 1,000V, where F is
the output frequency in hertz and V is the control voltage in volts. With this
relation, the audio range would be covered with a control voltage range of
20 mV to 20 V. The 1% error alluded to earlier would amount to 200 J.LV at
the low end of the range and 0.2 V at the upper end.
Actually, 20 V is a little high for convenient use of modern linear ICs.
A maximum of 10 V would be more reasonable. The 100-p.V error now