of Microprocessors

80 MUSICAL ApPLICATIONS OF MICROPROCESSORS
VOLTAGf­
CONTROLLED
OSCILLATOR
AUDIO SIGNAL
TO FURTHER
PROCESSING
OUT
MIXER
A
B
VOLTAGE­
CONTROLLED
OSCILLATOR
Fig. 3--3. Added modules for vibrato
impresses a different contour onto the spectrum of the sound through the
voltage-controlled filter.
As a result, these three fundamental parameters are directly, and most
important, visibly controlled by the three control voltages. More complex
patches generally add modules in the control voltage paths, although a few
more might also be added to the signal path. As an example, let us take this
basic patch and add vibrato. Since vibrato is a small wavering of frequency
during extended steady states, the control voltage connection between the
keyboard and the oscillator will be broken and additional modules inserted as
in Fig. 3-3. First, another oscillator is added, which is the source of the
low-frequency vibrato waveform. For this example, no control voltage is
supplied to the oscillator; its mechanical controls are used to set the vibrato
frequency. The vibrato voltage is combined with the keyboard output with
the mixer module shown. Mechanical controls on the mixer determine the
proportion of each input signal that appears at the output. For naturalsounding
vibrato, the controls are set for only a small contribution from the
vibrato oscillator. The resulting control voltage is then sent to the original
oscillator and the remainder of the system as before.
It is easy to imagine further growth of the patch, since the vibrato
produced by the example patch is rather crude. Typically, one might want
shorr notes and the beginnings oflong ones to be free ofvibrato. After a delay
period on long notes, vibrato should commence gradually. Addition of
another voltage-controlled amplifier and envelope generator in the frequency
control voltage path will allow this effect to be produced. Similar additions
may also be made in the amplitude and spectrum control paths. Once a good