of Microprocessors

92 MUSICAL ApPLICATIONS OF MICROPROCESSORS
key (basis voltage). Although these are redundant with VCO controls, it is
very convenient to have them right at the keyboard itself particularly if it
feeds several VCOs. The third control is for portamento. When rotated away
from its zero position, the change in control voltage output from one level to
another when a new key is pressed slows down, resulting in a pitch glide
from the first note to the second.
Recently, a somewhat more sophisticated keyboard was developed that
allows two keys to be pressed simultaneously. Two sets of outputs are provided,
one for the lowest key pressed and the other for the highest key. When
properly scored and played with care, two independent voices with different
characteristics may be played on one keyboard. Another simple and often
employed scheme is to simply use several keyboards such as with theater
organs. Digital scanning techniques have also been applied to synthesizer
keyboard design such that truly polyphonic playing is possible. One decision
that has to be made in the design of such a keyboard is the assignment of keys
to control voltage-trigger-gate output groups. If the voices controlled by the
keyboard are not all the same, the assignment of voices to keys may vary
greatly according to playing technique or even random chance. Digital
keyboards will be studied in greater detail in Chapter 9.
Other Modules
The modules that have been described so far are those that can be
expected to be present in any synthesizer, often in multiples. There are also a
number of specialized module types for special effects, to perform functions
normally requiring a number of interconnected modules, and "utility" versions
ofthe standard modules already discussed. Utility modules, parriculary
VCOs and VCAs, are less flexible, less accurate, and therefore quite a bit less
expensive than their standard counterparts. They are used freely in complex
patches where their characteristics are adequate for the task.
Sequencer
One specialized module that adds a measure of automation to the
synthesis process is called a sequencer. Interesting repetitive effects can be
created with one or more VCOs set to their very-low-frequency range and
used as sources of control voltages. However, if a specific sequence such as a
short melody is desired, it is not normally obvious how a group of oscillatOrs
might be set up to generate it. A sequencer allows the user to directly
determine and easily modify specific, arbitrary sequences of control voltages.
The sequencer is usually designed to simulate a keyboard; thus, it has
trigger and gate outputs as well as the control voltage output. The simplest
sequencers might have 16 potentiometer knobs and a "speed" conrrol that
determines the frequency ofa clock oscillator. Each cycle ofthe clock causes a