of Microprocessors

298 MUSICAL ApPLICATIONS OF MICROPROCESSORS
A Microprocessor-Based Keyboard Interface
Not only can digital logic solve the polyphonic keyboard problem and
thereby effect an efficient, completely general interface to a microcomputer,
but also a dedicated microprocessor can replace the logic. In this section, a
five-octave velocity-sensitive keyboard interface will be described, which uses a
6502 microprocessor to perform all of the needed logic functions. Using a
dedicated microprocessor results in an interface that uses a minimum of
parts, is easy to build, is flexible in that the operational characteristics may
be altered by reprogramming, and is actually inexpensive.
Velocity Sensing
Befote delving into the actual circuitry and programming, let's develop
a set ofspecifications for the unit. First, what is velocity sensing and how is it
implemented? Basically, velocity sensing is a very inexpensive way to obtain
additional information about keystrokes beyond simple duration. The keys
on a piano, for example, are velocity sensitive. The actual speed of key travel
downward at the exact instant of key "bottoming" solely determines the force
with which the hammer strikes the string. In fact, the hammer "coasts" a
finite distance from the point of key bottoming to actual contact with the
string. Thus, variations in velocity or pressure while the key is going down do
not affect the sound! Unfortunately, a velocity-sensitive organ keyboard will
not feel at all like a piano keyboard because the inertia of the hammer is
absent, but the same kind of information will be available. Note that since
we are using a synthesizer the velocity information need not necessarily
control the amplitude of the note. It could just as well control timbre,
vibrato, or the envelope.
The mechanical modification necessary to allow velocity sensing on an
organ keyboard is really quite simple. All that is required is a second
keyboard bus spaced a fixed distance above the standard bus and positioned
such that the keys' spring wires make contact with it when in the up position.
Now, when a key travels down, the wire will first break contact with the
upper bus after a short distance, float freely between the buses for the
majority of the travel, and then finally contact the lower bus just before the
key bottoms. The average downward velocity of the wire and thus the key
may be determined by measuring the time interval between breaking the
upper contact and making the lower one! If desired, the speed of release at
the end of a note may also be determined, which might indeed be used to
vary envelope decay. For monophonic keyboards, it is relatively easy to
design analog timing circuits that will produce a control voltage output
proportional to velocity. For polyphonic keyboards, however, only digital
scanning logic can cope with the problem.
The actual characteristics of standard two-bus commercial keyboards
are not quite ideal but can be lived with. Contact resistance, for example, is