of Microprocessors

ORGAN KEYBOARD INTERFACE 293
that a key is pressed. Typically, these two outputs would go to an envelope
generator, which in turn controls a VCA to shape the amplitude envelope of
the notes.
Figure 9-1 is a simplified schematic diagram of such a keyboard. The
heart of the unit is the keyboard itself. Organ keyboards are usually constructed
with a straight gold-plated spring wire attached to each key. When
a key is pressed, this wire contacts a stationary gold-plated rod running the
length of the keyboard. This rod is called a "keyboard bus" in organ-builder's
terms. For the synthesizer interface, the spring wire on each key connects to a
tap point on a series string of resistors designated R in the diagram. A
current source sends a constant current through rhe string, creating an equal
voltage drop across each resistor. For 100-ohm resistors (a common value)
and 1/12 V/resistor (l2-tone scale and 1 V/octave output) the current would
be 0.83 rnA. Thus, the keyboard bus picks up a definite voltage when a key
is pressed against it. If two or more keys simultaneously contact the bus, the
voltage corresponding to the lowest key pressed appears on rhe bus due to the
action of the constant current source. The remainder of the interface circuit
essentially looks at the bus voltage and produces proper gate, trigger, and
control voltage outputs.
Gate and Trigger
The gate output is the easiest to generate. If no keys are pressed, R 1 (in
the megohm range) tends to pull the bus down toward - 15 V. D 1, however,
limits the fall to about -0.5 V. When any key is pressed, the bus is
immediately pulled up to a positive voltage dependent on the key pressed.
The gate voltage then may be taken from a comparator referenced to ground,
which will produce a logic one for positive bus voltage and a zero for negative
bus voltage. Cl is a noise filter in the range of 200 pF, while Al is a unity
gain buffer, which prevents loading of the keyboard bus.
The trigger circuit must provide a short (1 to 10 msec) pulse at the
beginning of each key closure. In all cases but one, this occurs when there is a
sudden change in keyboard bus voltage. In the circu.it shown, a transition
detector generates a pulse whenever such a sudden change happens. This
pulse would be the trigger output if it were not for the case that occurs when
a single contacting key is lifted. In this case, the trigger pulse should be
suppressed. To solve the problem, the transition detector output is delayed
slightly and logically anded with the gate signal. The result triggers a
one-shot, which provides the actual trigger output. Thus, when the transition
to no keys is detected it is blocked from producing a trigger. The delay
need only be long enough for the gate comparator to respond.
The control voltage output from the keyboard normally follows the bus
voltage. However, when no keys are pressed, it should reflect the voltage
level of the last key released. This is necessary because most envelope
generators do not begin their decay until the gate voltage has gone away. In