of Microprocessors

ORGAN KEYBOARD INTERFACE 295
PARALLEL
INPUTS
INTREQ
GATE
KEYBOARO
TRIGGER
'---------1 COMPUTER
CONTROL
VOLTAGE
OUTPUT
ANALOG
INPUT
ADC
PARALLEL
ADe DATA
Fig. 9--2. Analog keyboard-to-computer interface
termine if the keyboard is requesting by examining the trigger and gate
signals. After reading the ADC output, the request flip-flop can be reset
through an output port bit. If a 6-bit ADC is used, the entire interface only
requires 8 input port bits and 1 output port bit. The interface is equally
applicable to software ADC methods. Once the interrupt service routine
determines that the keyboard caused the interrupt, the ADC subroutine can
be entered to read the keyboard voltage.
Polyphonic Keyboards
In spite of its vast audio-processing power, most analog synthesizers are
inherently monophonic (one note at a time) instruments. In fact it is usually
the keyboard that causes this limitation. One can always use multiple
keyboards with a voice for each but an increasingly popular musical application
of microprocessors is as polyphonic keyboard controllers that allow a
synthesizer player to become a real-time one-man band. Before discussing
such a keyboard controller, let's examine a fairly common analog technique
used to obtain two simultaneous independent notes from a single keyboard.
Two-Note Keyboard
Figure 9-3 shows the idea behind a two-note analog keyboard using a
single-keyboard bus. As mentioned in the single-note interface, when more
than one key is pressed, the bus voltage will correspond to the lowest key
pressed. What is needed is an additional circuit to generate a voltage proportional
to the highest key pressed as well. Examining the siruation with two
keys down, it is seen that all of the resistors between the two keys are shorted
out (three resistors in the example). Since the string is driven by a constantcurrent
source, the voltage at the top of the string, E2, will decrease by an
amount equal to the voltage that would normally appear across the shorted