of Microprocessors

294 MUSICAL ApPLICATIONS OF MICROPROCESSORS
effect, a sample-and-hold function is needed. S1 and C2 form the sampleand-hold,
which is updated whenever a trigger is generated. C2 should be a
low-leakage low-dielectric absorption type (polystyrene) and A3 should be a
low-bias type so that the control voltage output does not audibly drift during
long decays. R2 and C3 provide adjustable portamento, which is a gliding
effect between notes.
The timing diagram illustrates a typical playing sequence on the
keyboard. For an isolated key closure, the gate goes high during the closure
time and the trigger is coincident with the rising edge of the gate. The
control voltage output goes from whatever it was to the level corresponding
to note G5. The next case is a three-note sequence in which the key closures
overlap somewhat. The first note (A5) starts the gate and trigger as before.
When B5 is initially struck, nothing happens because A5, being lower than
B5, takes precedence and no voltage change occurs on the keyboard bus.
When A5 is finally released, the change in bus voltage is detected and
another trigger is generated and updates the control voltage output. Response
to the following G5 is immediate, however, since it is lower than B5.
At the end of the sequence, when G5 is released, the trigger is suppressed
and the control voltage output remains at the G5 level. Also shown is a
typical ADSR envelope that might be generated in response to the illustrated
gate and trigger sequence.
Computer Interface
Interfacing such a keyboard to a computer is fairly simple. Basically, all
that is needed is an ADC connected to the control voltage output and two
input port bits connected to the trigger and gate signals. Whenever a trigger
occurs, an analog-to-digital conversion would be initiated. If keyboard operation
is restricted to the 12-tone equally tempered scale, then the ADC need
only be accurate enough to determine which key is pressed. Thus, a 6-bit
ADC is sufficient for a five-octave keyboard provided that the voltage step
from key to key is matched to the ADC step size.
Once interfaced, it is a simple matter to write a program loop that
looks at these inputs and controls the synthesizer in response to them. If
keyboard activity is to be stored for later editing and recall, a real-time clock
is needed. The program would then note the time and the keyboard vQltage
whenever a trigger occurred or the gate changed. Maximum program flexibility
is attained if triggers and gate changes are connected to the interrupt
structure on the computer. Then the program may do other tasks and still
respond to the keyboard quickly when necessary.
Figure 9-2 is a simplified diagram of a suitable interface. The gate,
trigger, and ADC output data enter through an input port. The flip-flop is
set whenever a trigger pulse or trailing gate edge occurs, which signifies a
significant keyboard event. When set, this flip-flop may request an interrupt
via the 7405 open-collector inverter. The interrupt service routine can de-