of Microprocessors

ORGAN KEYBOARD INTERFACE 299
quite low and perfectly suitable for carrying any kind of logic signal. The
contact time differential used for velocity sensing varies from a minimum of
around 5 msec to a reasonable maximum of 50 msec. Attempting to pound
the key for shorter times results in severe bounce of the key itself as well as
possible damage. Gently easing the key down for longer times requires so
much care that it is unlikely to be done. Variations in the differential from
key to key and even the same key from stroke to stroke are in the range of
20% up or down. The very light touch characteristic of organ keyboards is
probably responsible for much of the variation.
Contact bounce is a problem that must be dealt with if spurious
outputs are to be prevented. Bounce on contact make is generally 1 msec,
although it can be as long as 3 msec. Bounce on break, however, can be as
long as 5 msec which normally occurs for the slower velocities. Thus, to
retain any degree of accuracy in velocity timing the keyboard controller logic
must define the time differential as being between the last bounce of contact
break on one bus to the first bounce of contact make on the other bus. With a
microprocessor doing the work, such sophistication is relatively simple.
Another option sometimes used by synthesizer manufacturers who also make
their own keyboards is to dispense with contacts altogether and use other
types of position sensors such as light source-photocell combinations, which
do not bounce.
Keyboard Events
The real purpose of the keyboard interface is to report all significant
keyboard activity to the using system. With velocity sensing, this requires
five pieces of information about each keystroke:
1. A number defining the key
2. When it was pressed
3. A number defining the depression velocity
4. When it was released
5. A number defining the release velocity
If the time of depression and release are defined as elapsed time from an
arbitrary point such as the beginning of the song, then the time relationship
among all keystrokes is retained and the keyboard activity can be stored and
reproduced exactly or edited.
For a real-time playing situation, each piece of information should be
reported as soon as it is available. It is possible to break each keystroke into
two parts, which will be called events. A depression event would therefore
consist of a key identification, its depression velocity, and the "time of day"
that it was depressed. Similarly, a release event would specify which key, its
release speed, and when it was released. With information in this form, it
becomes easy for the system control computer to operate the synthesizer in
immediate response to the keyboard and/or record the playing sequence.