of Microprocessors

ORGAN KEYBOARD INTERFACE 297
one of the gates would go away and the corresponding voltage output would
not change. Which one should it be? These problems would be particularly
important if the voices were different, such as a rich timbre for the low notes
and a thin timbre for the high notes. If the answer is consistent such that the
high note always takes precedence when only one key is down, the circuitry
can be designed easily enough. But then the question becomes whether that
choice is appropriate for all music that will be played. In a nutshell, this
defines the assignment problem that is difficult for two voices and purely a
matter of compromise for more than two.
Ultimate Limitations
If the trigger and gate circuitry used on the one-voice keyboard is
applied to each channel of the two-voice case, the behavior will be far from
ideal. First, the two gates would be identical because both would be looking
for a positive bus voltage. In addition, if only one key is pressed, both voices
would trigger and both would output the same control voltage. If a second
key is struck, one voice will retrigger and update its output depending on
whether the second note is lower or higher than the first.
So far the problems are not particularly bad if both voices have the same
timbre. The real difficulty occurs when two keys are down and the player
attempts to release them simultaneously, expecting the two-note chord to die
out during the envelope decay. What will almost surely happen instead is
that one of the keys will release first, and the voice that was assigned to that
key will trigger and update itself to the other key. The end result will be that
the two voices will decay while playing the same note and it is not even
predictable which note it will be!
Obviously, logic and delay circuits can be added to obtain performance
closer to the ideal. The most important element would be a circuit to
specifically detect when only one key is down and modify the gate and
trigger action according to the chosen set of rules. Since one set of rules may
not be appropriate for all playing situations, a selector switch might be added
to allow changing the rules. Even with its limitations, the two-voice analog
keyboard is an inexpensive feature often found in prepackaged synthesizers
and used primarily as a selling point.
Beyond two voices, it is necessary to return to the digital domain.
Actually, an all-digital keyboard interface would make the most sense for
musical input to a computer, since a digital-to-analog operation (series resistor
string and keyswitches) and an analog-to-digital operation can both be
bypassed. It should be intuitively obvious that a suitable digital circuit can
constantly scan all of the keyswitch contacts, track the state of each one, and
report significant events to the computer. Assignment of keys to voices would
then be done in the control computer, where as little or as much intelligence
as necessary can be applied to the task.