of Microprocessors

DIRECT COMPUTER SYNTHESIS METHODS 103
The multitrack recorder would seem to be a complete solution to the
simultaneous sound problem with the only disadvantage being that the
synthesis process is taken out of real time. However, this is not the case. As a
simple example, consider a rapid run (fast series of notes) on an instrument
with the amplitude envelope characteristics of a piano. Assuming initial
silence, the envelope of the first note struck would rise rapidly and then fall
slowly. Shortly afterward, the second note would be struck, but, since the
first note has not yet died away, there are now two simultaneous sounds. The
third note would increase the number to three and so forth. At some point,
the first note would be so faint that it could be ignored, but it is not clear
exactly how many simultaneous sounds were built up, although it is certainly
not more than the number of notes in the run.
A similar run on a synthesizer keyboard would create a very different
result even if the timbre and envelope of a single note was identical to the
previous instrument. In this case, the first note struck would start an envelope
identical to the previous case, but, when the second note was struck,
the pitch of the tone would immediately be updated to the second note's
frequency and a new envelope segment would be started. Thus, simultaneous
sounds are not created, which should be obvious since only one tone oscillator
was involved.
Theoretically, this could be corrected with the multitrack tape recorder
by recording each note of the run on a separate track, although such a
procedure would not really be practical. The problem could also be partially
solved by using a digital scanning polyphonic keyboard and enough modules
to create up to, say, eight simultaneous iridependenttones. Even if in the
ideal case there were more than eight simultaneous sounds, the effect of
abruptly terminating the first when seven others are still sounding would
probably go unnoticed. The important point is that such a need for large
numbers of simultaneous sounds may occur only rarely in a composition.
Either all of the equipment or effort needed for the worst case must be
available or the problem must be ignored.
In direct computer synthesis, a virtually unlimited number of simultaneous
sounds may be built up when circumstances dictate. The cost of this
capability is borne only when the event occurs. During other times, when the
sound is simpler, the ability to handle large numbers of simultaneous sounds
is free! This applies to other capabilities as well. The effect is like being able
to rent additional synthesizer modules by the minute from a company with
an essentially infinite inventory.
Programmability
The difficulty in controlling a number of simultaneous sounds to a
great degree by a single performer is also overcome in direct computer
synthesis systems. The control problem is actually one of time: during realtime
performance, there is simply not enough time available for the per-