of Microprocessors

690 MUSICAL ApPLICATIONS OF MICROPROCESSORS
the next line, follow. Parameters probably should not be split across line
boundaries (whether this is a requirement depends on the interpreter implementation),
and besides, splitting makes the statement hard to read.
Parameters that specify a duration are always in units of milliseconds.
Parameters that specify an amplitude are in terms of percent. Depending on
the parameter type, the values of related parameters may have to add up to
100% or less. With other parameter types, the percentage figures merely
establish ratios and can add up to any value, although individual parameters
must be less than 100%. With these conventions for numerical parameters,
integers can normally be used; however, a decimal point and fractional part
can be tacked on for additional precision.
Figure 18-21 shows an example voice statement and a list of the
keyword parameters available. The number immediately following the
VOICE keyword is the 10 number for the voice. Note statements using the
voice will specify the 10 number. During the course of the music, a
particular voice number may be redefined as often as desired. There is no
restriction on the numbers that can be used.
The AD, DO, and RD parameters are used to specify the durations, in
milliseconds, of three phases of the ADSR envelope. The duration of the
sustain phase depends on the note being played. The first letter identifies the
envelope phase and the D is a mnemonic for duration. If a parameter is
omitted, zero (actually 1 msec) is assumed. SA refers to sustain ampJiwde
and gives the sustain level relative to the peak overshoot of the attack. Note
that the percent figure refers to voltage amplitude; thus, a specification of 50
would give a sustain level 6 dB below the attack peak. Omissi/on of this
parameter gives a default of 100%.
The voice amplitude (VA) parameter is very important. Its basic function
is to specify the amplitude of the voice relative to that of other sounds.
The importance is due to the requirement that the sum of all of the voice
amplitudes of all simultaneous sounds must not exceed 100% if total freedom
from overflow is desired. This means that if at some point in the musical
score 15 simultaneous sounds have been built up, the VA parameters associated
with each sound should not add up to more than 100%. Observing
this rule will virtually guarantee (since the filtered nois() used in percussion
instruments is random, it cannot be absolutely guaranteed) that overflow will
not occur when the sounds are summed in the sample buffer. In most cases,
with a lot of simultaneous sounds, the amplitude sum could substantially
exceed 100% without undue risk of overflow because of the low probability
of envelope and waveform peaks all coinciding at the same time.
The remaining parameters define the four waveforms associated with
the four envelope phases. These are organized into groups of parameters. A
group starts with a keyword, such as AW for attack wave, followed by an
equals sign, which signals that a group of parameters follows. Within the
group may be several parameter subgroups, each of which corresponds to a