of Microprocessors

SOME REAL ApPLICATIONS 749
name followed by several parameters, which specify where the inputs come
from and where the output goes. Available unit generators include several
kinds of oscillators, amplifiers, mixers, several different filters, delay lines,
mathematical operators, nonlinear distorters (table lookup elements), noise
generators, and the content of existing sound files. As many unit generators
as desired may be combined in an instrument definition, and, since 32-bit
floating-point arithmetic is used, error accumulation is generally not a
concern.
Figure 19-15 shows a schematic representation of the OSC unit
generator along with a prototype OSC statement. The statement begins with
the keyword, OSC, followed by the name of an I/O block that will receive its
output. I/O blocks are different from constants and parameters and are used
to pass sample values among the unit generators in an instrument. The
reason they are special is that signal samples are calculated in blocks to
improve computation speed. I/O blocks are identified as B1, B2, ... up to
the limit of memory. The second and third fields specify the output
amplitude and the increment for the table lookup pointer, respectively. Each
may be parameters from note statements (P-type variables), V-type variables,
constants, or other I/O blocks. The resulting amplitude value simply
multiplies sample values read from the wavetable. The frequency of the
oscillator is I*SR/FL, where I is the resulting increment value, SR is the
prevailing sample rate, and FL is the length of the wavetable. The next field
specifies which wavetable to use. Wavetables are created later in the functiongeneration
section and are identified by number. Like the amplitude and
increment fields, variables may be used to specify the function number to be
used.
The last field effectively specifies the initial value of the waveform table
pointer. If the letter D (for "dynamic" variable) is used, then the initial value
is zero. Use of a P-type or a V-type variable allows the oscillator's initial phase
to be controlled. Internally, a temporary storage location is allocated to hold
the wavetable pointer when a note using the oscillator starts and is freed
when the note ends. Every time a note starts, a new storage location is
allocated so an instrument may play any number of notes simultaneously.
Also shown in Fig. 19-15 are some valid OSC statements. Example 1 is
the simplest possible OSC statement. It outputs to I/O block number 1, uses
function number 1 for its wavetable, and always starts its waveform at the
beginning of the table. The amplitude of the note is specified by the fifth P­
field in note statements that play it, and frequency is specified by the sixth P­
field. Example 2 is similar except that the wavetable to be used may be
specified in the note statements rather than being constant in the i,nstrument
definition. In example 3, the amplitude is the content of another I/O block
from a previous statement that presumably has samples of an envelope shape.
The same can be done for frequency as in Example 4 in which the content of
B1 sets the instantaneous frequency.