of Microprocessors

642 MUSICAL ApPLICATIONS OF MICROPROCESSORS
For example, amplitude envelope needs are often satisfied by the ADSR
shape, which can be completely described by five parameters. It is the
responsibility of a Level 2 routine to generate samples of the shape as needed,
given the parameters, and then pass these samples as parameters to the Level
1 routines. A different Level 2 routine may produce arbitrary contour shapes
using a piecewise linear or curvilinear approximation. With such approximations,
the higher-level routine specifies segment endpoints, the amount of
time spanned by the segment, and the segment shape (linear, concave,
convex, etc.). Yet another might interpolate between tabulated samples of a
shape that could be the result of a source-signal analysis, for example.
Level 3 routines comprise the bulk of the coding in the system. Their
job is to accept music language statements, which are character strings,
extract the information contained in them (or flag an error if the statement is
incomprehensible), and supply parameters to Levels 1 and 2 routines. It is
also responsible for setting up waveform and envelope shape tables, if used,
in response to voicing statements in the score. It is hard to pin down the
exact rare of execution for a Level 3 routine, but it is generally at the rate that
notes are played. This is often called the "syllabic rate" from the corresponding
level in the hierarchy of routines that comprise a typical speech synthesis
system.
Level 4 programming is a superset of Level 3. In most musIc, there Is a
considerable amount of repetition that when added to the many conventions
of standard music notation results in a conventional score much shorrer than
the corresponding computer score expressed as character strings. The concept
of macroinstructions is useful in taking advantage of these properties of the
score to reduce its size. Basically, a macroinstruction is a command to the
maeroexpander (the Level 4 routine under consideration) to convert a
statement such as "repeat the following three-note sequence eight times" into
eight repetitions of the three-note statement, which are then interpreted by
the Level 3 routine just as if the user had actually written the sequence eight
times. Most macroexpanders allow loops, conditional tests, counters, and
procedure calls (subroutines), suggesting their use as a composition tool as
well.
Even though they are not really part of the music software system,
operating system functions on LevelS have been included for completeness.
These are the routines that allow the user to prepare arrd edit the score file,
link and load the music system programs into memory, and control the
reading of the score file and writing of the sample file in a delayed playback
system. Depending on the installation, the operating system may range from
a paper tape editor and loader to a full disk operating system. In a loosely
structured music software system, the operating system is vital in its role of
maintaining the data base.
Implementation of the Leoels
Current thinking in computer science dictates that each of these levels
be independently implemented as a separate program and linked together