of Microprocessors

640 MUSICAL ApPLICATIONS OF MICROPROCESSORS
SCORE
OURATION
LEVEL 5
SCORE
SECTIONS
LEVEL 4
NOTE
TEMPO
LEVEL 3
ENVE LOPE ,-- -L- ----,
SAMPLE
RATE
LEVEL 2
SOUND
SAMPLE
RATE
LEVEL I
SOUND
SAMPLE
RATE
LEVEL 0
Fig. 18-1. Music software system hierarchy
and, in most respects, more difficult than real-time performance usmg
external synthesis hardware.
Organization of Music Software Systems
Like most nontrivial software systems, a music synthesis software system
is organized into a hierarchy of programs and functions. Figure 18-1
illustrates that at least five distinct levels can be identified ranging from the
lowest level sample-by-sample computations to the highest level operating
system functions. The levels are distinguished by the time scale at which
they operate. The lower levels operate in the micro- and millisecond terms of
individual waveform and envelope samples, while the higher levels operate
on the scale of seconds associated with notes and phrases in the musical
performance. Information flow is generally from the higher levels to the
lower levels, although in some cases signaling in the opposite direction may
be done.