What's
in a polymetric expression?
May
be you liked the way this item sounded (on a "Koto" patch of the D-50, or
vibraphone in QuickTime music) and want to understand its time structure.
Click the "Data" window (or type cmd-d) and select the item with the mouse.
Then click button "Expand display
". You get this:
Fig.17 Expanding
a polymetric expression Expressions
with curled brackets {} and commas are called
polymetric
expressions
and represent polyphonic sound-object structur
es.
These expressions are incomplete descriptions of the actual precedence
relationships between sound-objects and/or notes in the structure. BP2 is able
to construct a
complete
polymetric expression
by adapting tempos in superimposed sequences. (This process is explained in
great detail in Bel 1990a-b,1991,1992.) The expanded display shown Fig.17 is
an outcome of the expansion algorithm.
BP2
expands
any
polymetric expressio
n
(including a simple sequence) before making sounds. Once the expression has
been expanded, the following score-like
phase
diagram
is constructed:
in
which it is clear, for instance, that the first occurrence of 'f' begins with
the first occurrence of 'a'.
Symbols
'_' prolongate sound-objects or notes. For instance, the
symbolic
duration
of the first occurrence of 'b' is three time units whereas the last occurrence
of 'f' is five time units. (In this particular example, a time unit is one
third of a beat.)
Symbols
<<f>>, <<chik>> and <<sync>> represent
out-time
sound-objects,
i.e. objects with no duration. Here for instance (from the information found
in the "-mi.abc1" file), <<f>> will be a NoteOn on key F4 followed
immediately with a NoteOff. <<chik>> is a chord plucked in the
same way. It is only audible if the NoteOff instruction does not suppress
sound immediately. Therefore we used a "plucked string" patch.
Out-time
sound-object <<sync>> produces a C5 on MIDI channel 15 that does
not make any sound but may be used to synchronise another BP2 running on the
same MIDI network (see §6.4).
Performance
controls
_vel(127) and _volume(40) are attached to the next object or silence appearing
in the same sequence, and are not displayed on graphics. Remember that you can
attach very subtle variations of all MIDI parameters, notably pitchbend,
modulation, aftertouch, to any structure of sound-object or/and simple notes.
(See §1.4-10 regarding performance controls.)
Many
polymetric
expressio
ns
are found in project "-gr.polyphony
1".
You
may select only part of the item (even a single terminal symbol) and listen to
the way it sounds (type cmd-p). Try to figure out the difference between
"striated" and "smooth" time by carefully looking at the graphics these
different settings generate. (See §9 regarding smooth tim
e.)