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Must Procedural Audio
be real-time?
understandings of production I use the terms prior,
concurrent and deferred computation. Those com-
ing from a computer animation background, such
as in the work of Zheng and James at Cornell, and
Moss and Yee at North Carolina, seem to embrace
the concurrent model in which realistic sounds are
a side effect of realistic physics based animation.
However, a fuller definition of procedural and
behavioural audio would add the requirement
of real-time computation and imply something
further about delivery as well as generation.
Procedural audio differs from longer established
computer music in several ways. It tackles the
general case of sound synthesis, not only those
sounds which are musically pleasing. It is real-time
and interactive. And it anticipates deferred form
(and unexpected interactions) as a philosophy. A
concurrent approach does not handle non-diegetic
sources, high energy events or, at present, more
complex systems than rigid body collisions and
simple fluid dynamics. We put the full power of
procedural audio to use in the context of computer
games when we extend it to the general case all
natural sounds in an unpredictable environment.
It is for live installment (arts and theatre), virtual
reality, or games that sound design with proce-
dural sound objects comes into its own because
we can make parametric decisions in real-time,
which means we can create sound for situations
that have not been planned in advance.
This way of thinking is a departure from con-
temporary sound design á la Hollywood. In some
ways it returns us to the era before Foley, of the
live theatre sound effects artist employing props:
a reactive, adaptive and intelligent sound maker.
So, as for nomenclature, we might better employ
the established expression “synthesised sound” for
prior media like music and film, while reserving
“procedural audio” for deferred applications with
a real-time element in which sound is created in
the moment, as required. In prior media, the use
of the word “sound” implies it has already been
realised (concretised) as a signal, while “proce-
dural audio” is still code (potential sound). You
Loose definitions of procedural audio might
omit the requirement for real-time computation
because behaviour and process can be described
and computed in an offline system. In some
ways the boundary between what is offline and
real-time is only a matter of degree, the amount
of CPU power available. In other ways there is a
stark discontinuity in the temporal design of some
models that means they cannot cross the divide. An
example is found in the Csound implementation
layer, based originally on an offline musical score
model in which event durations are given a priori .
Although advances in machine technology now
allow real-time deployment, sound objects must
be structurally redesigned to incorporate “on the
fly” MIDI events. The underlying Csound model
requires different operations for each approach,
which cannot be mixed. One must therefore ap-
proach a Csound design with this in mind at the
start.
For film, pre-recorded radio and other static
media, parameters are applied up-front, then the
sound is recorded. Computer sound has been used
in this way for decades, and indeed this inherits
from the MUSIC-V lineage, which includes
Csound. For computer sound, this rendering
predates similar CGI concepts by a long time. A
slightly different use is also possible. Where the
synthesis parameters accompany a visual time-
line, such as in animation, and where sufficient
offline computing power is available, we have a
powerful editing system in which changes to the
visual elements are automatically reflected in
the audio track, which is rendered along with the
images. This changes the temporal status of what
is traditionally post -production, because sounds
don't have to be created after the picture but are
designed as part of it, or even beforehand. Here is
a subtle change in the use of the technology rather
than the deeper nature of the technology itself.
To distinguish these (past, present, and future)
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