CCRMA OVERVIEW - CCRMA - Stanford University

6.2.4 Parameter Manipulation for Composing with Physical Models
Abstract
Juan Reyes
The problem with Physical Models and their appeal to composers is not merely perceptual or aesthetic.
Furthermore is not a question of understanding the physics and parameters of the actual instrument. It
is a question of achieving musical textures, realistic articulations and extending the qualities of a family
of sounds given by a single characteristic timbre in a Physical Model of an instrument. This can be
achieved modeling and manipulating expression parameters for musical gesture. This paper describes
some composition techniques for rendering a Computer Music piece using Physical Models as the primary
source technique for sound synthesis in non real time environments and parameter manipulation by means
of envelopes, randomness and chaotic signals for expressiveness.
Introduction
In this paper. Musical Expression is the synthesis of meaningful musical gestures which are usuallycategorized
on the frequency domain as vibratos, trills, grace notes, appogiaturas, dynamic changes, etc.
Gestures on the time domain are tempo, rubato, rhythmic patterns and durations. The articulation
and phrasing of a sequence of notes depends upon combinations of gestures in the time and frequency
domains. A performance of a musical event is the relation among the Physical Model of the instrument,
a set of rules which describe a sequence of sounds plus their expressive parameters to play the notes.
Expressiveness can be applied on a single note basis or to a whole sequence of notes as in musical phrase.
Synthesis of Expression
In the composition context, real time is not an issue instead creative ideas are seduced by various degrees
of freedom and parameters in the Physical Model algorithm. Although a traditional representation of
the acoustical instrument is highly desirable, a non conventional behavior gives new fresh possibilities
impossible in the real world by morphing the traditional sound of the instrument into different wave
shapes or variations [Smith, 1996]. In order to obtain this desired control over the rendering of a composition,
all of the expressive parameters need to be computed a priory therefor providing a score file
which is later combined with the synthesis of the modeled acoustics of the sound. These parameters
can be computed manually or automatic by means of envelopes, functions or algorithms. Several applications
of automatic parameter manipulation by mathematical functions in the fields of Randomness,
Chaos, Markov Models, Bayesian Nets and Neural Nets provide a high degree of variety for musical
expressiveness.
Expression Modeling
A practical and flexible interface for rendering meaningful musical expression is part of most sound
synthesis software packages. In our research, we have used the Common Lisp family of Computer
Music Composition programs developed by Bill Schottstaedt and Rick Taube at CCRMA and known as
Common Lisp Music (CLM) , Common Music and Common Music Notation. These programs provide
a very effective connection between the Physical Model specifications and Algorithmic Composition. In
these environments Expression Modeling can be an optional part of an algorithmic composition or it
can be a separate process. Physical Models of the flute, piano, plucked string, clarinet and maraca have
been used for this purpose.
Expression and Random or Chaotic Behavior
Given that acoustic sounds have a tendency to behave as random or chaotic systems, it seems natural to
manipulate parameters in this way in order to achieve variety in the synthesis of a musical gesture. When
randomness and chaos are applied to spectral parameters they produce a characteristic aggregate noise
found in woodwinds or bowed instruments [Chafe, 1995]. When applied to a sequence of pitches, they
give elements of unpredictability and constant change [Sapp, 2000]. Randomness depends on probability
distributions for obtaining a parameter or value. In musical applications we want discrete or integer
values to map to a sequence of notes.
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