Chapter 8
Musical Design I: Evolutions

The typescript for chapter 8 is provided in AutomatedComposition08.pdf.

Chapter 8 introduces evolutions, whereby the quantity controlling a compositional attribute can be allowed to vary over time. Each evolution is constituted as a sequence of segments, where a segment is defined by a starting time, an ending time, an origin, and a goal. Evolutions come in several computational modes, including linear and exponential; these types are distinguished by the method of calculating intermediate segment values. Within a linear segment, values increase or decrease by equal increments over equal units of time. Within an exponential segment, values change by equal ratios over equal units of time.

The implementation explained in the typescript is static and array based, with evolutions for different attributes share common segment starting and ending times. This static implementation would subsequently give way to a dynamically allocated framework, and I would come to substitute the term contour in place of evolution. For example the ASHTON score-transcription utility employed dynamically allocated contours with independent time coordinates to represent tempo, dynamics, and evolving custom parameters.

Historical precedents for composition using mathematical functions of time include Johannes Kepler's Harmonices Mundi and Joseph Schillinger's melody graphs. Two examples of evolutions implemented in composing programs would subsquently find publication in “Automated Composition in Retrospect” at the pages indicated: John Myhill's Scherzo a tre voce (p. 173) and Thomas DeLio's Serenade (pp. 179-180). A third example, James Tenney's Dialogue, is one of several pieces recounted in Tenney's own monograph “Computer Music Experiences”, pp. 28-31. The TENDENCY feature of Gottfried Michael Koenig's Project Two used linear evolutions to plot lower and upper bounds for a region of uniform probability; this would in turn drive the selection of supply elements.

John Myhill's composition Toy Harmonium, one of the suite of projects created for the U.S. Pavilion at Expo '85 in Tsukuba, Japan, used parameter graphs to control tempo, syncopation, part density (which indirectly determines register), and chroma. These controlling graphs were reproduced in Perspectives of New Music. Further implementation details about the composing program developed (to Myhill's specifications) by Leonard Manzara appear in the 1985 ICMC Proceedings. A Kurzweil 250 realization of Myhill's composition is available on this site in MP3 format.

The practical programming content of Chapter 8 was provided by Demonstration 6: Evolutions. This process illustrated the principle of top-down design in that contours describing the overall shape of the form as a whole were consulted to determine specific facts about content. Piecewise exponential contours control both average note durations (affecting tempo) and the ratio between maximum and minimum durations (effecting transitions from periodicity to free randomness). Piecewise linear contours control register. The program also employs two piecewise linear contours where it adapts G.M. Koenig's TENDENCY feature to control articulations.

© Charles Ames Page created: 2017-03-12 Last updated: 2017-03-12