IMG/1: An Incidental Music Generator

Abstract

Computer programs are available to aid musicians in the composition of music-e.g,, Music Mouse (Spiegel 1985); Jam Factory, M (Intelligent Music 1988); and others-but there are circumstances in which a person with little or no specific musical expertise (i.e., not a musician) needs to be able to produce simple musical works fitting a simple set of criteria. For example, a research chemist may want to assemble a videotape presentation showing how some new process works. There may be long sequences with no narration, e.g., while the solution changes color or the chart recorder runs. Perhaps someone is compiling a series of interviews that share a common title/credit sequence. In these sequences, some background or incidental music would be a perfect accompaniment and would lend continuity. The music required need not be very sophisticated or elaborate; it merely needs to have the right and be the right length. However, unless the project has a very large budget, or the person making the video happens to be a musician with recording facilities available, the resulting sound track is likely to be silence or something pirated from a commercial recording. Researchers have been exploring algorithmic composition for many years (Hiller 1970, 1981; Bolognesi 1983; Dodge and Jerse 1985; Loy 1989), but as Kemal Ebcioglu says in a recent article on harmonizing chorales, It seems that musical composition is a hard mental task that requires a substantial amount of knowledge, and any serious attempt to simulate 'noncomputer' music composition on the computer would have to face the task of constructing a formal model of considerable complexity. We have found that even the algorithmic representation of the knowledge underlying the seemingly simple Bach chorale style is a task that already borders the intractable. (Ebcioglu 1988) Recent work at the Bellcore research center (Langston 1986; 1988; 1989a; 1990) has produced short pieces that, although devoid of semantic content, adequately embody the syntactic structures of music and result in acceptable music. IMG/i (Incidental Music Generator number one) is a set of programs built on this work that extend it to address a range of musical styles. The central goal in IMG/1 is to allow musically-naive users to produce pieces of background music of arbitrary length using algorithmic composition techniques. Simplicity was an important design goal for IMG/1; the intended users, computerand music-novices, must be able to compose and play an original piece of music with little or no training. As a result, IMG/1 is extremely easy to use; with a few clicks of the mouse, it can compose and play an original piece of music. The user simply selects a musical style, sets the tempo, and types a length (in seconds) to determine the exact duration of the piece. Most people are more sophisticated in listening than in creating music (I don't know anything about making music, but I know what I like). Since IMG/i can generate arbitrarily-many variations, all different (and, thanks to the built-in musical knowledge, surprisingly pleasant), the user need only pick from among them. The dozens of parameters that provide detailed control of the composition process default to reasonable values if not set explicitly by the user, but may be manipulated as expertise with IMG/1 and its music increases. Melodies are generated by a set of algorithms with expert knowledge of various musical styles. The techniques employed range from probabilistic traversal of graphs to simulation of the mechanics of a specific instrument. In all cases, the melody generation uses some stochastic information (i.e., none of the algorithms are deterministic). Accompaniments are assembled by transforming a set of predefined fragments to fit the progression of the overall harmonic structlre. Although this scheme is deterministic, the gen ration of the harmonic structure is not; thus the accompaniment also changes for each piece (unless the user specifically asks to keep it fixed). Music generated by IMG/1 has been used as background music in videotaped technical demonComputer Music Journal, Vol. 15, No. 1, Spring 1991, C 1991 Massachusetts Institute of Technology.