- Description-Based Design of Melodies
Most current approaches in computer-aided composition (CAC) are based on an explicit construction paradigm: users build musical objects by assembling components using various construction tools. Virtually all technologies developed by computer science and artificial intelligence have been applied to CAC, thereby progressively increasing the sophistication of music-composition tools. Composers can choose between many programming paradigms to express the compositions they "have in mind," from the now-standard time-lined sequencers (e.g., Steinberg's Cubase) to advanced programming languages or libraries (e.g., OpenMusic; Assayag et al. 1999).
Although these explicit constructions do benefit from abstractions of increasing sophistication (objects, constraints, rules, flow diagrams, etc.), CAC always remains based on an explicit construction paradigm: Users must give the computer a clear and complete definition of their material. This approach has the enormous advantage of letting users control all dimensions of their work. However, it also requires from users a fine understanding of the technicalities at work. For instance, composing music with object-orientation requires the understanding of objects, classes, and message passing. Using constraints requires the understanding of constraint satisfaction, filtering, and of the basic constraint libraries, etc.
An interesting attempt to escape these technical requirements is the Elody system (Letz, Orlarey, and Fober 1998) in which the user can create arbitrary abstractions by selecting musical material together with a specific dimension of music (e.g., pitch or rhythm). These abstractions can then be applied to other musical material to create yet more complex objects. But here again, the user must mentally maintain a model of the abstraction algorithm at work, a task that can be particularly difficult as the complexity of the composition grows. Other approaches propose construction tools that do not require explicit programming skills. For instance, Hamanaka, Hirata, and Tojo (2008) propose a morphing metaphor in which melodies can be created as interpolations between two given melodies. But this approach is limited to the context of the generative theory of tonal music (Lerdahl and Jackendoff 1983), and is not extensible to arbitrary categories, as we will show later.
We propose here a novel approach to music composition called description-based design that attempts to remove the need for the user to understand anything technical related to the target objects. In this article, we focus on the creation of simple musical objects—unaccompanied melodies—as a working example, but our paradigm is general and can be applied to many other fields of design.
First, we introduce the general description-based design mechanism, and then we describe the type of melodies we target. Finally, we describe experiments demonstrating the functionality of the algorithm and its potential.
Description-based design stems from the paradigm of Reflexive Interaction (Pachet 2008). The idea is to let users manipulate images of themselves, produced by an interactive machine-learning component. The creation of objects (musical objects in our case) is performed as a side-effect of the interaction, as opposed to traditional interactive systems in which target objects are produced up-front as the result of a controlled process. A typical example of reflexive interactive system is the Continuator (Pachet 2004), a system that continuously learns stylistic information coming from the user's performance and generates music "in the same style" in the form of real-time answers to, or continuations of, the music performed by the user. The Continuator was shown to trigger spectacular interactions with professional jazz musicians (Pachet 2004) as well as with children (Addessi and Pachet 2005) involved in free, unstructured improvisation. However, this type of interaction shows limitations when users want to structure their production—in other words, [End Page 56] when they want to shift from improvisation to composition.
Description-based design adds a further component to the Continuator-like interaction by introducing an explicit linguistic construct, precisely aimed at addressing this "structure" problem inherent in free-form improvisation systems. The idea is as follows. In the first phase, the system generates objects—melodies, in our case—randomly or according to specific generators. The user can then freely tag these objects with words, "jumpy," "flat," "tonal," "dissonant," etc. Each object can be tagged by several...