BeatBender: Subsumption Architecture for Rhythm Generation Aaron Levisohn Simon Fraser University 250-13450 102 Ave. Surrey, British Columbia V3T 0A3 +1 604 782 7474 [email protected] ABSTRACT capacity for solving problems beyond human capabilities; they The development of artificial intelligence techniques have have produced paintings and music; they have outwitted the best allowed artist to develop computational artworks that are human chess players. While questions remain about the nature capable of autonomous creative behavior. Such artworks are of intelligence and how machine intelligence differs from now commonly referred to as metacreations. Unlike traditional biological intelligence, it is now an accepted fact that computers artistic methods in which the artist maintains control over the can exhibit intelligent behavior. The questions now being asked final form and representation of their work, these new methods seek to define the limits and possibilities of computational are more open ended. The artist’s role in development of a intelligence. metacreation becomes that of the programmer, developing a system that will ultimately produce the artworks themselves. Working with metacreations allows for the application of AI This paper presents a computer music project called BeatBender techniques in unusual and interesting ways. Unlike AI that explores a new method for generating emergent rhythmic researchers who are often focused on the development or drum patterns using the subsumption architecture developed by refinement of techniques to solve a specific problem or emulate Rodney Brooks. Rather than explicitly coding symbolic human biology, artists developing metacreation have more intelligence into the system using procedural algorithms, freedom to explore creative space. The incorrect application of BeatBender uses a behavior-based model to elicit emergent an AI technique can result in a Metacreation that is highly rhythmic output from six autonomous agents. engaging and aesthetically pleasing. And the results of such experiments often generate new ideas or techniques that can be Categories and Subject Descriptors shared by all AI researchers. Metacreations have utilized various AI techniques in General Terms accomplishing specific goals. Often, multiple AI techniques are used collectively to engender greater complexity or creative Keywords behavior. Artificial neural networks, autonomous agents, H.5.5 Sound and Music Computing: Systems cellular automata (CA), genetic algorithms/programming, as well as various artificial life techniques have all been utilized in 1. INTRODUCTION the production of metacreations. The development of artificial intelligence techniques have This paper presents a computer music project called BeatBender allowed artist to develop computational works that are capable that explores a new method for generating rhythmic drum of autonomous creative behavior. Such artworks are now patterns using the subsumption architecture developed by commonly referred to as metacreations. Unlike traditional Rodney Brooks. Rather than explicitly coding symbolic artistic methods in which the artist maintains control over the intelligence into the system using procedural algorithms, final form and representation of their work, these new methods BeatBender uses a behavior-based model that facilitates are more open ended. The artist’s role in development of a emergent expression. Subsumption architecture systems are metacreation becomes that of the programmer, developing a designed to respond to stimuli in the environment using a system that will ultimately produce the artwork itself. hierarchical set of rules. Depending on the state of the environment, different rules of varying complexity are invoked, The development of a metacreation has research goals as well as generating the behavioral output of the system. Due to the artistic ones. In particular, metacreations are often designed to design of BeatBender as a closed system, the environment is explore a particular aspect of human or computer creativity. In virtual and is embodied in the state of the system. this regard, the development of metacreations bridges the artistic and scientific world, providing insights into both the biological The BeatBender system is comprised of six autonomous agents, and cultural aspects of creativity. each responsible for the production of one beat of every measure in an ongoing rhythm. Each agent is designed to react The ability of computers to exhibit intelligent and creative individually to the state of the system at different, but sequential, behaviors is no longer questioned: they have demonstrated their points in time. By encoding different rules into each layer of the subsumption architecture, emergent behaviors are elicited 2.2 Prior Work resulting in varied rhythmic output. This paper describes the As a metacreation, BeatBender positions itself within two design of the BeatBender system, and the explorations different disciplines. First, BeatBender must be situated among undertaken using the subsumption architecture to generate works that explore rhythm generation. Second, it must be rhythms that exhibit emergent characteristics. situated among general electronic music applications that utilize AI techniques. 2. BACKGROUND Research on rhythm generation is not a new subject in art or 2.2.1 Rhythm Applications research, however, the subsumption architecture has never been Rhythm generation applications take various forms and serve a applied as the foundation for such a system. The subsumption variety of purposes. In general, most of these types of architecture is well suited for rhythm generation since the applications are designed to produce rhythms as accompaniment articulation of beats does not require any form of symbolic to a human performance. One example of this is Haile, the representation. Using this architecture allows autonomous anthropomorphic robotic percussionist designed by Weinberg agents in a closed system to exhibit emergent behavior. and Driscoll [4]. Haile is capable of both mimicking and Together, these agents are able to generate rhythmic patterns collaborating with a live human performer. One of Weinberg’s similar to those produced by human participants in a drum primary goals was the implementation of a system that would circle. allow Haile to develop meaningful representations of music being listened to. Haile uses complex analysis software to both 2.1 Subsumption Architecture establish the characteristics of the live performance and to The use of the subsumption architecture as a means of eliciting compute rhythmic output. One of the benefits of subsumption emergent behavior in robots was introduced by Rodney Brooks architecture is the significant reduction of computational load in his seminal paper “Intelligence without Representation” [1] since there is no need for symbolic representation. Complex Rather than utilizing explicitly programmed algorithmic behavior can be elicited using simple behavior-based rules. solutions, subsumption architecture systems focus on developing sets of behaviors; these behaviors are implemented as layers, Several systems, including Eigenfeldt’s Kinetic-Engine [5], with some layers given priority over others. Low level layers are utilize agents as a means of representing individual drummers given the lowest priority and usually invoke non-critical within a composition. Like BeatBender, Kinetic-Engine is a behaviors. Higher level layers generally invoke behaviors that metacreation and requires no real-time input from a user. are more complex and often vital to the functioning of the Kintetic-Engine uses networked agent architecture to emulate a system. Specific layer behaviors are often selected based on percussive ensemble. When activated, the system assigns environmental data gathered using sensors; however other data “personalities” to the agents who collectively personify the can be used as well. human elements in a drum circle. Eigenfeldt’s agent system requires a much higher level of complexity than systems While the subsumption architecture methodology has been utilizing the subsumption architecture. Kinetic Engine uses a primarily utilized in the field of robotics it is equally applicable social model of agent interaction to emulate the behavior of live to other endeavors in which emergent system behaviors are human performers. Agents “make eye contact” with one another desired. For example, Bryson et al used the architectures in the and, once connected, adjust to each other’s performance. In development of a musical accompanist [2] and Nakashima and addition, a special agent type called a conductor is used to Noda incorporated it into their design of intelligent agents oversee high level organizational elements. Eigenfeldt claims capable of playing soccer games [3]. While the subsumption that this level of complexity is necessary in order for a system to architecture has been applied in musical systems, it has been be “musically successful.” He cites Brown’s work [6] with solely toward the production of tonal output. This paper presents rhythm generating cellular automata as evidence of this. BeatBender, an exploratory project in which the subsumption BeatBender explores emergence as an alternate means of architecture was implemented for the purpose of exploring generating musically successful rhythmic output. emergent rhythm generation. Pachet also developed a multi-agent system for generating and The subsumption architecture is particularly suited to the