Structured interactive scores: From a structural description of a multimedia scenario to a real-time capable implementation with formal semantics

By Mauricio Toro-Bermudez,
Université Bordeaux I – Ecole doctorale de mathématiques et d’informatique
September 2012


Technology has shaped the way on which we compose and produce music: Notably, the invention of microphones and computers pushed the development of new music styles in the 20th century. In fact, several artistic domains have been benefiting from such technology developments; for instance, Experimental music, non-linear multimedia, Electroacoustic music, and interactive multimedia. In this dissertation, we focus on interactive multimedia.

Interactive multimedia deals with the design of scenarios where multimedia content and interactive events are handled by computer programs. Examples of such scenarios are multimedia art installations, interactive museum exhibitions, some Electroacoustic music pieces, and some Experimental music pieces. Unfortunately, most interactive multimedia scenarios are based on informal specifications, thus it is not possible to formally verify properties of such systems. We advocate the need of a general and formal model.

Interactive scores is a formalism to describe interactive multimedia scenarios. We propose new semantics for interactive scores based on timed event structures. With such a semantics, we can specify properties for the system, in particular, properties about traces, which are difficult to specify as constraints. In fact, constraints are an important part of the semantic model of interactive scores because the formalism is based on temporal constraints among the objects of the scenario. We also present an operational semantics of interactive scores based on the non-deterministic timed concurrent constraint (ntcc) calculus and we relate such a semantics to the timed event structures semantics. With the operational semantics, we formally describe the behavior of a score whose temporal object durations can be arbitrary integer intervals.

The operational semantics is obtained from the timed event structures semantics of the score. To provide such a translation, we first define the normal form of a timed event structure in which events related with zero-duration delays are collapsed into a single one. We also define the notion of dispatchable timed event structures: Event structures such that its constraint graph can be dispatched by relying only on local propagation.

We believe that operational semantics in ntcc offers some advantages over existing Petri nets semantics for interactive scores; for instance, the duration of the temporal objects can be arbitrary integer intervals, whereas in previous models of interactive scores, such durations can only be intervals to represent equalities and inequalities.

In this dissertation, we also introduce two extensions of the formalism of interactive scores: (1) one to handle audio processing using the Fast AUdio Stream (Faust) language and (2) another one to handle conditional branching, allowing designers to specify choices and loops. For the first extension, we present a timed event structures semantics and ideas on how to define operational semantics. For the second extension, we present an implementation and results comparing the average relative jitter of an implementation of an arpeggio based on Karplus-Strong with respect to existing implementations of Karplus written in Pure Data. We also define a XML file format for interactive scores and for the conditional branching extension. A file format is crucial to assure the persistence of the scores.

Ntcc models of interactive scores are executed using Ntccrt, a real-time capable interpreter for ntcc. They can also be verified automatically using ntccMC, a bounded-time automata based model checker for ntcc which we introduce in this dissertation. Using ntccMC, we can verify properties expressed on constraint linear-time logic. Ntcc has been used in the past, not only for multimedia interaction models, but also for system biology, security protocols and robots. We argue that simulation and verification tools for ntcc are useful for interactive score and they are also useful for applications in other domains.