| Résumé |
The control of directivity represents a new stake in the reproduction of the sound sources by electroacoustics devices. Nowadays, thanks to signal processing, current loudspeakers can respect quite well spectral and temporal properties of the signal. The remaining step is to account for spatial information – radiation – of the real sound source they intend to reproduce. In a larger frame, this research direction fits in the field of studies devoted to the spatialization domain, from a technological and perceptive point a view. The introduction, sums up a previous work that proposed a general method for reproducing the acoustical field radiated by a source, using a multichannel device controlled by digital signal processing. The general principle is to approximate a given directivity pattern by linear combination of the fields radiated by a set of loudspeakers laid out on the faces of a polyhedron. The introduction reminds mainly the following points : the experimental measurements, the theoretical hypotheses of the method and the optimization procedure that leads to the design of digital filters. This work, undertaken in parallel using simulations and prototype developments, showed the scientific and perceptive interest of the process with two main conclusions : first, the good agreement between numerical predictions and experimental results, and secondly, encouraging performances of radiation reproduction for several basic directivity patterns, although being limited to a restricted frequency range. The second part presents ongoing developments, putting the emphasis on signal processing issues. Studies are devoted, on the one hand, to the general improvement of the system (particularly, its bandwidth) and, on the other hand, to the optimization of the directivity control. This one can be measured in terms of perceptive relevance and real time processing that are significant characteristics when considering the integration of the method in a musical context. A new prototype has been designed, with six independent loudspeakers mounted on a cubic frame. The directivity control rests on the preliminary constitution of a set of basic directivities – namely the first spherical harmonics – with which it is possible to tune the cardioïcity and the three-dimensional orientation of the directivity pattern. Actually, this original approach, derived from the initial method mentioned in the introduction, raises new issues as regards to the digital signal processing. First of all, given the equally importance of phase and magnitude properties in the design of the directivity filters, different modeling approaches have been investigated, and their influence on the reproduction error is discussed. In other respects, the combination of the basic directivities is an efficient method to realize a continuous variation of the radiation pattern, for it avoids usual interpolation and commutation problems with filters. However, it leads to a new level of constraints, because it requires a design coherence – especially for the phase – of the whole set of filters related to each elementary directivity. Finally, together with usual computer cost optimization, foreseen applications during music performances impose real time processing constraints and the lowest latency. The third part is dedicated to the simulation of musical instruments. For this application, it appears significant to tune independently the spatial and spectral characteristics. In particular, listening in a room tends to average the spectral contributions radiated by the instrument in the different directions. Then, it seems crucial to develop an accurate method for analyzing and synthesizing the power spectrum. A practical procedure, based on room acoustics diffuse field theory, allows the automatic design of a correction filter to be applied on near field recording of the instrument in order to create a signal carrying the power spectrum information. Choosing this signal as an input for the multi-loudspeakers device allows to maintain a faithful power spectrum reproduction, while offering control on the directivity pattern. The results will be discussed by considering reproduction examples of several instruments (harpsichord, flute, human voice,…). The last part, reviews musical experience already acquired through the collaboration with a composer. The musical application is driven by software written in Max/MSP and includes several specific tools such as graphical interfaces or MIDI controllers that will be briefly described. Various esquisses consider different ways of playing with directivity : spatial counterpoint, chimerical instruments, duo between a real instrument and its realistic or distorted image, reverberation enhancement, real-time control by an instrument player, … The short term of this cooperation takes shape by involving this new sound diffusion system in a music performance programmed for summer 2001. |
