Page  00000002 Jansen (1997) derived mathematical formulae for synthesising moving sound sources. He took into account the Doppler effect and showed that for its application one would need to have continuously time-varying delays. He also showed that for slow moving sources the Doppler effect is negligible and one can resort to updating locations and calculating filters for each location and changing those in time. This approach was chosen in this project. Additionally, in order to avoid clicks in playback, an option was built in to crossfade between two locations to make the movement sound smoother. 3 System setup at the TU Berlin The prototype system in Berlin was created with the specific aim to make a system for the use in electronic music (Weske 2001). The system consists of a LINUX PC, driving 24 loudspeakers with an RME Hammerfall Soundcard. allows composers to define the movements of their sounds, independent of the system on which it eventually will be played. That is, the composer should be bothered as less as possible with the actual calculations for each loudspeaker, but instead be able to focus on defining paths through space for his sounds. The current version of the program, WONDER (Wave field synthesis Of New Dimensions of Electronic music in Realtime), allows the composer to do so. It allows the composer to work in two ways with the program: either he creates a composition of all movements of all the sound sources with WONDER, using the composition tool, or he defines a grid of points that he wants to use in his piece and controls the movement from another program using the OpenSoundControl protocol. The main part of the program is the play engine which can play the composition created or move the sources in realtime. Each part of the program will be discussed separately in the next subparagraphs. The array configuration can be set in the program. It is possible to define the position of various array segments through a dialog. WONDER includes a simple room model for calculation of reflections. The user can define the position of four walls of a rectangular room, an absorption factor and the order of calculation. The calculations are done with the mirror image source model (see also Berkhout 1988). 4.1 Composition tool The user can define various sources, each with their own characteristics. A source in this context is the virtual source from which sound emanates in space and whose spatial parameters can be given by the user. For each source the user can set the type of source (a point source having a specific location or a plane wave having only a direction), whether it is moving or stationary, its location or angle, the sound input channel at which the sound will be supplied and in the case of a point source, whether high frequency damping of the air has to be taken into account and whether reflections have to be calculated or not. If reflections have to be calculated, room characteristics can be defined (these can be different for each source and even vary in time). In the case of a moving source, one can define a path through space and choose to let the movement loop along the path. All input can be either typed in or drawn with the mouse on a graphical overview. In figure 4 a screenshot of the source definition dialog is given. After supplying all information and storing it, WONDER checks the input for any problems and after that the user can make the calculations for the filters. He can then store the filters for all positions calculated and a score for playback. Figure 3. The loudspeaker array in the studio at the TU Berlin. For the calculation (in real time) of the loudspeaker signals the program BruteFIR by Torger2 is used. This program is capable of making many convolutions with long filters in realtime. The filter coefficients can be calculated with the interface software described in this paper. With the current prototype system it is possible to play a maximum of 9 sound sources with different locations in realtime, even when the sources are moving. This is the maximum amount of sources; the exact amount of sources that can be used in a piece depend on the maximum filter length used. A detailed overview of the capacity was given in last year's paper (Baalman 2003). 4 Interface software In order to work with the system, interface software was needed to calculate the necessary filter coefficients. The aim was to create an interface that 2 Torger, A., BruteFIR, http://www. Proceedings ICMC 2004

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