Measured Data

When an electroacoustic system and a spatial processor are used, the source signal either comes from a live source through a microphone pickup, or it comes from a synthesizer or sampler. In the latter case, the global (artificial) acoustic quality perceived by the listener is simply defined by the cascade of two filters: the spatial processor itself and the transformation of the reproduced signals due to the frequency response of the loudspeakers and their coupling with the listening room (which depends on their directivities, positions, and orientations). In the live case, an additional input filter is added (associated to the sound capture), and the natural acoustic quality (due to the live source radiating in the listening room) is superimposed on the artificial acoustic quality.

For both applications, the spatial processor should integrate a compensation procedure ensuring that the DSP parameters will not be directly derived from the specified acoustic quality, but will be set so that the combination of the above effects produces, at the listening position, the desired acoustic quality. When this is possible, it means that spatial effects defined at the composition stage of a musical work can be preserved irrespectively of the loudspeaker setup, sound capture technique, or listening room where the work will be performed.

An approximate "deconvolution" procedure has been implemented in Spat_OPer for this purpose. Spat_OPer can receive different types of control messages containing data which will configure this procedure (most often, these data will be derived from impulse response measurements made at the reference listening position):

• context: This data describes the acoustic behaviour of the reproduction system coupled with the listening room (derived from the acoustic quality produced by each loudspeaker at the reference listening position).

• live : This data describes the acoustic quality naturally produced by a live source in the listening room (along with the source-to-microphone transfer function). In this case, Spat~ is used as an elaborate sound reinforcement system for modifying the natural acoustic quality of the sound source (including amplification of the direct sound and/or reverberation enhancement).

• target : This data describes the desired acoustic quality. The automatic deconvolution procedure will compensate for the context acoustic quality and the live acoustic quality to compute appropriate values of the DSP parameters in order to achieve the target acoustic quality.

A target message causes an update of the DSP parameters (taking into account the context and live data provided that these have been loaded previously) and of the values of the perceptual factors displayed in the perceptual control interface. A measured acoustic quality can alternatively be loaded by sending a series of messages to Spat_OPer in order to update the value of each perceptual factor (but each message then causes DSP parameters to be re-evaluated and sent whereas a target message is equivalent to updating all factors at one time).

Experimental measurement and analysis hardware and software tools which allow to perform the necessary measurements and convert them to control messages for Spat_OPer exist at Ircam. Their integration into the Spat~ library is planned for future versions.