High Level Control and the Spat Oper

By associating the signal processing modules described in Chapter 3, a complete spatial processing chain can be constructed, starting from the captured or synthesized sounds to the distribution of the processed sounds to the loudspeakers.

For the musician or the sound engineer, it is preferable to use a control interface that is not simply the reunion of the low-level control interfaces of the different signal processing objects, but rather is made of a selection of high-level command controls (see figure). The Max graphical programming environment allows the user to build his/her own custom control interface, capable of sending control messages to the DSP objects, according to each object's control syntax.

Spat_OPer is a high-level control interface of this kind. Its role is to provide a reduced set of controls which describe the reproduced effect through quantities that are intuitive to the user and perceptually relevant from the point of view of the listener. The core of Spat_OPer is a perceptual control module based on research carried out in the Ircam room acoustics team on the objective and perceptual characterization of room acoustic quality [6, 10]. This control interface can be set to imitate the acoustics of an existing room and allows to interpolate or extrapolate naturally towards a different acoustic quality.

Furthermore, Spat_OPer can take into account measured data describing how the loudspeaker system, coupled with the listening room, will "filter" the output signals, observed from the reference listening position. These data will be used by an automatic compensation procedure to set the DSP parameters so as to ensure, as much as possible, that the specified acoustic quality be realized at this reference position.

The graphical user-interface in Spat_OPer is divided into four sections (see figure below):

• perceptual description of the acoustic quality,

• localization of the virtual source,

• radiation of the virtual source (directivity and orientation),

• configuration of the loudspeaker system.

Spat_OPer is a high-level control object (according to the definition given in Chapter 2) which can receive three types of remote control messages:

• High-level remote control: A remote control syntax is provided for each one of the sliders and number boxes which compose the Spat_OPer user-interface (the displayed positions or values of the sliders and number boxes are updated, unless this display is disabled by the user). These messages are translated to low-level control messages sent to the DSP objects.

• Measured data: These messages are also interpreted by Spat_OPer. However, they are not intended for continuous variation, but rather for loading data describing a configuration or acoustic quality (see section for details). These messages do not necessarily cause an update of DSP parameters or displayed values.

• Low-level remote control: Any incoming message that does not match the syntax for high-level control or for loading measured data is transmitted to the DSP objects with no modification. Some low-level parameters may have an equivalent in Spat_OPer, whose displayed value will be updated by the incoming message (if desired by the user). However, if other low-level control messages are received, it cannot be guaranteed that Spat_OPer will faithfully reflect the effect perceived by the listener.