Technology
Digital Directivity Control (DDC)


What is DDC?
Digital Directivity Control is a parametric, electronic beam control concept. DDC-driven Intellivox-type loudspeaker arrays show a strong, almost frequency independent, directional behavior. This results in a uniform audience coverage, even over very long distances (up to 80 m).

The far field directivity pattern can be controlled by a number of beam parameters: 'opening angle', 'elevation angle' and 'focus distance'. So, the 'throw' and the aiming of the beam can be changed electronically, i.e., without any mechanical adjustment of the array.

The on-board DSP hardware takes care of the required signal processing. Apart from the DSP, each unit is equipped with a micro controller that takes care of all the surveillance routines, DSP management, storage and logging and RS-485 network functionality. All beam and surveillance settings can be configured using the WinControl software.


In order to realize a frequency independent main lobe and also to avoid grating lobes, the following criteria should be met:

The effective length of the array must be proportional to the wave length, in formula:

The distance Dz between adjacent loudspeakers must be smaller than half the wave length (i.e. spatial Nyquist-criterion):

It can be shown mathematically that combining both requirement results in a special non-uniform ('logarithmic') positioning scheme of the transducers. Note that the minimum distance between the loudspeakers in a line array is limited by the diameter of the loudspeakers, which means that the second condition cannot always be fulfilled for high frequencies in practice. Applying this patented positioning scheme (in the larger models of the Intellivox series), reduces the total number of loudspeakers channels that is needed for a given frequency range.

Since the main lobe can be steered electronically, i.e. without the need of physical aiming, the array can be mounted vertically, even flush-mounted. This will allow an easy integration into many architectural environments. Vertical mounting also has an acoustic benefit. The backward radiated energy of the loudspeaker array is reflected in the same direction as the direct sound and will contribute to the direct sound pressure level in the intended listening area. In case of a mechanically aimed array, the backward radiated energy would be reflected upwards (towards the ceiling) as shown in Fig.1. So, using an electronically aimed array the level of detrimental (late) reflections can be significantly reduced, yielding a better speech intelligibility and clarity.


The beam parameters determine the coefficients of the output filters and delays which control the shape and the aiming of the main lobe respectively, as shown in Fig.2.

MAIN LOBE
Elevation Angle
Vertical far-field aiming angle of the main lobe in degrees. Negative values mean that the beam is steered downwards with respect to the perpendicular of the Intellivox unit. The setting of the elevation angle should be chosen in relation to the height of the acoustic center of the array with respect to the height and length of the listening plane, as explained Choosing DDC beam parameters. The range of the Elevation angle as well as the other beam related parameters is depending on the unit type.

Opening Angle
Vertical far-field opening angle (- 6 dB) of the main lobe in degrees. By increasing or decreasing this value, the 'throw' of the array can be enlarged or reduced respectively. The minimum opening angle is limited by the (acoustic) length of the array.

Focus Distance
Distance of the acoustical reference point ("acoustical center") to the focal point in meters. The focal point is the position in space where all loudspeaker contributions arrive in phase. The acoustical reference point is taken as:

The acoustical center of the lowest transducer in case of an asymmetric array.

or

The acoustical center of the transducer located in the middle of a symmetrical array (or the average of the two transducers closest to the middle in case the array consists of an even number of transducers).

The Focus distance and the Elevation angle define the optimization point that is used to calculate the output channel delays for the main lobe. In Choosing DDC beam parameters guidelines are given to choose the optimum values for the beam parameters.


In Fig.3 the basic DDC processing scheme is shown. Analytical expressions have been derived to calculate the DDC delays tn and output filters Wn as a function of the beam parameters.


When considering the Intellivox type which is most suited for a specific application, following issues have to be taken into consideration:
Room Height
As pointed out in the next paragraph, the Intellivox needs to be mounted at a specific acoustic mounting height well above the listening plane. Sometimes other regulations also dictate a minimum mounting height. Of course the room has to allow this mounting height.
Array length and room properties
Acoustical room parameters like reverberation time and total room volume put a constraint on the minimum required directivity properties of the loudspeaker array(s) in order to reach an acceptable intelligibility. To obtain a frequency independent vertical coverage pattern, the effective array length is adapted to the reproduced frequency by means of the implemented digital filters (see also DDC basics). In this way the effective array length has to increase for lower frequencies. For high frequencies this process is limited by the outer diameter of the individual transducers (which is around 105 mm (4.1")) for all Intellivox models except the 1608 and the 808). For low frequencies on the contrary, the limitation is due to the actual physical array length. This means that increasing the physical array length offers the possibility of:


Lowering the frequency below which control of vertical coverage is lost while retaining the same vertical opening angle above this frequency.

OR 


Reducing the vertical opening angle while retaining the same frequency below which vertical coverage is lost.

Listening Area
The size of the listening area that must be covered is probably the most important criterion to select the Intellivox type. In general the following rule holds: The larger the acoustic length of the array, the larger the 'throw' of the array (i.e., the larger the covered audience area). As an example, the smallest Intellivox model (Intellivox-DC115) can cover up to 10 m, while the largest model (Intellivox-DC500) covers about 70 m. Refer to the appropriate product data sheet for details about other models.



The acoustic mounting height is a very important parameter in the specification of acoustic design based on loudspeaker arrays. Changing the mounting height after installation, leads to a lot of frustration and extra costs.

The acoustic mounting height is defined as the height of the acoustic center relative to the floor. For the asymmetric Intellivox units (such as the Intellivox-DC115, DC180, DC280, DC430, DC500, DC808 and DC1608) the acoustic center is the position of the lowest loudspeaker, while for the symmetric units (Intellivox-DC360SF, DC360SV) the acoustic center is between the two center loudspeakers. It should be realized that the height of the bottom of the array usually differs from the acoustic mounting height, because the electronics module is usually at the bottom of the array (see Intellivox data sheets for model-specific details).

In order to realize a constant direct SPL over a large distance, it is essential that the acoustic center of the array is located above the listening plane. In many practical situations, a height difference of 0.5 to 1.0 m is a good choice. As an example, consider a seated audience on a horizontal floor. On an average, the listening plane is 1.2 m. In this situation the 'optimum' acoustic mounting height would be approx. 2 m.



In DDC mode, the response of an Intellivox column can be controlled by the beam parameters. To facilitate the choice of the optimum beam parameters, the main design steps are summarized.


MAIN LOBE

Opening Angle
From a coverage point of view, choosing the default opening angle for the Intellivox unit is a good starting-point (see Range of DDC beam parameters for an overview of the default settings). However, in some situations it may be desirable to adjust the opening angle.

For example, in a very reverberant space, decreasing the opening angle may further improve the direct-to-reverberant ratio. Be aware that the coverage close to the array may degrade due to the smaller opening angle.

Vice versa, suppose that for some aesthetical or architectural reason the height of the acoustic center is too high , it may be advisable to increase the opening angle to obtain a better HF coverage of the area near the array.

Elevation Angle
The setting of the elevation (or steering angle) angle should be carefully chosen in relation to the height of the acoustic center zc of the array with respect to the height zli and length D2 of the listening plane, as shown in Fig.5. As a good starting-point, the elevation angle a can be calculated using this formula:

It's important to realize that a negative elevation angle means that the beam is steered downwards with respect to the perpendicular of the Intellivox unit.


Focus Distance
Initially, the focus distance DFocus should be chosen equal to the distance between the acoustic center of the Intellivox and the farthest listening position (see Fig.5). Usually, this gives an optimum SPL coverage.

In some situations however it is necessary to reduce the focus distance. For example when an acoustically hard (i.e., reflecting) back wall produces a strong echo in the hall, reducing the focus distance (and/or decreasing the elevation angle) makes the echo less audible.



The following table lists the ranges and the default values for the DDC related beam parameters.



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BROCHURES (PDF)

USER GUIDES (PDF)