9th July 2002 - Digital Sound Projector

The Digital Sound Projector is a new loudspeaker system designed to provide full surround-sound reproduction from a single unit. It is a striking-looking flat panel, about the size and shape of a large plasma TV display, with an array of 254 small transducers visible. The first product is aimed squarely at the top-end home cinema market - it is expected that lower-cost mass-market products will follow.

Paul began his talk with an overview of the essential principles employed in the Digital Sound Projector. There are two key elements firstly the production of steerable beams of sound using the transducer array and secondly the reflection of these beams off the surfaces of the room to produce loudspeaker images at the required locations for surround-sound reproduction.

Paul showed how the beam produced by an array of loudspeakers, such as in the familiar column loudspeaker, could be steered by delaying the feeds to each loudspeaker by the correct amounts. He explained the constraints on the dimensions of such an array The overall size of the array determines the lowest frequency which can effectively be steered - below this frequency the unit becomes omni-directional. The spacing between the individual elements determines the highest frequency that can be reproduced without creating an interference pattern or 'grating lobes'. These two constraints together lead to the requirement for an array with a large number of small transducers.

Paul pointed out that these techniques are not new, and have been widely employed in steerable antennas for radar and communications. He illustrated this with a picture of an enormous US military UHF radar antenna - and observed that the wavelengths involved at UHF are comparable to those in audio.

To be able to produce convincing loudspeaker images, we need to be able to control the focussing of the beam in order to simulate listening in the near field, where the wavefront tends to be spherical. This can be achieved by appropriate adjustments to the delays feeding each transducer. By this method, a focal point can be created in front of or behind the loudspeaker panel.

The required loudspeaker images are created by reflecting the beams for the five channels from the surfaces of the room - usually the side walls for left and right front and a combination of ceiling and side and rear walls for the surround channels. Paul mentioned the need to avoid the Haas effect dominating. This requires that the beams are sufficiently well-controlled that the level of reflected (and therefore later) sound is at least 12dB higher than any sound arriving direct from the loudspeaker.

Clearly, a large amount of digital signal processing is employed to create the drive signals for each transducer. Not only do the correct delays have to be calculated and applied, but each beam is equalised to compensate for the different reflection characteristics of the room surfaces. With this size of array, it is not possible to steer low frequencies below about 300Hz - these are just summed for all channels and time-aligned with the beams. A fairly elaborate, although largely automated, set-up procedure must be carried out by the installer. This uses a measurement microphone and laptop computer and generates a configuration file which is then loaded into the system via a serial link. Different settings can be stored - for example, to allow for the differences between curtains being open or closed.

Paul then described the construction of the Digital Sound Projector. The panel is a single aluminium casting into which the transducers are press-fitted. The transducers themselves are small moving-coil units with a long throw, made for 1Limited by Aura, better known for in-car loudspeakers. These are driven by 2W Class D amplifiers, built in groups of 10 on narrow circuit boards fitted directly behind the transducers.

Paul outlined some of the other possible applications for the technology Adding height information (periphony) would be relatively easy, public address systems could be zoned or targetted, and outdoor sound systems could automatically compensate for wind-shear. One of the more amusing possibilities was what Paul described as the "Oi, you!" application, where a security guard watching a CCTV system in a shop could focus the beam on a shoplifter and make an announcement that would appear to come from within the victim's head!

The session ended with a fascinating demonstration, which clearly showed the system working, even in a less-than-ideal room (too wide and with an absorbent ceiling). As usual at AES lectures, some alternative applications were suggested Could a tightly focussed beam be used to break a wine glass? Could a multi-lingual cinema presentation be given with different language beams focussed on different sections of the audience?

The Digital Sound Projector is expected to be available in September - the retail price is £25,000.

Chris Sleight