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The Long Road To Digital Loudspeakers

IS THERE anything left in consumer or pro AV that hasn't had the adjective ?digital? slapped in front of its name? Loudspeakers are one exception, but it's not for lack of trying. After more than 30 years of R&D work, there's still no industry-standard definition for ?digital loudspeaker,? let alone a variety of commercial products.

IS THERE anything left in consumer or pro AV that hasn't had the adjective “digital” slapped in front of its name? Loudspeakers are one exception, but it's not for lack of trying. After more than 30 years of R&D work, there's still no industry-standard definition for “digital loudspeaker,” let alone a variety of commercial products.

“You'll sometimes hear people refer to ‘digital speakers' that aren't really digital in the true sense at all,” Alex Salvatti, senior R&D engineer at Northridge, CA-based JBL Professional, told attendees at an October 2002 meeting of the Los Angeles section of the Audio Engineering Society. “Usually it's just a conventional analog driver with some digital signal processing (DSP) in front of it. I've even heard it applied to a studio monitor that happens to have a jack in the back where you can plug in a Sony/Philips Digital Interface Format (S/PDIF) signal.”

Even so, the prototypes and products usually can be plugged into one of two categories:

  • DSP Active Loudspeakers – “These are essentially conventional, ‘powered' loudspeakers that include some kind of digital signal processing (DSP) to improve their performance,” says Ken Kantor, CTO of Tymphany, a Cupertino, CA-based company that's developing alternative loudspeaker transducer designs. “The key issue is that they still use digital-to-analog converters (DACs), power amplifiers, and, finally, conventional, analog transducers, which convert electrical signals to air movement in a basically linear way.”
  • “True” Digital Loudspeakers – In a traditional speaker design, an analog electrical signal moves the diaphragm, so forgoing a DAC means figuring out a way to use the digital bits themselves to move air. “Each digital bit drives either a single transducer, or some fraction of a single transducer's motor, in an on/off manner,” Kantor says. “No such system is really near commercialization, and many challenges remain. As such, there's a still a tremendous competition of ideas for how this process can be best accomplished.”

What's wrong with a DAC? As Texas Instruments explained in a 2002 patent application for a digital loudspeaker system, “DACs introduce noise and distortion that adds to that already present in the system, and also add extra cost.” Dumping the DAC is one of the reasons why the quest for digital loudspeakers continues after three decades of trying.

Thirty years, many designs

The quest for an all-digital loudspeaker has produced some unusual designs. For example, in 1980, Sony patented a “fluid flow control speaker system,” which generated puffs of air to produce sound. Each puff came from an array of exponential horns, where the air flow was essentially coughed up according a Pulse Code Modulated (PCM) input signal.

The first digital loudspeaker appears to have originated in 1973, when IBM applied for a patent in Germany. IBM's design consisted of a piezoelectric beam, with a cone perched at one end. As each digital bit's signal hits the beam, it arches, moving the diaphragm. Each bit hits the beam in a different spot, with the position determined by the sound that's supposed to be created.

Another, more recent design is similar to the Digital Light Processing (DLP) technology now used in variety of consumer and pro video applications. A DLP chip has up to 2 million micro mirrors, each of which tilts thousands of times per second in response to a stream of bits that make up the image.

A digital loudspeaker could use a similar approach, but with microscopic pistons instead of mirrors. Each signal would trigger a particular piston to move air to create the sound — a design that's as complex as it seems. For example, 65,536 pistons would be required to reproduce 16-bit PCM audio.

Even so, manufacturing such a complex system might be possible over the next few years. In 2002, Texas Instruments — which also developed DLP — patented a digital speaker array that's effectively “grown” out of silicon. TI's “semiconductor digital loudspeaker array” is a sandwich consisting of two electrically conductive membranes, with a gap of air in between forming a chamber. That approach could reduce the cost of manufacturing, and in turn improve the chances that a digital loudspeaker based on that design has a business case good enough for commercialization.

So where are they?

The concept of digital loudspeakers was hatched more than 30 years ago. So the obvious question is, why aren't they commercially available today?

“There are several technological hurdles, depending on implementation,” Salvatti says. “For example, in segmented motors, you need to make all the components exact multiples of two because the errors are cumulative. However, in arrays this isn't the case, as the errors are independent. But then the problem is wiring up a huge number of drivers and making sure none of them fail.”

Another challenge is mindset: In AV and other high-tech industries, such as computers, circuitry and software have evolved rapidly and exponentially. That creates the expectation that mechanical devices can be improved at a similar pace. But that's an impossibility, one that TI noted in its patent application in order to explain how it chose the design for its semiconductor digital loudspeaker array.

“Innovating mechanical devices is much more challenging,” Kantor says. “Physical systems don't scale as readily as electrical systems do. It's very difficult to get a bunch of mechanical/acoustical devices to work together seamlessly, down to 20 bits of precision. That's the minimum dynamic range you need in a hi-fi transducer, which must be greater than the medium it's reproducing, if you want your volume control to work.”

Digital loudspeakers also have to vault a few non-technological barriers, including a price premium for first-gen products that could limit their market potential.

“The biggest problem to commercialization is the cost/benefit,” Salvatti says. “Working prototypes have been built and presented to the audio engineering community over the years, but they have in general been just science experiments. The reality is that all the shortcomings of standard loudspeakers — like power handling and directivity control — are still present, except now you have the additional complexity of ‘going digital' on top of it.”

Think small — for now

That begs a fairly obvious question: Will digital loudspeakers solve real-world problems that existing technologies can't, or are they R&D for R&D's sake?

“I would definitely characterize current digital loudspeakers as more of a science project rather than a development project,” Salvatti says. “In the last three years or so, there has been very limited, but continuous, research with several papers presented at the Audio Engineering Society conventions.”

So although R&D work on digital loudspeakers currently is heavier on the R than the D, that level of activity suggests that eventually some of the designs will be turned into commercial products. But the initial digital loudspeakers probably won't be aimed at the pro AV market, mainly because the first-gen products will fall short in terms of power to weight.

“The amount of acoustical output you get for the size, weight, complexity, and cost will most likely not make sense,” Salvatti says. “My prediction is that it will most likely be a small consumer device — like a cell phone or PDA — that will get it first.”

Tim Kridel is a freelance writer and analyst who covers telecom and technology. He's based in Kansas City and can be reached at tkridel@kc.rr.com.

 


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