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Able Planet: Constructive Distortion

Hearing loss ultimately is about hair loss: the death of the tiny hairs that line the cochlea of the inner ear. Sound waves bend the hairs and thus the cells from which they sprout, and that motion c 3/03/2008 5:53 AM Eastern

Able Planet: Constructive Distortion

Hearing loss ultimately is about hair loss: the death of the tiny hairs that line the cochlea of the inner ear. Sound waves bend the hairs and thus the cells from which they sprout, and that motion creates electrical signals that auditory nerves pass on to the brain, which interprets them as sound.

HEARING LOSS ULTIMATELY IS ABOUT hair loss: the death of the tiny hairs that line the cochlea of the inner ear. Sound waves bend the hairs and thus the cells from which they sprout, and that motion creates electrical signals that auditory nerves pass on to the brain, which interprets them as sound.

But if the sound pressure level (SPL) is too high, it can whip the hairs so far in one direction that they're damaged. In some cases, the damage can be severe enough that the hair cell dies. Once a hair cell is gone, it's gone forever.

Able Planet's Linx technology has been used in headphones, but the company is thinking bigger when it comes to enhancing audio.

The effect of that hair loss varies, but one common result is that certain sounds become difficult to pick out. For example, when listening to speech, high-frequency sounds such as the letters “S” and “H” might no longer be distinguishable.

The founders of Able Planet wanted to find out how to get those high-frequency sounds back. It was a quest born out of personal experience: all of them suffered hearing loss, one of them profound.

The Wheat Ridge, Colo.–based company developed techniques and technology—since branded as Linx Audio—that restores high-pitched sounds such as “Th” and “F.” The end result is that Able Planet essentially expanded the concept of hearing aids from a stand-alone product aimed at the hearing-impaired to a technology that could be embedded in headsets and loudspeakers intended for people who don't necessarily have hearing loss. For example, Linx's users include linguists and language labs because the technology preserves intonation and other nuances.

“Our target group is the audiophiles—people who love music and are spending $20,000 on speakers, cables, and home stereo systems—[as well as] gamers and people 55-plus,” says Kiva Allgood, Able Planet's vice president of marketing.

HARMONICS AS ASSETS

Over the past two years, Able Planet's Linx technology (www.ableplanet.com/linxtechnology.html) has built a buzz among reviewers and racked up recognition such as back-to-back Consumer Electronics Show (CES) awards. The difference that consumers and reviewers say they notice is due largely to a technique that Able Planet calls Harmonic Enrichment.

It's based on the fundamental concept of harmonics, where a sound can be spontaneously produced at multiple frequencies. For example, a tone at 500 Hz can produce harmonics at 1 kHz and 3 kHz. In the ear, the original, fundamental tone and its harmonics each whip different sets of hair cells.

The catch is that the ear doesn't distinguish between the original tones and its harmonics. Instead, the cochlea aggregates the electrical signals from each set of hair cells, so to the brain, they literally all sound the same.

Linx leverages that ability: If one set of hair cells is damaged or dead, there's a chance that other sets are still alive and well. There's also a chance that if one of the dead sets covers a particular fundamental tone, a set that can “hear” its harmonic is still functioning. So unless a person is completely deaf, Linx can create harmonics that the remaining hair cells pick up and interpret as the fundamental tone.



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Able Planet: Constructive Distortion

Hearing loss ultimately is about hair loss: the death of the tiny hairs that line the cochlea of the inner ear. Sound waves bend the hairs and thus the cells from which they sprout, and that motion creates electrical signals that auditory nerves pass on to the brain, which interprets them as sound.

Able Planet uses a variety of analogies to explain how the Harmonic Enrichment technique works, including adding chord notes to a base tone, but it stops short of going into details about how exactly it achieves those effects. “The technique creates a kind of constructive distortion,” says Christopher Schweitzer, Ph.D., the company's chief audiology officer. “I'm not at liberty to reveal precisely the method, but the outcome is that the redistribution of energy across various harmonic regions adds the perception of greater high frequencies, at least through the crucial speech frequencies.”

Distortion is the reason why it's possible to hear sounds that are beyond the capabilities of a device, such as a telephone.

“Since it's well known that we cognitively ‘hear' the fundamental voice frequency by means of the harmonic structure, some aspects of the spoken word are detected in a telephone application that exceed the passband of the phone,” Schweitzer says. “These are coherently represented as distortion products in the receiver. That's part of the key—coherence—in this case a logically structured relation of the harmonic components to the input.”

Another analogy is the differences between transistors and vacuum tubes that many argue are audible. “Clearly there is something about the type of distortion that matters probably as much as the measured quantity,” Schweitzer says. “For example, a 500 Hz tone input is treated by two processing methods. There's obvious differences in the distortion properties—and they are clearly audible—but the overall level of distortion components is the same.”

One reason why Able Planet won't go into detail about how exactly its technology works is because it's still figuring it out.

“Not to sound cagey, but there is also some evidence that additional, let's say, ‘mysterious' psychoacoustic properties result with use of our circuit approach,” Schweitzer says. “We are in the process of understanding them better and in tightening our intellectual property protections.”

The Linx technology has a couple of side benefits. One is that it removes undesirable sounds and noise, partly because it redistributes energy into the desired harmonics. Another side benefit could help reduce the number of people with a hearing loss.

“This energy redistribution also contributes to a non-trivial loudness increase for a given sound pressure level,” Schweitzer says. “That's why listeners tend to turn it down, which, as an audiologist, I find very encouraging for the health of headphone-equipped ears.”

SCALING UP

In a sense, Linx Audio is a happy accident, a byproduct of one of the company's founder's struggles with using a hearing aid with telephones.

"Older hearing aids tried to obtain a magnetic coupling from the telephone receiver by means of a coil in the hearing aid," Schweitzer says."When that was engaged, and sufficient magnetic leakage from the receiver was available, it had the advantage of isolating the phone signal from surrounding noise and bypassing an acoustic feedback path. But newer and lighter phones tended to have a weaker magnetic spill, and T-coils on hearing aids got less and less effective. So the work was primarily focused on boosting the magnetic properties of a phone receiver into a hearing aid magnetic pick-up mode."

In the process, Able Planet's founders identified the acoustical properties now used in its headphones and other acoustical output transducers.

"Since there is a much larger universe of non-hearing-aid users of sound processing, the focus shifted," Schweitzer says. "However, current and future work will include more investment back into hearing aid products, in addition to expansion of products in the consumer market."

So far, Able Planet has commercialized Linx in the form of multiple headset models, and Allgood says that earbuds are set to debut this quarter. The company also sees potential applications in cell phones,where the ability to enhance speech?especially for users in noisy environments?would be a plus.

Able Planet's interest in the cell phone market is noteworthy because of the market size: If it can get its technology into even just a fraction of the hundreds of millions of handsets sold each year, it would have revenue to fund expansion into other sectors. For example, in theory, the Linx technology also could be ported to loudspeakers, although some work remains before that can be productized.

"There are some definite nonlinear properties in Linx Audio," Schweitzer says. "Lower levels of signal processing tend to generate less interesting psychoacoustic effects. That may change with some tricks under development. Hence, the straightforward porting to loudspeakers, whether by means of our proprietary circuit, or via digital signal processing[DSP], is not without some challenges."

Able Planet sees those challenges as opportunities rather than problems. "That's half the fun, isn't it?" Schweitzer says. "Solving the next problem, and possibly discovering yet another surprise. So I expect to be able to say yes, we are porting it into loudspeakers sometime in the near future."

One catch is that Able Planet is a small company. Although it's managed to foster a cult following among audiophiles, that's not the same as winning over integrators and other AV pros. That's why the company would consider licensing its technology to established players as one way to enter the loudspeaker market.

"We'd definitely be interested in licensing it," Allgood says. "Speakers are an industry where we'd definitely need a partner. We've had some initial discussions, but we're still living the dream in a small company, so we've got to focus a bit."

Tim Kridel is a freelance writer and analyst who covers telecom and technology based in Columbia, Mo. He can be reached at tim@timkridel.com.



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