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NY TIMES | MAY 29 2003
www.nytimes.com/2003/05/29/technology/circuits/29next.html
WHAT'S NEXT
Analog Over Digital? For a Better Ear Implant, Yes
By ANNE EISENBERG
COCHLEAR implants that restore some hearing to the profoundly
deaf have improved steadily over the past two decades. Although
they are called implants, however, these systems still lie
mainly outside the ear.
Most of the apparatus - including the microphone, processor
and batteries that transform speech into electrical signals
passed on to electrodes embedded in the cochlea - is still
typically worn behind the ear or in a shirt pocket.
Researchers hope that one day the entire apparatus, which
is designed to stimulate the auditory nerves of people who
have lost or damaged cells in the cochlea, can be implanted
in the body. But before that goal can be reached, cochlear
implants will need to use far less power. Currently the batteries
must be changed as often as every four hours.
Now a researcher at the Massachusetts Institute of Technology
has devised a processor for cochlear implants that he says
consumes only about half a milliwatt, one-tenth of the processing
power of current devices. Such an acoustic processing chip,
if proven to be effective, might be suitable for next-generation
cochlear devices that are fully implanted.
"There might be a small bump behind the head,"
said Rahul Sarpeshkar, an associate professor of electrical
engineering and computer science at M.I.T., who with his group
created the low-power processor. "But otherwise you won't
know from appearance that the person is deaf."
To save power, the new processor reverses the traditional
pattern for chips used in cochlear implants: it does most
of the work with analog circuits, not digital ones.
"Most people digitize the signal immediately as it comes
from the microphone," turning the information into bits
that a digital signal processor then handles, Dr. Sarpeshkar
said. "We did the opposite." The signal remains
in analog form for most of the processing, including filtering
the sound, and is digitized only at the last interface to
drive the control circuitry of the electrodes.
In the digital age, it turns out, there are still jobs at
which a well-designed analog circuit can excel. Yannis Tsividis,
a professor of electrical engineering at Columbia University
who specializes in merging precision analog and digital circuits
on single chips, said that Dr. Sarpeshkar had alighted on
such an opportunity.
"The ear does impressive things," Dr. Tsividis
said, "but not at high speed." It processes information
not in the gigahertz range of say, a typical Intel chip, but
in the far more leisurely kilohertz range.
"Analog circuits can be profitably operated here,"
he said, because the design does not demand the high current
needed for digital operation.
The physical world is basically analog, he said, but at some
point chip designers must convert those analog processes to
the zeroes and ones of digital design. "The larger question
is, when do you do this?" he asked. "There is a
lot you can gain from doing much of the initial work in analog,"
avoiding the dissipation of power that occurs in digital number
crunching, where each of millions of elements handles part
of the computation. That process can quickly empty a battery.
Dr. Sarpeshkar and his group have been working on the processor
project for three years and have written papers that document
the circuits built for each block in the new design. The analog
circuits make unusual use of complementary metal-oxide silicon,
or CMOS, transistors, which are usually thought of as digital
components but are in this case wired into analog circuits
in a way that draws little power.
The project was underwritten by industry sponsors, and Dr.
Sarpeshkar expects the chip to be available commercially within
two years.
Reducing the power that cochlear devices draw is a crucial
issue today as well as for the next generation of devices,
said Philip Loizou, an associate professor of electrical engineering
at the University of Texas at Dallas who does research in
cochlear implants. "You could be in the middle of a meeting,
and you have to say, 'Hold on, I have to change my batteries,'
" he said.
In the digital part of the process, Dr. Loizou said, a lot
of computing is required quickly. "The more complicated
the algorithm, the more power it consumes," he said.
While Dr. Sarpeshkar's processor is based on analog circuits,
it includes digital outputs so that it can be used with other
parts of the system like the programming interface. Being
able to reprogram the processor is crucial because each patient
has different auditory needs that are translated into instructions
to each electrode that stimulates a nerve ending in the cochlea.
Andreas Andreou, a professor of electrical and computer engineering
at Johns Hopkins University, said that Dr. Sarpeshkar's circuits
were unusual examples of precision engineering. "Analog
does not necessarily mean low power; it's the careful engineering
that does it," he said.
Dr. Andreou expects low-power analog circuitry like Dr. Sarpeshkar's
to have other applications besides cochlear implants. "Some
day with circuits like these, people will have a whole MP3
player in an earbud," he said, or an entire translation
or speech recognition system.
Dr. Sarpeshkar's interest in imitating natural systems with
analog circuits is not new. As a doctoral student at the California
Institute of Technology a decade ago, he worked with the microelectronics
pioneer Carver Mead, creating analog circuit models of the
cochlea.
"Rahul did even better circuit models than we did,"
said Richard Lyon, who also worked with Dr. Mead on cochlear
circuits and is now vice president for research at Foveon,
a company founded by Dr. Mead that has used analog circuits
for computer modeling of another natural process, vision.
"Rahul could always analyze the heck out of everything,"
Mr. Lyon said. "He took those circuits and came up with
real innovations."
Copyright © 2003 by The New York Times Co. Reprinted
with permission.
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