Noisy Medicine

SINCE SAMUAL Morse tapped out his message (24 May 1844) ``What hath God wrought?" on the first telegraph line the problem that dominated the scene was how to detect the signal above the background noise, which was generally attributed to static electricity. The ideal is a clear signal free from interference. A measure of the quality of a communication circuit is defined by the signal-to-noise ratio.

In every day life, noise is unavoidably present. But it is usually viewed as being determential to the detection of signals and transmission of information. It is the hiss and crackle that you want to get rid of, when you tune in on short-wave radio. Or you look for a clear, clean reception, when you talk on the cellular phone. So noise filter is a necessary party of many equipments.

Since the early 1980, in the work of global climate modelling, physicists discovered the phenomenon of `stochastic resonance': stochastic, because noise is essentially random; and resonance, because the noise works with the signal to boost it. The possible benefits of noise in non-linear system, in a report of the above year in Physics Review Letters, are highlighted.

In real life the phenomenon indeed works. So the obvious place to start is neurobiology. Sensory neurons pick out sights and sounds from background noise. They encode information in the form of electrical impulses, but the way they respond to stimulic is highly non-linear: that is, gradual changes in sound or light intensity result in a disproportionate increase in perception.

James Collins, a bio-medical engineer at Boston University experimented with neurons in living organism, he found they do not operate in isolation but work as units in a network. He set up a computer model of such a network and showed that a minimum noise level is necessary to enhance the system's ability to detect a range of signals.

In a system built with a ring laster (a non-linear device) stochastic resonance can operate. Here it is as if the faint signal surfs on a sea of background noise, hitting random waves that lift it over the threshold of the system.

Peter McClintock (Lancaster University) observes, ''stochastic resonance is an attractive concept, because noise pervades life. Rather than seek impossible silence, we might learn to live with and use the noise around us``. Similarly, chaos, which is a deterministic phenomenon, was frequently mistaken for noise and was viewed in the past as something to be avoided.

Van Weiggeren and Roy (Science Vol 279, 20 February 1998) believe that chaos can be harnessed for a variety of applications in chemical, electrical, optical and biological systems. They have demonstrated that optical chaos may be ideally suited for communicating at high data transmission rates.

They have used a practical set up consisting of a ring laser and an erbium-doped fiber amplifier (EDFA) producing chaotic light with a broad spectrum. A tiny message injected into the transmitter folds the data into the chaotic frequency fluctuations. The receiver reverses this process and recovers a high-fidelity copy of the message. Some level of privacy from eavesdroppers is provided, because the tiny message is hidden in the broad spectrum of the chaotic carrier.

The old notion that noise is a disruptive nuisance in communications in now challenged. It looks as of noise could turn out to be a valuable ally in engineering new applications.

R.Parthasarathy

Send this article to Friends by E-Mail

Section  : Science & Tech
Previous : PATHS OF INNOVATORS - Carl Bosch (1874-1940)
Next     : Penicillin resistant gene in pneumonia bacteria
           identified