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Peter Cochrane's Hard Drive 1999 Fresh air or codes for sale? THE earliest radio systems employed crude spark transmitters and cat's whisker crystal receivers. High voltages were generated using wire-wound inductors in much the same way we generate the ignition spark for today's internal combustion engines. The key characteristic of signals generated in this way is their lack of frequency constraint. Today we would detect them at the very low frequencies currently used for our international time standards in the region of 10kHz, right through to our highest frequency TV channels at 850Mhz and beyond. In fact, they would interfere with every radio system in the vicinity in the same way a car with a faulty suppressor does today. A significant advance occurred with the arrival of the thermionic valve in 1915, when electronics allowed signal selectivity to be engineered. This established the systems we use today with radio and TV signals arranged as distinct channels across the frequency spectrum. Our analogue and digital telephone conversations are also still conveyed in this manner - stacked in frequency space like AM or FM stations on a domestic radio. During and immediately after the Second World War, the military was concerned about signal detection and jamming, and in a significant rush of déja vu, spread spectrum was born. While on the secret list for decades, this technology was developed to far exceed the old spark transmitters. Instead of blasting energy into the ether in an uncontrolled manner, this new generation employed digital techniques to spread the signal evenly over a broad, but limited, range of the radio spectrum so it remained hidden in the background noise. Only a receiving station that knew it was there, and had the necessary code could extract the signal. So for example, a telephone conversation occupying 3kHz of bandwidth could be spread over 30MHz or more. The signal energy would be 1 10,000 of its original intensity, and an enemy would be unable to detect it. In addition, by using different spreading codes it is possible to overlay thousands of conversations in the same frequency space. Unfortunately, practical systems required huge amounts of hardware, rendering terminals expensive and prohibitive for anyone other than the military. But recent advances in chip technology have seen such systems realised in hand-held units, and this will be the basis of Third Generation Mobile Systems. Meanwhile, governments worldwide have become accustomed to making significant revenues by selling fresh air - the radio channels used for broadcast, fixed and mobile telephone services. And this is wholly on the basis of the old analogue and digital systems with channels stacked in frequency space. Unfortunately, it looks as though they are unable to understand that they now need to sell code space. In sticking with the old way they will waste all the advantages of spread spectrum. The new operators will not be encouraged to compete in the code and service space, they will not be sharing towers and transmitters, costs will remain high and there will be more visual pollution. If only we could get governments to understand just the rudiments of this vital technology we might see a sensible decision. Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is: |
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