Call the cops
Question: Are the voice modulators used over the phone by kidnappers any use? Because the gadgets electronically alter the caller鈥檚 voice, is it possible to 鈥渄emodulate鈥� the signal and reveal the true voice?
Answer: As a contributor to Last Word noted recently, 鈥渉umans are a social species and, as such, it is very important for an individual to express his or her feelings and emotions to fellow individuals鈥�. Accordingly, human speech has developed into an information transmission channel of enormous capacity. At the same time, much of the information transmitted is redundant鈥攜ou can distort the physical signals used and still get the essence of the message faithfully transmitted. A kidnapper calling another party is usually sending extremely simple messages and is not interested in preserving all the fine detail of the speech.
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An easy way to scramble the speech is to pass the analogue electrical speech signal through a frequency filter, such as low-pass, high-pass, band-pass or graphic equaliser. In this way much of the information which could be used for identification purposes is lost permanently, and there is no way to recover it unambiguously. If, in addition, the scrambler includes, for instance, an electronic delay line which is adjusted randomly, the character of the speech is also altered鈥攁gain in an irrecoverable manner. The kidnapper鈥檚 speech would now be completely unrecognisable while still leaving the message intact and intelligible to the listener.
To make it even more difficult for the cops to identify, a different person can compose the text of the message, using words the caller might not. Of course, all this does not really matter much, if the police are able to locate the caller geographically, as they often are.
Therefore there is no way to demodulate the speech so that the original voice is recovered unambiguously鈥攗nless the original modulation is very simple indeed. Additionally, the means outlined above relate to conventional analogue signals. Today it is quite possible to carry out all these tricks digitally so that the listener has no idea that any scrambling is being done.
Heikki Collan
Espoo, Finland
Power house
Question: Why don鈥檛 gyms sell power to the national grid by converting treadmills, stationary bikes and weight machines to drive generators via a variable ratio device? The exerciser could set the workload and determine just how much of their electricity bill to work off.
Answer: It is true that you could generate power with an exercise machine. However, the amount and value of the power is surprisingly small. A steady exerciser can produce about 250 watts of power. This corresponds to riding a bicycle up a hill 300 metres high in 15 minutes.
In my locality, electricity sells for 10 cents per kilowatt hour. This means that the retail value of the power generated by a single machine is only around 2.5 cents per hour. The price the power company pay for electricity amounts to no more than one-half the retail price, or 1.25 cents per hour. If a machine were in use for eight hours a day, every day, it would generate about 70 cents鈥� worth of energy per week. Since the cost of equipping an exercise machine with a generator is well over $100, it would take nearly three years just to pay off the initial outlay. The additional cost of maintenance, depreciation and interest would ensure that, overall, the gym would lose money by installing generators.
Barry Spletzer
by e-mail, no address supplied
Answer: The amount of energy we are capable of producing when on the exercise machine is quite insignificant when compared to the energy we consume in our daily lives.
The amount of power you would need to run your television each time you watched it would keep you fit and trim, if, indeed, it didn鈥檛 kill you. In fact, if everyone had to power a single light bulb for a month via the apparatus suggested, the educational effect, as people learnt just how much energy is required to power our national grids, would solve the world鈥檚 energy problems.
James Prichard
by e-mail, no address supplied
Not easily LED
Question: LEDs have advantages鈥攖hey are long-lasting and efficient. What limitations prevent their use in domestic lighting?
Answer: The only limitations on LEDs are economic. For domestic use you need expensive high-power LEDs. Also, LEDs produce monochromatic light, but usually we need white light which requires a colour mix of LEDs, including blue. Sadly, blue LEDs are the most expensive.
Ahmad Allatif
T眉bingen, Germany
Answer: LEDs are used in traffic signals in Japan. They cost more than usual bulbs, but last longer鈥攁t least 10 000 hours.
Adolph Smith
Santa Cruz, California
Answer: LEDs are used in traffic signals in remote areas, where bulb replacement costs are high.
Alex Lu
Edinburgh
LEDs have been used by the motor industry in vehicle brake lights. They are brighter and light marginally faster than normal bulbs, increasing available reaction time for following drivers鈥擡d