Questionable Science, Questionable Logic, Questionable Morals: London Transport’s 1988 Attempt to Erase Cassette Tapes on the Tube
Could the engines of tube trains on London’s Underground have been adapted to erase any recorded material brought on board? Maybe not, but somebody thought it was a good enough idea to bring all the way to the testing stage.
Was it technically possible to adapt the motors of tube trains to transmit a high-frequency 100kHz bias signal that would wipe clean any recorded material from magnetic tape? Apparently London Transport (now Transport For London) thought so — and allegedly tested the system on Good Friday, 1988.
According to an article in the 31 March 1988 issue of New Scientist, a system that would not only have erased magnetic tape but could also possibly deactivate bank and credit cards was to be tested on the upcoming Bank Holiday. However, the article also claims that dogs and cats could hear the supposed bias signal, which makes us question this system’s validity.
Nevertheless, we feel it is our duty to reprint the article for you in its entirety, for you to be the judge…
FORUM
How your recordings could go down the tube
An experiment on London’s Underground could have serious consequences for music lovers, as Barry Fox explains
If you are thinking of taking your Walkman player on the Tube tomorrow morning, think again. A report leaked from the government’s communications headquarters in Cheltenham reveals that electronics engineers have come up with a way to stop passengers annoying each other with noisy headphones. But the price of silence will be high. The technology works by erasing magnetic recordings. It could effect not just audio cassettes, but computer data tapes, master tapes used by recording studios, and even bank or credit cards. That is engineers intend to test the system on a Bank Holiday morning, when the Underground is less crowded than usual.
Since Sony invented the Walkman, the Japanese electronics industry has flooded the world with millions of similar gadgets. Although they envelop the user in a cocoon of personal music, the headphones let enough tizzy sound leak out to infuriate anyone sitting or standing close by.
London Transport has been heavily criticised for allowing passengers to get away with impersonal listening. Bylaws arc unenforceable, so an electronics wizard has come up with a technical solution. Like all the best ideas, it is simple.
All tape recorders rely on a high-frequency signal, called bias, to erase tapes for re-recording. This bias signal, of up to 100 kHz, and thus far too high-pitched to be heard by humans, affects the magnetic particles on the tape. Their magnetic poles take up a random pattern, erasing any previously recorded signal for ever.
The plan now is to superimpose a similar hi-frequency audio signal on the 630-volts DC supply which runs alongside the rails on the Underground, and which powers the trains’ drive motors. When this high-frequency signal feeds through the coils of a train’s drive motor, along with the normal DC power, the motor drives the train as usual. But at the same time the motor behaves like the erase head of a giant recorder. Any tapes near the motor will be wiped clean. So the personal stereo goes quiet and no sound leaks from the headphones to disturb other passengers.
The BBC has for many years recognised the risk of carrying magnetic tapes by tube, even under normal circumstances when there is no high-frequency bias on the normal DC supply. Occasionally, the motors and pick-up create and generate brief bursts of magnetic field which can erase tapes. Producers at the BBC are advised always to take programme tapes by taxi. Often, however, they go by tube, cross their fingers and claim expenses for a cab. “If LT does try out the bias system tomorrow morning,” chortled one broadcaster at the BBC, “there will some red faces when people claim for a taxi and then find they have ended up with a blank tape and no programme.”
Critics of the system warn that if it works well enough to erase music tapes, it could wipe computer data tapes clean, too. Many bank and credit cards now rely on a magnetic strip on the rear which carries digital information that identifies the authorised user. Erase this coding, and the card will prove useless when the owner tries to extract money from a cash dispenser or make a credit-card telephone call.
Bill Foster, a specialist in tape recording, sits on the technical committee of Professional Recording Studios, the association of professional recording studios, the trade body of the professional recording industry. Foster advises pop groups to play safe with master tapes which may have taken months to produce in a studio.
“The motors are underneath the car,” explains Foster “so the best thing I can suggest is that if anyone has to travel with a tape tomorrow morning, they should keep it as far away from the floor as possible. The same goes for personal stereos. Stand up, rather than sit for the journey, and hold them at shoulder height.”
London Transport has a fleet of nearly 4000 tube cars, of which 2500 are motorised. Every tube train contains two or three cars with motors; the rest are unpowered. The magnetic field from a motor will not stray all the way down a car. So it is a case of pot luck whether you risk erasure or not. Because the bias signal is so high-pitched. only dogs and cats would know which areas to avoid.
If you are travelling by tube train in London tomorrow morning, Good Friday, our advice is to play safe. Switch off your Walkman, walk tall and hold it high — as far away from the motor as possible.
