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Thread: Tape erasure

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    Tape erasure

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    I've been working with old tapes recently - and have had some fantastic help on the forum from Miroslav and other guys trying to remaster a track from a digital copy made 15 years ago from 40 year old studio tape. This got me thinking about an odd incident from the time of the recordings which I'd appreciate some thoughts about. It is kind of weird - maybe not quite but interesting. Apologies in advance if it is not quite the sort of thing for the forum.

    My late brother, Mick, was recording at a studio in Cambridge here in the UK during April-August 1979. One night, i think it would have been in June '79, he was driving back from the studio early one morning - about 3 am and an strange incident occurred (will say more later if anyone is interested!) As a result, the car lost power for a few minutes and slowed to a crawl despite attempts to accelerate. After finally being able to pull away he came home (in a state of shock) and found that the cassette (TDK SA C60) copy of his sessions which he had with him in the car had been wiped - that is completely erased, no sound at all, zilch. The car was subsequently checked and found to have been magnetised - to the extent that its magnetic field could be mapped. So for 40 years I've wondered what level of external magnetic field you would need to completely erase a tape within a car? My limited experience of erasing tapes has been that you need a pretty high field from a head in intimate contact with the tape and that bull tape erasers applied externally aren't very effective. Anyone any experience/ideas on this? Any thoughts appreciated.

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    Well, I suspect it's complete hokum, to be honest. The magnetism required to wipe TDK SA 90s is pretty high. In these days I was feeling the kit and we had complaints about SA90's not being completely erased in some machines not designed to work with type II tapes. So the actual erase head, touching the tape couldn't quite delete the content when the tape was passed slowly past it's head gap. Magnetic fields don't follow the usual inverse square law we normally use to deal with electrical fields and audio, but the inverse cube law. As a result of this, the power to erase a cassette at a distance is immense. Magnetic fields can build up in metal structures through constant exposure to an outside influence, and in a 70s car, the likely ONLY source would be a dynamo, rather than alternators. However - a cassette is a substantial distance from the dynamo and the residual magnetic field in the chassis and body would seem an unlikely cause due to again, distance. If you do the maths the field strength required to erase a cassette completely is the kind of level only tape demagnetisers, with their designed in cavity can produce.

    you need a field strength of at least 450-500 Oersteads to erase a cassette. The physics actually says this level will erase ferric and chrome tapes, but some early cassette machines didn't use this high a figure. Once type II came along, most used the same figure for erasing for simplicity. My maths isn't up to calculating the field required to erase a tape at a distance inside a car, but it is good enough to know that for a car to be this strongly magnetised something very powerful would have been required, and not something possible to have been generated by the car itself.

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  4. #3
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    Thanks for your thoughts Rob. I thought it would need a pretty huge magnetic field to do this. A reel was wiped too. As you say it couldn't have been done by the car itself. The other aspect of this story gets a bit Close Encounters-ish! He was driving on a dark country road near Thaxted and saw two bright lights suddenly appear high up above the car which followed him until the car started slowing and almost came to a stop. After hovering for a while they disappeared and he managed to get speed up. Maybe it was some kind of electrical storm. He reported the sighting to the police and got a visit "from the Ministry" - there had been quite a few sightings that night over Essex and it was the MOD who measured the flux pattern from the car. Whatever it was it must have produced a heck of a field!

    PS I've just done a little reseasrch and found that Ford did some tests a while back and established that fields of around 16,000 Gauss (more or less 16,000 Oersteads) could cause problems with car ignition coils that could lead to power problems - so this would have been enough to cause the erasure I guess (32x the 500G required). If anyone is interested, I also found I made an error with the date - it was August 1977 and the event appears to have been listed in a few UFO journals!
    Last edited by Findlay; 06-02-2019 at 08:56.

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