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Author Topic: Credit/Debit Cards & cellophane tape  (Read 2890 times)

Offline seanturvey

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Credit/Debit Cards & cellophane tape
« on: 10/09/2008 19:00:34 »
Why is it that when my debit card is getting old the cashier puts tape, paper or a plastic bag over the magnetic strip?

I have used this trick many times but have never known the answer.


 

Offline RD

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Credit/Debit Cards & cellophane tape
« Reply #1 on: 14/09/2008 04:32:20 »
I've never seen this done, but if true it suggests the distance between the head of the card reader and the magnetised iron in the card is critical for the information on the card to be read.

Adding tape to a worn card could increase the reduced head-iron distance caused by wear, back to the correct value.
 

Offline techmind

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Credit/Debit Cards & cellophane tape
« Reply #2 on: 15/09/2008 21:22:52 »
Why is it that when my debit card is getting old the cashier puts tape, paper or a plastic bag over the magnetic strip?

I have used this trick many times but have never known the answer.

Some of these "tricks" are used but have no effect (people even polish RFID cards to help them read - when there is no plausible reason at all for it to help). Just "having another go" may be all that is required.

Having played around with reading magstripes myself using an old tape-head from a cassette player and connecting it to my computer (to the soundcard/microphone input) and watched the waveforms, I can assure you that as a general rule you'll get a better-defined signal by having the head as close as possible to the magstripe.

However, if the card has "lumpy" deposits on it then that could cause the card to snag on the head as it goes through the reader - which would upset the timing (and impair the decoding) of the signal.

Also I suppose it is plausible that a small lump may cause an abrupt change in amplitude of the signal which might upset the automatic-level thresholding in the decoder. The application of a thicker covering, while decreasing the overall signal, would reduce the proportionate change due to a small lump which I guess might improve things. But it might just be the "have another go" effect you're seeing.

By the way, the data on the card is very primitive digital data - all the data is in the timing of the flux reversals, so the exact amplitude shouldn't matter much. For far more information than most people need, do a websearch for an old web document entitled "A day in the life of a flux reversal"...
 

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Credit/Debit Cards & cellophane tape
« Reply #2 on: 15/09/2008 21:22:52 »

 

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