Re: credit card magic

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 07:16 PM 10/12/2003 -0400, James, you wrote:
> //Periodically my ATM
> card won't register on the "swiper" and the clerk will put it inside a
> plastic bag. When the plastic is pulled taut and the card is run through
> again, the magnetic stripe is read successfully.//
> My guess is that the plastic bag's thickness moves the card away from the
> magnetic pickup head on the "swiper" which is just enough to make the
> sensitive zone of the head's sensing gap just a tiny bit smaller, thus
> perhaps overcoming some smearing of the magnetically encoded data.
//
> Jim
>
> --
> James R. Frysinger


Jim seems to be proposing an enhanced bandwidth of the sensor
with a distancing film, which would reduce the smearing together o
 knows that swipe speed variability entails variable signal amplitude
 as a direct consequence, and so builds in a wide range gain control,
quite possibly specifying sufficient gain to maximize the signal
against upper and lower amplitude peak limits, a signal where zero
crossings might be sensed.

If this were the case, the signal noise tends to trigger false zero
crossings on low amplitude local minima (perhaps in worn spots?)

Spacing the magnetic stripes away from the head reduces the amplitude
markedly, and so would place a low amplitude signal crossing below the
threshold for detection.

This leads to a developed strategy for forcing recalcitrant card readers:
first try one thin layer (of adhesive tape?) then two then three....



Brian Whatcott    Altus OK    Eureka!