90 lines
4.3 KiB
Plaintext
90 lines
4.3 KiB
Plaintext
Subject: Re: fonecards
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Date: Thu, 14 Sep 95 18:04:28 GMT
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References: <DEwv47.Cv8@spuddy.mew.co.uk> <43blpt$9ke@columbia.acc.brad.ac.uk>
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Here Ya go:
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---------START OF TEXT -----------
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HOW BT PHONE CARDS WORK - Transcribed by Archaos - June '93
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----------------------- ---------------------- --------
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Contrary to popular belief, BT phonecards do not work using a magnetic
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strip system. The reason for this being that a magnetic strip would be
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read only.
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So how do they work then?
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Well, examine a phonecard - preferably a used one if you are going to
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scratch it or dissect it. If you look on the printed surface (the green
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side - which is the front) you will find two lines which form a thick band.
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Underneath this area is a "track" which holds the information about the
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number of units used up and how many are left. A used phonecard will have
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some tiny bars marked on the track near one end.
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On the reverse side of the phonecard (the black side) you can see a shiny
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black strip in contrast to the matt black which has text on it (on older
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phone cards the whole of this side is shiny black). Anyway, this shiny strip
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is "opposite" the band on the front and acts as a "window" to the information
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on the track - for the simple reason that it is no ordinary shiny black
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plastic. This special black plastic is not like all the others (which do
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not let normal light or infra-red light pass through) but is transparent to
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infra-red light. When a phonecard is in the machine, an infra-red beam is
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shone through the back of the card and the reflected beam is checked to
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detect the time units remaining.
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Now to explain the track itself which is protected by a layer of paint that
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also serves as the base for printing text and figures visible to the user.
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On a 20-unit card, the track has 20 tiny rectangular areas (called
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diffraction gratings - you might have come across them if you took physics)
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which affect the light reflected by the cards. As the time units are used up,
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the ares are destroyed by an eraser head. The design of the assembly enables
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the progress of the erasing operation to be checked. in fact, the 20
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rectangular areas touch each other and form a continuous strip on the card.
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The area which is read is wider than the track. This makes it possible to
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detect a reduction in track width.
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Each unit is separated from its neighbour by a distance of 0.6mm. the erase
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area is greater than the width of the track so that the unit is always
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completely erased. The dimensions of both the card and the time units
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suggest 140 as the theoretical maximum number of units possible.
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The read-and-erase mechanism consists of a moving carriage on which are
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fixed the eraser head and the optical components for reading. the carriage
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is driven by a stepping device which moves along the track to determine
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whether each unit is god or erased. when a unit has been consumed by the
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cardphone, the area is erased in its turn and the carriage moves on one step.
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OK, for those that weant to know, here is an ascii graphical representation
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of the read and erase geometry :
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Time units
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---------------------------------------------------------
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Track | | | | | | | | | 1.2mm
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---------------------------------------------------------
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<0.6mm>
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Area read Area erased
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*** *********
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---------------***------------------*********------------
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| | | *** | | | *|*****|* | | 1.6mm
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---------------***------------------*********------------
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*** *********
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0.4mm 0.7mm
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Well I hope you all understood that! Most of the information in this text
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file was obtained from British Telecom <spit> sources so is quite likely to
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be correct (after all, they should know their own cardphones!).
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Archaos.
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------EOF--------------
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okay?
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Hope I didn't offend anyone by upping this.
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-- DreamshadoW --
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Damn me father, for I must sin
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Finger for PGP Key.
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