MagTek InSpec 9000-2005 - Manual (Page 62)

InSpec 9000-2005 Encoded Card Tester
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DIGITAL MAGNETIC RECORDING
Digital recording utilizes the region of the magnetic tapes characteristic past the saturation point
where the signal amplitude does not change much with encode current. This is important, not
because encode currents vary, but because encode heads and magnetic tape are not always in
perfect, intimate contact during the encoding process. The magnetic field strength from the
encode head gap decreases significantly as the distance increases from the gap. Thus, digital
recording can withstand small “spacing losses” between the encoding head and magnetic tape.
Saturation recording (digital recording) also produces fast, sharp rise times in the read back
signal, making detection of the positions of the flux transitions more accurate.
Spacing losses occur when the encoding head leaves contact with the magnetic tape.
Contamination on the magnetic tape surface or the encoding head can produce spacing losses.
Contamination can be classified in two categories. One is loose debris (dust, plastic shavings),
which resides on the surface of the magnetic stripe causing the head to bounce or otherwise lose
contact with the magnetic stripe surface. The second type is deposits of oil, plasticizer or other
film type deposits which create a surface that causes the encoding or reading head to stick then
slip across the magnetic stripe. These oily film deposits can be thick enough to cause spacing
losses and thus can cause read or encode failures.
F2F Encoding
There are several schemes used to record or encode ones and zeros in the computer industry.
(We use the term record and encode synonymously.) In the card industry the International
Standards Organization (ISO) has defined F2F as the encoding scheme for cards. F2F stands for
frequency - double frequency, or for the purist - two frequency coherent phase recording. F2F
encoding provides for self-clocking data. That is, the serial data stream consisting of one and
zero bits provides the timing information for the reader to determine which bit is which.
The key feature of self-clocking data is that the data bits can be extracted from the serial data
stream without the need to control the speed of the magnetic media past the encoding head.
Thus, card swipe readers, where a human hand is passing the card through the read slot can work
regardless of how fast or slow the card is passing through the slot. Serial data merely means that
the one and zero bits, that form the desired characters, are stored on the same track, one bit after
the next bit.
F2F is an encoding technique which places flux transitions on the magnetic stripe separated by a
defined distance for zero bits and one half that distance for one bits.
The defined distance for Track 1 is 0.0047619 inches for zero bits. This value is the reciprocal
of the bit density of Track 1 - 210 bits per inch. For one bits - 0.0023809 (1/2 the zero bit
distance.)