MagTek TTL - Manual (Page 11)

7
SECTION 2. 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
Organization for Standardization (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 and Track 3 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.)
The defined distance for Track 2 is 0.013333 inches for zero bits. This value is the reciprocal of
the bit density of Track 2 - 75 bits per inch. for one bits - 0.0066665 inches (1/2 the zero bit
density.)
Figure 2-1 illustrates this concept.