MagTek InSpec 9000 - Manual (Page 65)

Appendix A. Magnetic Encoding
55
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 A-8 illustrates this concept.
Figure A-8. Bit Cell - Flux Transition
The encoding device has the responsibility to separate the flux transitions at the proper distance.
Variations in the proper distance between flux transitions is what is commonly referred to as
jitter. Jitter is described later in this section; but first, the following is a description of the
general encoding format on each of the ISO tracks.
Before the desired information can be stored (encoded) on the card, many things must happen.
The information itself (the alphanumeric characters for Track 1 and the numeric characters for
Tracks 2 and 3) needs to be coded. That is, the information must be converted to one and zero
bits. The one and zero bit code for each character can be found in the Coded Character Set
tables in ISO/IEC 7811-2. These tables define the pattern of the one and zero bits for each
character. There is a special bit that is added to each character code for the purpose of error
detection. This special bit is called a parity bit. The parity bit is either a zero bit or a one bit
depending on whether the number of one bits in the character is an even number or and odd
number. The ISO standard has defined that for all tracks odd parity is maintained on each
character.