MagTek TTL - Manual (Page 12)

I/O Interface for TTL Magnetic Stripe Readers
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Figure 2-1. 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.
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, see Appendix A. 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.
This means that if the character has an odd number of one bits in the character code for that
character, the parity bit is then a zero bit. On the other hand, if the character has an even number
of one bits in the character code for that character, the parity bit is a one bit causing the total
number of one bits to be odd. Thus the term odd parity.