MagTek InSpec 9000-2005 - Manual (Page 65)

Appendix A. Magnetic Encoding
55
There is one more important parameter that the Tester measures. It deals with the position of the
Start Sentinel from the physical right hand edge of the card. The ISO standard requires that the
“center line” of the first bit in the start sentinel on either Track 1 or 2 to be 0.293 inches ± 0.020”
from the right hand edge of the card. The ISO standard allows the tolerance for the Start
Sentinel position on Track 3 to be ± 0.040”. The reason the start sentinel position is important is
because it provides the reader with enough leading clocking bits for synchronization and an
expectation for where valid data starts from the physical edge of the card.
The F2F encoding technique also permits reading in reverse. That is, instead of passing the card
across a read head with the start sentinel first, it is possible to pass the card across the read head
with the LRC and end sentinel first. Whether any particular reader can function in reverse
depends on whether it was designed to do so.
Next, the parameter “jitter” and why it is an important parameter to measure and control. The
Tester measures jitter and reports values for each of the jitter terms described below.
Jitter
Jitter is a term used as a short version for: Variations in Flux Transition positions. Jitter is more
precisely stated in the ISO standard as Flux Transition Spacing Variation, Subinterval Spacing
Variation, Adjacent Bit Cell Variation, and Adjacent Subinterval Spacing Variation, Bin+1 for
Adjacent Bit Cell Variation and Sin+1 for Adjacent Subinterval Spacing Variation. We will use
the ISO shortened terms (Bin, Sin, Bin + 1, and Sin +1) in this section. All four parameters have
one thing in common. They are all distance measurements between flux transitions. While all
four parameters are distance measurements, it is common to state the measured value in terms of
a percent.
Average Flux Transition Spacing Variation - Ba
This term means a measurement of the overall or average bit density for the encode track.
Individual Flux Transition Spacing Variation - Bin.
Flux Transition Spacing Variation is the distance measurement between clocking flux transitions
or Zero Bit flux transitions over the bit density for the track.
For example, if the distance between the flux transitions for a Zero bit on Track 1 measured to be
0.004350, the Flux Transition Spacing Variation would be stated as -8.7% meaning that the flux
transition that made a zero bit were 8.7% too close together.