Rare Apples, eBay, Goodwill

Dwight K. Elvey dwight.elvey at amd.com
Wed Mar 23 12:24:00 CST 2005


Hi
 I meant to jump in earlier when we were talking
about the data formats on disk but got side tracked
with work ( ick! ).
 It was mentioned that one could have no more than
two concecutive zeros. It was mentioned that this
was a speed issue of the disk. Actually there is
another reason. The level of the signal from the disk
is constantly changing. Think of driving over a rough
road and you want to count the rocks but not the
larger bumps. If you just set a fixed threshold
on the shocks, every time you went over a bump,
you would miss a bunch of small rocks while the
suspension system was absorbing the larger bump.
Now, if you made the system track the slower larger
stuff, you could ignore the large bump and still
see the rocks. The problem is that the system now
sees the occational rock when there was none for
some time because it is drifting along. By making
sure that there are occational rocks, the system
can stay adjusted to be most sensitive to the
rocks without having false detects.
 The disk read is the same, it needs to see data
from the disk surface to keep the threshold detection
circuit from having false detects from noise and random
level shifts.
Dwight





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