Hardware Reference
In-Depth Information
tostoredataandisoftenindicatedasanumberofbitsperinch(BPI).Unfortunately,these
types of densities are often confused when different disks and drives are discussed.
Media Coercivity and Thickness
The
coercivity
specification of a disk refers to the magnetic-field strength required to
make a proper recording. Coercivity, measured in oersteds, is a value indicating magnetic
strength. A disk with a higher coercivity rating requires a stronger magnetic field to make
a recording on that disk. With lower ratings, the disk can be recorded with a weaker mag-
netic field. In other words, the lower the coercivity rating, the more sensitive the disk.
HD and ED media demand higher coercivity ratings so the adjacent magnetic domains
don't interfere with each other. Higher coercivity media is actually less sensitive and re-
quires a stronger recording signal strength.
Anotherfactoristhethicknessofthedisk.Thethinnerthedisk,thelessinfluencearegion
of the disk has on another adjacent region. The thinner disks, therefore, can accept many
more bits per inch without eventually degrading the recording.
Floppy Disk Media
The 3 1/2-inch disks differ from the older 5 1/4-inch disks in both construction and phys-
ical properties. The flexible (or floppy) disk is contained within a plastic jacket. The 3
1/2-inch disks are covered by a more rigid jacket than are the 5 1/4-inch disks. The disks
within the jackets, however, are virtually identical except, of course, for size.
The rigid plastic case of 3 1/2-inch discs helps stabilize the magnetic medium inside.
Therefore, the disks can store data at track and data densities greater than the 5 1/4-inch
disks(see
Figure10.8
).Ametalshutterprotectsthemedia-accesshole.Thedrivemanipu-
lates the shutter, leaving it closed whenever the disk is not in a drive. The medium is then
completely insulated from the environment and from your fingers. The shutter also obvi-
ates the need for a disk jacket.
Figure 10.8
Construction of a 3 1/2-inch floppy disk.
