1084 Chapter 28
computer or storage system to furnish startup or diag-
nostic information, much like a boot ROM in today’s
computers. Over the next ten years the product evolved
from an 8 inch diameter single-sided read-only device
to a 3.5 inch double-sided read/write device with
twenty times the capacity of the original diskette.
Although the original diskette operated on only one
side of the disk, the media had magnetic coatings on
both sides to promote flatness. The symmetric construc-
tion of the disk was eventually exploited to double the
data capacity by recording on both sides.
By the end of their evolution, floppy disks utilized a
very thin coating of cobalt doped gamma ferric oxide.
The coating thickness was about one-fiftenth the thick-
ness found on our analog mastering tapes, and the
particle coercivity was about twice as high.
One important difference in floppy disk media is that
the magnetic particles for a spinning disk must not be
oriented in a single direction as on our audio tapes.
Recording characteristics degrade several dB when an
oriented tape is operated crosswise to the intended
direction. A linearly oriented disk would therefore see
large peaks and troughs in output at twice the rotational
speed. To avoid these fluctuations, the floppy disk
coating process is optimized to either disperse the
magnetic particles in a random orientation or orient the
particles circularly.
The floppy disk operated with the magnetic head in
direct contact with the magnetic media, just as in a tape
recorder. As a result, the floppy disk system was subject
to head wear and head clogging due to dirt and debris.
28.5.6.2 Rigid Disks
Tape and floppy disk recorders utilized contact
recording with the head touching the recording medium.
This continuous sliding contact produced wear that
limited the life of the heads and medium. The flying
head of a hard disk drive eliminates this contact, greatly
extending the life of the head and disk.
High digital densities require very low flying heights
on the order of 0.4μinch (10 nm) to avoid excessive
spacing loss. Any surface irregularity that sticks up
more than the flying height will impact the flying head.
To make matters worse, even smaller defects can upset
the aerodynamic flow of the air around the head enough
to cause instabilities that can make the head crash into
the disk surface. To avoid these problems, the disk
substrate and the magnetic coating must both be
extremely smooth.
Aluminum/magnesium alloys and glass are the
preferred substrate materials, with glass rapidly gaining
popularity as disk sizes decrease. Plastic disks, some
with servo patterns pressed into the surface during the
molding process, are also entering the market.
Aluminum disk substrates are cut from special
aluminum sheet that is optimized for flatness and
surface smoothness. The disks are polished and then
plated with an undercoat of nickel phosphorus (NiP).
Glass and glass/ceramic substrates are rapidly
displacing aluminum disks. Glass offers a very smooth
surface and a higher stiffness than aluminum. The bene-
fits are a lower flying height with fewer surface defects
and a disk that is more robust.
The aluminum or glass disk is coated with multiple
layers that include foundation layers, the active
magnetic surface and protective overcoats. Although
earlier disks were spin coated with a slurry resembling
the coating for magnetic tape, modern disks are prepared
by plating and ion bombardment. Hard diamond-like
overcoats and surface lubricants protect the magnetic
layer from accidental contact with the head.28.5.7 Hard Disk DrivesRotating disk drives offer very rapid random access to a
huge array of data, Fig. 28-54. This yields two very
important benefits. First, the rewind, fast forward, and
autolocate functions of a tape recorder become nearly
instantaneous. This speeds up operation, especially
during editing sessions.
A second and much more important benefit is the
ability to rearrange the output data. Assuming a fast
host computer with versatile digital audio workstation
(DAW) software, the user can construct a song from a
multitude of track segments almost as if he or she cut
each track of a reel of multitrack tape into a thin ribbon
and then chopped and spliced the individual ribbons
back together to arrange the song. This incredible versa-
tility has fueled the rapid replacement of analog audio
tape recorders in recording studios. Even when an
analog recorder is employed for the initial capture of the
music, the analog tracks will probably be digitized and
loaded into a DAW for editing and mixing.
To demonstrate how data are stored on a spinning
disk, consider the inner workings of a representative
single-platter drive. Although this unit is only a
single-platter, 15 Gb entry-level drive under $100, this
drive’s areal density of 22.5 Gb/in^2 led the industry
when the drive was introduced in early 2001. We will
look at the major subsystems to rotate the disk, position