Hardware Reference
In-Depth Information
magnetic domain, the higher the density of data that can be stored on the drive.
When the magnetic field passes through the medium, the particles in the area below the head gap are
aligned in the same direction as the field emanating from the gap. When the individual magnetic
domains of the particles are in alignment, they no longer cancel one another out, and an observable
magnetic field exists in that region of the medium. This local field is generated by the many magnetic
particles that now are operating as a team to produce a detectable cumulative field with a unified
direction.
The term
flux
describes a magnetic field that has a specific direction or polarity. As the surface of the
medium moves under the drive head, the head can generate what is called a
magnetic flux
of a given
polarity over a specific region of the medium. When the flow of electric current through the coils in
the head is reversed, so is the magnetic field polarity or flux in the head gap. This flux reversal in the
head causes the polarity of the magnetized particles on the disk medium to reverse.
The flux reversal (or flux transition) is a change in the polarity of the aligned magnetic particles on
the surface of the storage medium. A drive head creates flux reversals on the medium to record data.
For each data bit (or bits) that a drive writes, it creates a pattern of positive-to-negative and negative-
to-positive flux reversals on the medium in specific areas known as
bit cells
or
transition cells
. A bit
cell or transition cell is a specific area of the medium—controlled by the time and speed at which the
medium travels—in which the drive head creates flux reversals. The particular pattern of flux
reversals within the transition cells used to store a given data bit (or bits) is called the
encoding
method
. The drive logic or controller takes the data to be stored and encodes it as a series of flux
reversals over a period of time, according to the pattern dictated by the encoding method it uses.
Note
The two most popular encoding methods for magnetic media are Modified Frequency
Modulation (MFM) and Run Length Limited (RLL). All floppy disk drives and some older
hard disk drives use the MFM scheme. Today's hard disk drives use one of several variations
on the RLL encoding method. These encoding methods are described in more detail in the
section “
Data-Encoding Schemes
.
”
During the write process, voltage is applied to the head. As the polarity of this voltage changes, the
polarity of the magnetic field being recorded also changes. The flux transitions are written precisely
at the points where the recording polarity changes. Strange as it might seem, during the read process,
a head does not generate exactly the same signal that was written. Instead, the head generates a
voltage pulse or spike only when it crosses a flux transition. When the transition changes from
positive to negative, the pulse that the head detects is a negative voltage. When the transition changes
from negative to positive, the pulse is a positive voltage spike. This effect occurs because current is
generated in a conductor only when passing through lines of magnetic force at an angle. Because the
head moves parallel to the magnetic fields it created on the media, the only time the head generates
voltage when reading is when passing through a polarity or flux transition (flux reversal).
In essence, while reading from the medium, the head becomes a flux transition detector, emitting
voltage pulses whenever it crosses a transition. Areas of no transition generate no pulse.
Figure 8.4
shows the relationship between the read and write waveforms and the flux transitions recorded on a
storage medium.
