Hardware Reference
In-Depth Information
localfieldisgenerated bythemanymagnetic particles thatnowareoperatingasateamto
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
orfluxintheheadgap.Thisfluxreversalintheheadcausesthepolarityofthemagnetized
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 re-
versals 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 Mod-
ulation (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 later in this
chapter 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 trans-
itions 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
