Chapter 9: Hard Disks and Floppy Disks^179
Because the read/write head only sends a signal on a flux transition, a device called
anencoder/decoder,orendec, is used to convert these signals to actual binary data and to
convert binary data into flux transitions. During a write operation, the endec focuses on
creating a signal pattern for the read/write head. In a read operation, the endec interprets
the read/write head signals converting them into binary data. To ensure that all of the
electronic devices involved in this process remain in sync, each data signal is preceded
with a clock signal that is used by the sending and receiving devices (the read/write head
and the endec) to make sure they are both working on the same signal. If one gets ahead
of the other, the clock signal is used to resynchronize them. Clock cells are actually placed
on the disk media between bit cells.Encoding Methods
The different disk media and head technologies used on disk drives directly control how
much data can be placed on a disk. Because of this, there are a number of different ways to
encode data, calledencodingmethods, so it requires a minimum number of flux transitions, in-
cluding clocking cells, to maximize the data storage capacity of the disk drive. Each encoding
method defines a particular scheme for how magnetic particles are arranged in a bit cell.
There are three primary encoding methods in use: FM (frequency modulation) This was one of the earliest methods used for
encoding data on disk storage. This scheme simply recordeda1ora0as
different polarities on the recording media. Although quite popular into the
late 1970s, FM is no longer used today.
MFM (modified frequency modulation) This is the encoding method still
used on all floppy disks, as well as many hard disks. It was developed to
optimize FM and to reduce the number of flux transitions used to store data.
MFM uses a minimum of clock cells, using them only to separate 0 bits only.
This resulted in twice as much data being stored with the same number of flux
transitions as the FM encoding method.
RLL (run length limited) RLL has emerged as the most commonly used hard
disk storage encoding method. It yields higher data density by spacing 1 bits
farther apart and specially encoding groups of bits to be accessed together. RLL
introduced data compression techniques, and most current disk drives (IDE,
SCSI, and so on) use a form of RLL encoding.Head Actuators
The read/write heads of the hard disk drive are moved into position by thehead actuator.
This mechanism is used to extend and retract the heads so that data can be read from or
written to the disk platters. There are a number of actuator types, but they can generally
be categorized into two groups: stepper motor actuators and voice coil actuators. There
are large differences in performance and reliability between these two actuator categories.
Stepper motor actuators are slow, very sensitive to temperature changes, and less reliable.