Frequency Modulation (FM)
Frequency modulation (FM) is the simplest method
of encoding data to include enough timing pulses so that
the controller and disk drive remain synchronized.
Using FM, each data bit is split into two clock periods.
A logic ONE is encoded as two pulses or flux reversals.
A logic ZERO is encoded as a pulse followed by no
pulse. Therefore the byte 11000101 would be encoded
on the disk as PPPPPNPNPNPPPNPP (P = pulse,
N = no pulse).
FM is an effective method for encoding data, but it
wastes a lot of space on the disk. To maximim data
storage on the disk, a method is needed that reduces the
number of pulses yet does not allow too many no pulse
Modified Frequency Modulation (MFM)
Modified frequency modulation (MFM) refines
data encoding to reduce the number of pulses written
on the disk. Using MFM, a logic ONE is always
encoded as no pulse followed by a pulse. A logic
ZERO, when preceded by a logic ONE, is encoded as
two no pulses. A logic ZERO, when preceded by
another logic ZERO, is encoded as a pulse followed by
no pulse. Using MFM, the byte 11000101 would be
encoded NPNPNNPNPNNPNNNP for a total of six
pulses or flux reversals on the disk. Compare this with
the 12 pulses required to store the same data using FM.
MFM is currently used with all floppy drives, most
large disk memory sets, and many fixed disk systems.
Run Length Limited (RLL)
The run length limited encoding schemes take data
encoding to a new level. Usually the RLL specification
will be followed by two numbers such as 1, 7 or 2, 7.
These numbers represent the minimum and maximum
run of O bits between two 1s. The most common RLL
scheme is RLL 2,7.
RLL 2,7 is a complex encoding scheme that groups
bits together and uses a table to encode the data in these
groups. for example, 1100 is encoded as
NNNNPNNN, 1101 is NNPNNPNN, and 111 is
RLL increases the density and transfer rate of data
by 50 percent. A 20M MFM drive can store 30M if
formatted as an RLL drive. Whether a drive is MFM or
RLL depends on the controller and not the drive.
FIXED DISK CONTROLLERS
The disk controller determines what encoding
scheme will be used and interfaces the disk with the
computer. You can change the disk controller to make
a 20M drive into a 30M drive by changing from an
MFM controller to an RLL controller.
RLL encoding requires that the drive work harder;
therefore, be sure your drive can handle the demands of
a new controller. Of particular concern is the type of
head actuator and the magnetic medium of the drive.
Stepper motor head actuators are slower and the
problems they can encounter with temperature can
cause the drive to be very unreliable if formatted as an
RLL drive. Iron oxide medium has a lower signal-to-
noise ratio than the thin film medium. The noise picked
up by the heads can be interpreted as data and result in
FIXED DISK INTERLEAVE FACTOR
The interleave factor is a method of numbering the
sectors on a fixed disk to provide the optimal transfer
of data between the controller and the computer. When
a freed disk is formatted, sector numbers are written on
each track. Interleaving refers to the relationship
between the physical sectors on a track and the logical
sectors on a track. Each sector on a fixed disk in a
personal computer has 512 bytes per sector. Most files
are larger than 512 bytes; therefore, it is assumed that
if you want to retrieve the data at cylinder 225, sector
1, you will next need the data in sector 2. Since the fixed
disk spins at 60 revolutions per second, the heads read
data at 512 bytes per sector, 17 sectors per track or a
data rate of over 500 kilobytes per second.
With no interleave factor, the head reads the data
from sector 1 and sends it to the controller. While the
controller assembles the data to send it to the computer,
sector 2 is under the head but the controller is not ready
to accept the data. So the disk must make another
revolution to retrieve the data from sector 2. To avoid
this problem, the disk is interleaved. This means the
logical sector numbers do not necessarily follow the
Figure 10-19 illustrates the sector numbering of a
disk with a 3:1 interleave. Physically the sectors are
numbered 1, 7, 13, 2, 8, 14, 3, 9, 15, 4, 10, 16. . .12,
and back to 1. With a 3:1 interleave, the head reads
logical sector 1 and sends the data to the controller.
While the controller processes the data, the next
physical sector and part of the following sector pass by