Introduction - Hard Disk Drive: Mechatronics and Control - page 17

Figure 1.7: A track is created by recording binary bits on a spinning disk.
1.3 Accessing Data in HDD
1.3.1 Arrangement of Data on the Disks
Bit is the smallest unit of recorded information on magnetic media. It is a
tiny piece of the disk surface and contains binary information. During writing,
the read/write head is positioned at radial distance from the center of the
disk while the disk is spinning. The read/write head is moved to the desired
location and is positioned there with as low variance is possible with the help
of the actuator operated under closed loop servo control. Recording/writing of
information is achieved by alternating the polarity of the current in the write-
head's coil. Since the disk is spinning and the head is held at a point, the write
current magnetizes a circular path on the disk with alternating polarity of
magnetization. Type of magnetization on the medium depends on the polarity
of write current. A transition in the write current waveform creates a transition
of magnetization on the disk. The circular pattern of magnetization created
on the disk is called a track (Figure 1.7). A new track can be created by
repositioning the write head to a new point on the disk radius. In a typical 3 2
inch HDD, 70,000 to 100,000 tracks exist on each surface of a disk.
The polarity of write current is altered according to the binary bits to be
recorded. A 1 in the binary data causes the polarity to be reversed, otherwise
it is unchanged. If the data is recorded at a rate of b bits per second then
a clock signal of frequency f W = b Hz is used to change the polarity of the
write current. Minimum separation between two consecutive transitions in the
write current waveform (one from +ve to -ve and the other from -ve to +ve) is
T W =1/f W . The minimum distance between two magnetic transitions on the