Figure 2.12: Dedicated Servo and Embedded Servo.
servo information. For an HDD with only 2 disks, which is quite common
nowadays because of large areal density, one out of four surfaces are used for
servo information if dedicated servo is employed, i.e., 25% of the available
storage space is consumed by servo. On the contrary, servo sectors in each
track of an embedded servo HDD occupies less than 10% of the track. So the
servo overhead is less than 10% for any surface, and does not depend on the
number of disks used in the HDD.
2.3.1 Servo Bursts
In an embedded servo HDD, the servo sectors are placed on all tracks inter-
leaved with data blocks. Small section of a track is illustrated in Figure 2.13.
This figure also shows different fields in a servo sector; each field contains a
specific pattern of magnetization. These fields are,
• DC-gap field,
• Automatic gain control (AGC) field,
• Servo timing mark (STM) field,
• Grey coded track number field, and
• PES burst pattern field.
Each of these fields in the servo sector has specific function. Only two of
them are directly related to the generation of position feedback signal and
are explained here. These are the track number field and burst pattern field.
Thegreycodedtracknumberistheidentification of a track; the outermost
track is tagged as Trk0 and it increases inward. So each track has a unique
grey coded track number field, and the same pattern is repeated in the track
number fields of all sectors of a track. It remains unchanged within the width
of the track and, therefore, does not provide any scope to measure the off-track