Introduction - Hard Disk Drive: Mechatronics and Control - page 21

1.3.4 Zoned Bit Recording

If the rate of data transfer between the electronics and the media is kept
constant irrespective of the radial position of the track then the read/write
electronics can be optimally designed to cater for this single data rate. Such
aschemeiscalledconstant data rate recording. Frequency of the clock signal
used during reading or writing of data remains the same on all tracks, i.e.,
T W and therefore f W =

1

T W is constant. If the angular speed of the disk is
kept constant, the linear velocity v = ωr ofthemediumwithrespecttothe
head increases with increasing radius. Then the dimension (L bit = vT W )ofa
single bit depends on the radius of the track where the bit is written; inner
is the track smaller is this dimension. If the linear density is optimized on
the outermost track then the transitions on an inner track are too close to
each other and can not produce signi fi cant read back voltage. On the other
hand, if the clock frequency is chosen to achieve optimum linear density on
the innermost track, the transitions are sparsely created on outer tracks. In
the constant data rate recording, same number of data blocks are stored on all
tracks. However, the circumference of the outer track is larger than that of an
inner track and, therefore, it makes better sense to store more number of data
blocks on outer tracks.
It is possible to achieve the ideal solution to this problem if either radius-
dependent clock frequency or radius-dependent spindle speed is used. In CD-
ROM, the speed of the spindle motor is continuously adjusted as the head
moves from one track to another. This ensures constant linear density of
recording and hence constant areal density. Radius-dependent clock frequency
is not used in any storage device. Hard disk drive employs a scheme, Zoned
Bit Recording (ZBR) or Zoned Density Recording, that groups the tracks into
several annular zones. Each zone has its own recording frequency which op-
timizes the linear density on the innermost track of that zone. Frequency of
recording is increased from inner zone to outer zone. All tracks within a zone
use constant data rate recording and contain equal number of data blocks. A
schematic illustration of zoned-bit recording in Figure 1.10 shows the surface
of a disk divided into 8 zones. There are more data blocks per track in an
outer zone than an inner zone, which is clearly shown in this diagram.
More is the number of zones, better is the utilization of storage space.
The extreme end is to assign one track per zone and the clock frequency is
optimized for each track individually to achieve optimum linear density on all
tracks. This also results in constant areal density. The marginal improvement
due to increase in number of zones is signi fi cant when few zones are used.
There is approximately 13.63% improvement in storage when we increase from
2 zones to 3 zones, but increasing number of zones from 7 to 8 gives only 1.59%
improvement. Commercially available drives use 16-32 zones.