Hardware Reference
In-Depth Information
Figure 2.20: Dual frequency burst pattern.
is always possible. The second method of increasing sampling frequency by
adding more servo sectors per track does not affect the spectrum of the noise
and disturbances. Neither it generates more heat. However, it takes up more
space of the disk that can otherwise be used for storing data. If the length
of the servo sectors can be reduced without compromising the quality of de-
modulation of bursts then more servo sectors can be added with no additional
servo overhead. One scheme with patterns of different frequencies for different
servo bursts reduces the length of servo sector. Only difference between this
scheme and the conventional embedded servo is in the burst pattern; all four
bursts in the conventional scheme uses patterns of same frequency.
Dual Frequency Burst Pattern
In the conventional method, the four servo bursts use the same pattern of
magnetization, i.e., a series of transitions created at regular spacing along the
track (down-track direction). Since these bursts have identical patterns, they
can not be distinguished from one another if two of them are aligned in the
radial direction (cross-track direction). This makes it absolutely essential to
place the bursts shifted circumferentially from one another, leaving many voids,
i.e., area with no pattern, in the servo sector (see Figure 2.14). If the pattern
of transitions in one burst is made different from that of another burst then
these bursts can be distinguished easily. Two such bursts can be placed along
thesameradiusofthediskwithnocircumferentialoffset, and yet without any
loss of their identities. Then the voids otherwise found between the bursts
of conventional patterns are eliminated. Schematic representation of the dual
frequency burst pattern is shown in Figure 2.20. The diagram shows both in-
phase and quadrature bursts, which are essentially the same pattern but are
placed with radial offset by half a track.
In the conventional method, waveforms of all four bursts are identical. But
for dual-frequency burst pattern, the two bursts of the in-phase (or quadrature)
burst-pair produce different waveforms. Both of them contain series of pulses
with alternating polarities, but the intervals between two adjacent peaks in
one burst waveform differ from those of the other. If the read head senses both
