U. Glaeser

FIGURE 34.39 Alternative burst formats where A′ and B′ are either orthogonal to or of opposite polarity of A and
B, respectively.
A and B are transformed into amplitude estimates. These amplitude estimates are then subtracted from
each other and scaled to get a positional estimate. As the head moves from track center n to track center
(n + 1), the noiseless positional estimate, known as position error transfer function, is plotted in Fig. 34.38(b).
Here, since the radial width of the servo burst is larger than the read element, any radial position falls
into either the linear region, where radial estimate is accurate, or in the saturated region, where the radial
estimate is not accurate [9]. One solution to withstand saturated regions is to include multiple burst
pairs, such that any radial position would fall in the linear region of at least one pair of bursts. The
obvious drawback of such a strategy is the additional format loss. The amplitude format just presented
does not suffer from radial incoherence since two bursts are not recorded radially adjacent to each other.
Because nonrecorded areas do not generate any signal, in Fig. 34.38(a) only 50% of the servo burst
format is recorded with transitions or utilized. In an effort to improve the position estimate performance,
the whole allocated servo area can be recorded. As a result, at least two alternative formats have emerged,
both illustrated by Fig. 34.39.
In the first improved format, burst A is radially surrounded by an antipodal or “opposite polarity” burst
A′. For example, if burst A is recorded as ++−−++−−… then burst A′ is recorded as −−++−−++…. For
readers familiar with digital communications, the difference between the amplitude and antipodal servo
burst formats can be compared to the difference between on-off and antipodal signaling. In on-off
signaling, a symbol “0” or “1” is transmitted while in antipodal signaling 1 or −1 is transmitted. Antipodal
signaling is 6 dB more efficient than on-off signaling. Similarly, it can be shown that the antipodal servo
burst format gives a 6-dB advantage with respect to amplitude servo burst format under the AWGN
assumption [17].
Instead of recording A′ to be the opposite polarity of A, another alternative is to record a pattern A′
that is orthogonal to A. For example, it is possible to pick up two sinusoids with different frequencies
such that the two waveforms are orthogonal over a finite burst length interval. The resulting format is
known as the dual frequency format [20]. Inside the read channel, two independent estimates of the head
position can be obtained from two estimators, each tuned to one of the two frequencies. The final radial
estimate is the average of the two estimates, resulting in a 3-dB improvement with respect to the amplitude
format, again under AWGN assumption.
Unlike the amplitude format, these more sophisticated formats are in general more sensitive to other
impairments such as erase band and radial incoherence.
A fundamentally different format is presented in Fig. 34.40. Here, the transitions are skewed and the
periodic pattern gradually shifts in the angular direction as the radius changes. The radial information
is stored in the phase of the period, so it is called the phase format. In Fig. 34.40 two burst fields A and
B are presented where the transition slopes have the same magnitude but opposite polarities. An estimator
makes two phase estimates, one from the sinusoid in field A and another one from the sinusoid in field
B. By subtracting the second phase estimate from the first, and then by scaling the result, the radial
position estimate can be obtained. Similar to the antipodal format, it can be shown that the phase pattern
is 6 dB superior to the amplitude pattern [17] under AWGN. A major challenge for the phase format is
successfully recording the skewed transitions on a disk platter without significant presence of radial
incoherence and erase band.
© 2002 by CRC Press LLC
track n
track n+1
A
A′
B′
B