Information Technology Reference
In-Depth Information
Transducer location
Recording layers
Soft Interlayers
H z (a.u.)
SILS
SUL
Distance across the thickness (a.u.)
Transducer location
Line cross thickness
(a)
(b)
Figure 6.13. (a) Diagram and (b) simulations to illustrate how the field could be
''discontinued'' in the region of SILs. (c) Diagrams of various bit configurations to
illustrate the effect of the stray field on the intersymbol interference.
strongly remind the prerecorded patterns. One notices here that various deconvo-
lution based signal encoding techniques could be implemented to further optimize
SNR of a multilevel system [55].
How Soft Interlayers Could Be Used to Further Separate Individual Layers
across Thickness: The purpose of SILs is to create field ''discontinuities'' to
facilitate information access not only in the top and bottom layers but also in the
intermediate layers. Each SIL is patterned with a pattern reflecting the pattern of
SUL (Figure 6.13a). Numerical simulations showed that a thickness (of SIL) of
smaller than 2 nm is sufficient to create a detectable ''discontinuity''; at the same
time it is not too thick to totally shield the field from penetrating across the
thickness. A typical field profile across the thickness of the media with adjacent
magnetic layers separated from each other with a 2-nm thick SIL is shown in
Figure 6.13b. The thickness of the magnetic layers and SILs could be optimized to
maximize the effective number of layers.
6.3. MULTILEVEL DATA ENCODING
Before going into details of the new multilevel coding method, it may help to recall
how conventional coding works.
6.3.1. Summary of Conventional Encoding Methods
FM encoding has a simple one-to-one correspondence between the bit to be
encoded and the flux reversal pattern. One needs only to know the value of the
current bit. So called modified frequency modulation (MFM) improves encoding
efficiency over FM by controlling more intelligently where clock transitions are
added into the data stream; this is enabled by considering not just the current bit
but also the one before it. This ''looking backwards'' allows improved efficiency
by letting the controller consider more data in deciding when to add clock
 
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