Hardware Reference
In-Depth Information
in these patterns, the clock rate can be further increased while maintaining the same basic
distance between the flux transitions on the storage medium.
IBM invented RLL encoding and first used the method in many of its mainframe disk
drives. During the late 1980s, the PC hard disk industry began using RLL encoding
schemes to increase the storage capabilities of PC hard disks. Today, virtually every drive
on the market uses some form of RLL encoding.
Instead of encoding a single bit, RLL typically encodes a group of data bits at a time.
The term Run Length Limited is derived from the two primary specifications of these
codes, which are the minimum number (the run length) and maximum number (the run
limit) of transition cells allowed between two actual flux transitions. Several variations of
the scheme are achieved by changing the length and limit parameters, but only two have
achieved real popularity: RLL 2,7 and RLL 1,7.
You can even express FM and MFM encoding as a form of RLL. FM can be called RLL
0,1 because as few as zero and as many as one transition cells separate two flux trans-
itions. MFM can be called RLL 1,3 because as few as one and as many as three transition
cells separate two flux transitions. (Although these codes can be expressed as variations
of RLL form, it is not common to do so.)
RLL2,7wasinitiallythemostpopularRLLvariationbecauseitoffersahigh-densityratio
with a transition detection window that is the same relative size as that in MFM. This
method provides high storage density and fairly good reliability. In high-capacity drives,
however, RLL 2,7 did not prove to be reliable enough. Most of today's highest capacity
drives use RLL 1,7 encoding, which offers a density ratio 1.27 times that of MFM and a
larger transition detection window relative to MFM. Because of the larger relative timing
windoworcellsizewithinwhichatransitioncanbedetected,RLL1,7isamoreforgiving
and more reliable code, which is important when media and head technology are being
pushed to their limits.
Anotherlittle-used RLLvariation called RLL3,9—sometimes alsocalled AdvancedRLL
(ARLL)—allows an even higher density ratio than RLL 2,7. Unfortunately, reliability
suffered too greatly under the RLL 3,9 scheme; the method was used by only a few now-
obsolete controllers and has all but disappeared.
Understanding how RLL codes work is difficult without looking at an example. Within
a given RLL variation (such as RLL 2,7 or 1,7), you can construct many flux transition
encoding tables to demonstrate how particular groups of bits are encoded into flux trans-
itions.
In the conversion table shown in Table 8.3 , specific groups of data that are 2, 3, and 4 bits
long are translated into strings of flux transitions 4, 6, and 8 transition cells long, respect-
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