Hardware Reference
In-Depth Information
Run Length Limited Encoding
Today's most popular encoding scheme for hard disks, called
Run Length Limited
, packs up to twice
the information on a given disk than MFM does and three times as much information as FM. In RLL
encoding, the drive combines groups of bits into a unit to generate specific patterns of flux reversals.
Because the clock and data signals are combined 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 transitions. 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.)
RLL 2,7 was initially the most popular RLL variation because it offers a high-density ratio 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 window or cell size within which a transition can be detected,
RLL 1,7 is a more forgiving and more reliable code, which is important when media and head
technology are being pushed to their limits.
Another little-used RLL variation called RLL 3,9—sometimes also called Advanced RLL (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 transitions.
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, respectively. The selected
