Sparta PCBoard Hard Disk Conference
                        (201) 729-7056 or 729-5377

                            Technical Bulletin
                      Run Length Limited Coding (RLL)

In a Winchester disk drive, data is recorded along a disk track
by generating changing magnetic fields.  Data is detected (read)
by sensing the presence or absence of transitions between these
fields.  Recording density is limited by how closely these magnetic
fields (flux reversals) can be placed on a given track.

Bit density is a somewhat different matter.  Depending on the 
encoding technique, a given number of flux reversals may represent
fewer or more bits.

Run Length Limited (RLL) data encoding affords an improvement in
bit density over the single density encoding of Frequency Modulation
(FM) and double density encoding of Modified Frequency Modulation (MFM).
This is achieved by representing a given bit stream with fewer flux
reversals.  

Both FM and MFM defind a bit as having both a clock and data component.
FM writes both a clock bit and a data bit for logical ones only.  MFM
writes data flux reversals for one bits but removes all clock bits
except between consecutive zeros.

RLL has nothing resembling clock and data bits, rather it turns
data patterns indo code patterns.  Any binary data string can
be broken up into combinations of just seven code patterns as shown
below in Table A. RLL offers the best trade-off between flux 
reversals and bit density and is used internally on the data channel
on some of the currently manufactured high capacity drives.

Some hard disk controller vendors, including Adaptec (ACB 2070A)
and OMTI (5527), have developed controllers that encode data in
RLL for use with those ST506 products which are designed to give
good error rates with 2:7 code, such as the Seagate ST-238 and
Toshiba MK-50 series. These controllers effectively increase the
capacity (and data rate) by 50%.

If we could graphically represent the evolution of FM through MFM,
and finally to RLL, you could see that RLL represents the best of
both worlds... minimum flux reversals with maximum bit density.
We could show you that the minimum time between flux reversals
using RLL is the same as when using MFM. Thus, the capacity and
data are increased by 50% without increasing track flux density.


                                  TABLE A

                         RLL Codes for Binary Data

Binary Data                            RLL Code
11                                         1000
10                                         0100
000                                      100100
001                                      001000
010                                      000100
0110                                   00100100
0111                                   00001000


                                  TABLE B
   
                       Comparison; FM, MFM, RLL 2,7

                                          FM         MFM      RLL
Capacity (MBytes unformatted)             43          86      129
Data Transfer Rates (MHZ)                2.5         5.0      7.5
Data Window (nsec/bit)                   400         200      133


This data taken from a technical bulletin provided from Toshiba America.
The data is accurate. My typing may not be. 

Richard Driggers
SysOp
Sparta PCBoard
(201) 729-7056
June 22, 1986

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