Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!sdd.hp.com!hp-pcd!hplabs!hpda!hpcupt1!hprnd!hprpcd!!patf
From: patf@.HP.COM (pat fung)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Perstor controller?
Message-ID: <510001@.HP.COM>
Date: 26 Jun 90 22:04:10 GMT
References: <780023@hpsad.HP.COM>
Organization: HP Roseville Site
Lines: 48

From pnl@hpfinote.HP.COM Sun Jun 24 15:26:40 1990 ,  Peter Lim writes:
 
>Not according to what I know. RLL increases bits/track by doing 2 things.
>(1): Re-arrange the flux changes so that there is less flux change for
>     the same number of bits stored (which is what you just stated
>     above).
>(2): Pack successive bits more than 50% closer than in MFM. The 2,7
>     encoding scheme requires that more bits to be stored for the same
>     amount of data than MFM. At the same time, RLL store 50% more
>     data than MFM  =>  more than 50%.  :-).   However, because the
>     2,7 encoding scheme reduces the flux change per bit, it worked
>     out to be about 14 % more  ---->  Don't ask me how I arrive at
                      ^^^^ ---> 140%
>     this number. It is from some second hand source.


  This is how you get 140% more capacity using RLL 2,7 encoding. RLL 2,7 uses
7 bits to encode 5 data bits with no less than two consecutive bits of the
same type. The worst case of flux change is every two bits compared with MFM,
the worst case is a flux change every bit when alternating 1's and 0's are
encountered. So in the space of 7 bit of MFM recording, you get 14 bits of
RLL recording using the same flux density. But the 14 bits of RLL data is only
representing 10 bits of actual data. The increase in capacity is:

                      10/7 = 1.42 or 142%

  By playing with the inter-sector gap, 26 sectors can be squeezed into a
track compare with 17 sectors with MFM. The capacity boost is more like
25/17=1.52 or 152%.

  Since the flux density is the same as MFM recording, there is no reason
why any disk drive cannot be use with RLL controllers. I have used ST225, 
ST251 with no problem. No need to pay more for RLL rated drives, they are
the same drives! There is an argument that the bit shift is more
critical with RLL data seperators. With the advance in data separator design
this is no longer a problem and this is why the early effort of ARLL designs
will not work with oxide coated drives.

  The ARLL increase the capacity by incresing the flux density by 25%. I do
not know the Perstor controller writes 33 or 34 sectors on one track. This
is almost 190% to 200% increase in capacity. Will it work with any hard disk
drives? Like to see anyone with the Perstor controller post their experiences.


Pat Fung

patf@hpranger.HP.COM