[lug] RLL vs. MFM. Quality vs. Coercivity.

[Peter Hutnick]

> Peter> And as a litmus test: when I upgraded my the controller for my
> Peter> hard disks from an MFM to an RLL controller did the new
> Peter> controller "compress" my disks?
>
> Actually, I would say that yes, it did "compress" your disks as RLL
> recording involves writing more bits per track than MFM.
>
> http://kb.indiana.edu/data/adlt.html :
>
>      RLL recording involves squeezing more bits into a track. This is
> accomplished by using a larger number of sectors (27 rather than 17
> or 19), and requires a better quality drive medium than that
>      required for MFM. RLL recording results in a roughly 50% increase
> in capacity over MFM recording, given the same physical hard
>      drive.

This is flatly wrong in one way and very misleading in another.

Yes, there are more sectors per track, but this DOES NOT require "better
quality drive medium" because the flux transitions occur at the exact same
density.  Don't take my word for it, but suspect your source on that one.

To take it to an even finer point: is (7 bit) ASCII a /compressed form/ of
(8 bit) EBCDIC?

I'm getting waaay off course here, but suspect anything that talks about
the "quality" of media regarding data density.  While the minimal level of
constancy and quality control required increases with data density, the
attribute in question when it comes to how many flux transitions can be
stored by area is coercivity.  That's why those punches that let you
format 720k floppies to 1.44M didn't work very well.  Not, as is/was
commonly thought, because 720k floppies were of lower "quality."

-Peter