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

[jjh-blug at vieorhythms.com]

>>>>> "Peter" == Peter Hutnick <peter-lists at hutnick.com> writes:

>>  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.

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

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

Yes, very good point.  My source is definitely suspect.  A bit more
searching finds this source, which is probably a bit more authoritative
and does a much better job of explaining the differences between MFM and
RLL.

        http://homepage.ntlworld.com/tim_n_clarke/FAQ55a3.htm 

I concede on this point and agree that the number of magnetic
transitions per linear unit is unchangeable, and as such the reasoning
in my argument about disk compression is wrong.

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

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

Not so off course, I appreciate the information.  

Peter> -Peter

thanks for the discussion.

-jeremy

-- 
========================================================================
 Jeremy Hinegardner                              jeremy at hinegardner.org