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From: walk@mrcnext.cso.uiuc.edu (Todd Walk)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: Disk space for FREE (was Re: super high density formatters)
Message-ID: <1990Nov20.201159.17450@ux1.cso.uiuc.edu>
Date: 20 Nov 90 20:11:59 GMT
References: <1990Nov18.100239.10040@corp.telecom.co.nz> <992@demott.COM>
Sender: news@ux1.cso.uiuc.edu (News)
Organization: University of Illinois at Urbana
Lines: 51

kdq@demott.COM (Kevin D. Quitt) writes:

>In article <1990Nov18.100239.10040@corp.telecom.co.nz> stephen@corp.telecom.co.nz (Richard Stephen) writes:
>>I think the "no difference in the media" statement could be debated a
>>while. In principle, I'm inclinded to agree if you view it from a disc
>>manufacturer's point of view. It would be horribly expensive to run two
>>production lines, but if you run one production and test *every* disc as
>>it comes off the line with a special rigorous test for 1.44 MB media
>>integrity, you have "guaranteed" 1.44 MB discs and the failures are, after
>>quality checks your 720 K discs. Sound reasonable ?

>    Most manufacturers run a single line.  Disks are tested for either
>high or low density; disks that fail high-density certification are
>usually tossed, rather than sent back for low-density certification (not
>cost-effective).  Manufacturers that maintain good quality standards
>will have very few dropouts of either type, and usually not
>significantly more on the high side than on the low (when something's
>wrong in a newly manufactured disk, it probably isn't subtle). 


>-- 
> _
>Kevin D. Quitt         demott!kdq   kdq@demott.com
>DeMott Electronics Co. 14707 Keswick St.   Van Nuys, CA 91405-1266
>VOICE (818) 988-4975   FAX (818) 997-1190  MODEM (818) 997-4496 PEP last

>                96.37% of all statistics are made up.


The only significant differences between th 720K and the 1.44MB disks are that
the 720K disk have a coercitivity of about 600 compared to about 700 for
1.44MB disks, and the extra notch in the 1.44MB disks.  (Coercitivity is a 
measurement of magnetic strength.)  The difference in coercitivity is small,
so that it can't be the problem, but that extra notch can definitely be a 
problem.  Think about how much easier it is to make the entire disk, test it,
and throw away bad ones than to stop the production line just before the 
extra notch is cut, test the disk, move the few medicore ones to the 
720K disk line, and then cutting the notches in the rest of the disks.
I believe it obvious why the 720K disks are make on a different line than the 
1.44MB disks.
						Todd Walk
						walk@mrcnext.cso.uiuc.edu

(Oh yes, I just about forgot to say that since the 720K disks have a lower 
coercitivity than the 1.44MB disks, they are marginally reliable formated
as 1.44MB disks.  This means that some will not format in 1.44MB, and the others
will format as 1.44MB disks, but since disks lose coercitivity over time,
they will lose their information MUCH faster than a true 1.44MB disk will.
A true 1.44MB disk has a average data lifespan measured in years, while
a 720K disk formated to 1.44MB have an average data lifespan measured in       
months.)