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From: tjr@cbnewsc.att.com (thomas.j.roberts)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: 360K in 1.2M drives
Message-ID: <1991Mar8.232132.20519@cbnewsc.att.com>
Date: 8 Mar 91 23:21:32 GMT
References: <1265@theseas.ntua.gr>
Organization: AT&T Bell Laboratories
Lines: 46

From article <1265@theseas.ntua.gr>, by terry@theseas.ntua.gr (Eleftherios Simotas):
> gettys@yacht.enet.dec.com (Bob Gettys) writes:
> 
> 
>>	One way to re-use a floppy without getting into this problem is to bulk
>>erase it with a bulk tape eraser. This destroys the 360k info allowing
>>the 1.2meg sized info to be visible without the old 360k info in the
>>"background".  
> 
>>	Typically (but not always!) you can use a virgin (or a reborn virgin
>>via bulk erase) floppy to take data from a 1.2meg drive to a 360k drive
>>if the disk is formated as a 360k disk with the 1.2meg drive. This lays
>>down the narrow track without the interference of the wide track in the
>>background. When it won't work it is the result of the signal (remember,
>>there is less of it) is tto weak for the particular 360k drive you are
>>trying to read it on. 
> 
> I've wondered about the ability of a 1.2meg drive being able to reliably 
> write info on 360k disks even if they have been used in a 360k drive.
> I am aware of the head width differences resulting in a 1.2meg drive
> overwriting only part of the track written by a 360k drive. Couldn't
> this problem be overcome by having the 1.2meg drive write every track 
> twice ? (now it skips every other track when writing 360k format).
> I have done a bit of low level programming on PC's but I am not in 
> position to write such a utility just to see if it would work.
> There is also the problem of signal strength. The 360k drives have much
> larger write currents resulting in a stronger signal. The 1.2meg ones
> having to cope with much higher bit densities use lower write currents
> coupled with better signal detection electronics.
> So there is a chance even by double writing the tracks that the 360k
> signal will still peek through.
>>	/s/	Bob Gettys

There's NO HOPE of getting higher signal strength by writing two
1.2-meg-width tracks in place of a single 360-kb-width track. To
do so would require that the two separately-written identical tracks
are aligned under the wider 360-kb-width head to within less than
half a bit width. The bit width for a 360-kb diskette is (outer track):
	(circumference) / (# bits) = (3.14*5") / (9*512*8) = 0.00042"
Aligning the rotational position of a diskette to within 0.0002 inches
is not possible using the 1/8" (or so) diameter index hole. The whole
point of the diskette format is to remove any necessity for aligning
the diskette very accurately.

Tom ROberts
att!ihlpl!tjrob  TJROB@IHLPL.ATT.COM