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In article <25AD7F7A.26599@maccs.dcss.mcmaster.ca> cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn) writes:
>In article <JACOBS.90Jan9093359@cmos.cs.utah.edu> jacobs@cs.utah.edu (Steven R. Jacobs) writes:
>$>Having just had this problem, I asked around and it seems that High Density
>$>disks and Double Density disks (used for 360k) do have physical differences.
>$The person that told you this was either joking or making it up to
>$disguise their ignorance.  Physically, the two types of disks are
>$similar, with oxide covering the entire surface of the disk.  The
>$difference is in the oxide itself.  On the high density disks, the
>$oxide is a different material which requires a stronger magnetic
>$field to record information.  The heads in many 360K drives are
>$not designed to produce a strong enough magnetic field to record
>$data on the high density floppies.
>
>   That sure sounds like a physical difference to me.  And also, as I
>understand it, it's actually the other way round.  A 1.2M disk actually
>uses a weaker magnetic field, which makes 360K disks written in a 1.2M
>drive unreadable in many 360K drives.

Well - you do have it backwards.  The HD media use higher coercivity oxide.
This requires a higher write current to be able to magnetize than the oxides
used in standards disks.   Because of the higher coercivity you are able to
write the bits closer together without the associated problems of bit-shifting
or self-erasure.

If you try to format a regular disk as high density the higher write currents
generate a greater flux field, and this field will erase the previous bits
just written.  If you are successful in writing high density to low density
media, the bits will be so closely packed and the magnetic orientation of the
particles so easy to change that adjacent bits will try to move away if they
are magnetized the same way.  This is the bit shifting phenomenon.  Think of
how like poles on a magnet repel each other.  The bits on the disk try to do
the same.

>   If the problem was that 360K drive heads didn't generate strong enough
>magnetic fields, then I should be able to format 1.2M floppies at 360K in
>my 1.2M drive.  Well, it doesn't work any better in that drive than it
>does in my 360K drive.

When you try to format 360 on a 1.2 meg disk the reason is that you aren't
driving the head with enough current.  The current DOES vary depending on what
you are trying to write.   So when you try to format a high density disk as
low density, the write currents aren't strong enough to overcome the higher
coercivity of the HD disk.  After all, the system thinks it has a low density
disk in it and is using reduced write current. 

A 360k drive has only one one write level.  The 1.2 meg drives writes at two
different levels, that is why the 1.2 meg drives works as poorly on hd media
when trying to format at 360 as does the 360k drive when using HD media.
-- 
Bill Vermillion - UUCP: {uiucuxc,hoptoad,petsd}!peora!tarpit!bilver!bill
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