Floppy disks again

From: Clint Wolff <vaxman_at_uswest.net>
Date: Tue Oct 12 08:25:30 1999

Hi Bill,

First off, 3:30 in the morning is too early to be reading mail. Go
to bed :)

Most of my knowledge is from hard disks, but should apply OK.

Basically, magnetic media is a tradeoff between coercivity (how
easy it is to flip the bits) and write current (how hard you have
to try and flip the bits). A material with high coercivity resists
being re-magnetized in a different direction. The more write current
you push through the head increases the electro-magnets power, but
also increases its SIZE. More write current means you are writing
bigger transistions. Remember, you can't read a 'north' as a one,
you can only detect when the 'north' changes to a 'south' (a
transistion).

Ok, so 48 TPI media has low coercivity, so a 96TPI head with higher
write current will cause the adjacent tracks to erase because of
the magnet fringes from the head. These magnetic fringes aren't
strong enough to effect 96TPI media because it has a higher
coercivity.

Finally, the head controls the size of the track. The width
of the magnetic poles sets the width of the track written,
plus a little fringing to provide track erasing to clean
the previously written data in case of track misalignment.

The bit density is controlled by how fast the head is moving
w/respect to the media, and how fast you can toggle the write
current. Think of it this way: if you have a stencil .1x.1 and
can fill it in in either red or blue. put the stencil down and
fill it in red. now move the stencil 1/3000 inch. put the
stencil down and fill it in blue. repeat a LOT. Now you have
10 TPI at 3000 bits/inch. Make sense?

I will look for a picture from a class I took many years ago
and put it somewhere. I think I still have a geocities webpage :)

clint


On Tue, 12 Oct 1999, Bill Yakowenko wrote:

> Okay, one final question before I drop this topic
> forever (or until tomorrow, whichever comes first).
>
> How is it possible that DD media could be of such
> poor quality that it can't (reliably?) do 96 TPI,
> while still being just fine at 48 TPI?
>
> I mean, it is recording something like 3000 bits per
> inch along each track, right? How could that same
> media not have enough resolution to keep the bits of
> adjacent tracks separate at less than 100 per inch?
> Can somebody who believes this can happen give me a
> mental model of what is going on there? I mean, in
> terms of physics or geometry or mechanics or anything
> measurable and specific.
>
> It sounds to me like it must be the drive, and not
> the media, that limits the number of tracks to
> anything below 1500 TPI.
>
> Thanks,
> Bill.
>
>
>
Received on Tue Oct 12 1999 - 08:25:30 BST