Re: HD 5.25 format?

[dempson@actrix.gen.nz (David Empson)]

Laine Houghton <laine@rainbow.rmii.com> wrote:

> What exactly is a HD 5.25 disk and why won't the //'s work with it?
> Guess I always thought HD ment 3.5's so know I'm curious.

"HD" in general means "high density"; "DD" means "double density".
There are also "single density" disks, but that is usually just a
quality distinction.  (I'll abbreviate everything in the following
discussion, to save space.)

Both physical sizes of disks come in both densities.  On a PC, a DD
5.25" disk holds 360K, while an HD 5.25" disk holds 1.2MB.  A DD 3.5"
disk holds 720K, and an HD 3.5" disk holds 1440K.

The density usually refers to the type of magnetic media used on the
disk surface.  An HD disk has finer metallic particles, so it is able to
record a higher frequency (and weaker) signal.

It is not possible to use an HD 5.25" disk in a drive which only
supports SD or DD disks.  The SD/DD drive requires a disk media that can
hold a strong signal, but an HD 5.25" disk only holds a weaker signal.
The drive cannot read back what it has written, because the signal isn't
strong enough.  (Assuming what it wrote was even written reliably).

If you use a DD 5.25" disk in an HD drive, the drive has to switch to DD
mode to be able to use the disk reliably.  With 5.25" disks, there is a
problem here: DD 5.25" disks have 40 tracks, while HD disks have 80
tracks.  An HD drive usually has a smaller head to allow for the
narrower tracks, and it therefore produces a weaker signal (compared to
a DD drive) when writing to a DD disk.  I have often found that DD 5.25"
disks written in an HD drive cannot be read reliably by DD drives.

3.5" disks tend to be less fussy: DD and HD 3.5" disks have the same
number of tracks (80), and therefore the drives have the same width
heads.  An HD 3.5" disk is able to fit twice as many sectors per track
as a DD disk.  The HD drive simply varies the strength of the write
signal and adjusts the bit cell timing according to the type of media.

Even with 3.5" disks, it is still not a good idea to use an HD disk in a
DD-only drive, because the signal is not recorded as well (due to the
physical characteristics of the media).  An HD disk written with a DD
drive is likely to develop errors sooner than a DD disk written in the
same drive.

It seems that there is a significant difference between the physical
media with DD and HD 5.25" disks, but less of a difference between DD
and HD 3.5" disks.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand