14 Oct
1999
14 Oct
'99
1:06 a.m.
I'm thinking of the 5.25" DS/DD floppy disks
commonly used
in PC's and formatted to hold 360K at 48TPI. I was recently
discussing with someone whether or not they could be used as
RX50's, which as you know have 96 tracks per inch, with each
track having the same number of bits as in the PC 360K DS/DD
format. Both are supposed to be 300 Oerstedts. But I think
Yes they are.
I remember hearing complaints that the
not-so-high-quality
DS/DD 360K floppy disks would not work okay with the higher
number of tracks per inch. And I am sure I read something
about the importance of these diskettes being "well honed"
to be used as RX50's, with the assertion that not all 360K
DS/DD disks are. (But "well honed" was left undefined, and
I'd love to know what exactly was meant, as well as what
physical property of those not-so-high-quality diskettes
was causing them to fail.)
Bogus. Actually the term is calendered and its a surface treatment
for smoothness and consistancy. But whatever. I regurally use non-rx50
media for RX50 use. the real problem people had was lack of erasure
(bulk erase if you reformat) and the systems sometimes used to format the
disks wer causing problems (bad formatting as DEC was a bit oddball to fit
the 10 sectors).
The point I'm after is about the limitation on how
many
tracks that disk can hold - does that limitation come from
something about the diskette itself, or is it a property
of the disk drive in which it is used? My thinking is that
the media can handle >3000 magnetic transitions (bits) per
inch, because it does that along each track. So that can't
be the limiting factor that prevents you from fitting 96
tracks into one inch. I'm not concerned with the drive in
which it is written; I only care about what the media could
hold if I had a precise enough disk drive.
It's largely a property of the drive as the media is the same (excluding
imperfections) across the entire surface. Its an issue of head width,
positioner and stablity of the frame/head assembly. FYI: 3.5" drives do
use 600o media and 135 tracks/in. The reason for that is as you make the
head smaller it's harder to get a good signal (with simple amplifers)
so you use a "stronger" magnetic material to get a bigger signal.
<also a stronger material is more resistant to head landing damage or
external fields too.>
Assuming that the magnetic granules in the oxide are
not
systematically shaped in any odd way, (not generally longer
than they are wide, or wider than long), then the media
itself should in principle be able to handle 3000 tracks
per inch - just as many transitions moving outward from
Could but they would be ajacent to each other and there magnetic effects.
Even had drives have space (even if small) between tracks that are far
wider than the bit density. But Even at 500 tpi to allow that you have
the problem of need a positioner that can hit .002" +-.0005"(or better).
Steppers and bands or lead screws really arent up to the task and also are
very slow. transversing 500 tracks at 3ms step rate will take 1.5
seconds! Thats far too slow and we would have to look at the servo
(winchester) technologies used for hard disks. that also implies the media
would have to have servo info recorded and we end up with the ZIP disk!
and any old never-written (or properly bulk-erased)
"PC 360K" diskette should be perfectly happy as an RX50.
Yep!
more or less like a UPC bar code - the width of the
track
is very much greater than the length of a bit in the track;
maybe more than 200 times greater. So say the track was a
1 meter wide sidewalk, the bits would each be 1m wide, but
only 5mm long. So even a tiny twist in the read head would
gum it up real good - it should be much more sensitive to
this than to alignment. Move 1cm to the left, and you are
still 99% on the sidewalk - no problem. But twist just one
degree, and when your left side is at one bit, your right
side is three bits ahead or behind - big trouble. Well,
maybe the forces acting on the head are less likely to
twist it than to shift it.
Yep. But the head to read that 1m side walk is only .5m wide
and has erase elements to clear to the 1.05m width.
The track width is not 1/n the nuber of tracks as there is "dead" space
inbetween and then the actual track is not used to the full width as well.
so for you example the bar code actually has a requirement of 1m width (so
you can find it) actually read or written to for .5m and the edges are
erased wider than the read/write area and the bar code must also have a
"clearence band" around it of say .5m to make the picture more complete.
So the 48tpi disk may have a track that is .0208333" wide and of that
I'd bet without checking less than 50% is even written (erase or data).
Asmuth is important too, it impacts read signal strength. In this case
the wider bars are bigger magnets for the reason of a bigger signal.
Most floppy heads are fairly rigid in their mounting so thats not much of
an issue(save for the manufacturer building it). (also read head gap
important too).
Allison