> 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?
???? Nonsense question, no context. I have no difficulty
nor is there any reason to expect difficulty with 96tpi DD
ops. I have expereinced using formerly formatted media of
96 or 48TPI that REQUIRED bulk erasure to be usable. I believe
that intertrack noise due to differnt track widths are why.
I regurally use PC360k, Vt180, RX50 and Visual1050 media and they
are 48 TS, 48 SS, 96ss, 96ss/TS all running at DD data rates.
> media not have enough resolution to keep the bits of
> adjacent tracks separate at less than 100 per inch?
The recorded track density is not he same as the TRACK WIDTH
or the TRACK position.
> 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.
For all DD the number of bits per track will be the same (generally)
so whats different? The spacing of the tracks and their width.
Both 48 and 96 tpi drives write their trace in the same ~1" band on the
media.
The older 48tpi drives sliced this into 40 bands of .025"(approx) wide.
To allow for error the head writes and area less than that.
Not the newer 96tpi drives also use that same ~1" but put 80 bands
in that space, they are .0125(approx) wide. The head is narrower to
align and fit in that space.
The old 48tpi heads literally straddle two of the 96tpi tracks if you go
from 96 down to 48. Worse is that if you dilberatly skip a track to
try and create a "48tpi compatable disk" you still write a track that is
less than half the width. When you read it there is less than half the
magnetized area and half the signal. This promotes errors.
Going the other way, works better as the narrow 96tpi head gets to read a
track wider than itself, lots of signal. Can't write though as the head
is narrower than the old wide track, this means writing with a narrower
head can only erase a tunnel in the data previously written. read it
back and likely it's ok only less the drive is a different one and
alingment is off ever so slightly allowing both old data nd new data to be
visible, errors result. Try to read that disk on a 48tpi drive and the
wide head reads old data and new data, results will be poor at best.
> 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.
It's the .667 mil (0.0006666) wide head and the positioner.
Yes, specifically head width. The media also limits things some
as very narrow head will tend to dig into the media and the width of the
track also defines signal level. Zip disks are high track density
floppies using special cobalt media for high bit density and also high
track density. They aslo have servo info on the media to aid in finding
the rather narrow tracks.
This is why 135 tpi has been the limit for absolute mechanical positioners
like floppies. At some point the track is so narrow you have to be able
to hunt for it and stepper based positioners are inexpensive but lack
resolution (and speed) for that task. The next generation will be some
form of voicecoil and the media will have embedded servo (Zip, LS120,
Jaz and friends). At some point you bridge the differences between floppy
and hard disk.
There are really several questions in your posting.
Allison
Received on Tue Oct 12 1999 - 09:33:08 BST
This archive was generated by hypermail 2.3.0
: Fri Oct 10 2014 - 23:32:32 BST