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EVERYTHING YOU EVER WANTED TO KNOW ABOUT FLOPPY DISKS.......
BY Ted Jensen
FROM: ARTICLES, "The KAY*FOG Online Magazine."
So, you've just spent 400 bucks for that super piece of software,
made your back-up and are working away with your working copy.
Suddenly, while you are working with a relatively unimportant utility
program on another disk, your disk goes bad. This is not a major
problem. You have a back-up somewhere, but it gets you to thinking
about your back- ups on your commercial programs. What happens if they
go bad? Should you have made them on some type of premium diskette to
guard against that?
You scour through catalogs and ads in magazines. There are sources
galore for diskettes, at all prices, and some of them even have
specifications. You run into one spec called "Clipping Level" and the
supplier claims that because his disks have been tested to a higher
clipping level they are superior. Should you pay a premium for disks
with superior specifications? What do these specs mean? Will your
back-ups be less likely to fail if you use premium discs?
These are difficult questions to answer. Perhaps an explanation of
some of the tests run on disks and what can happen to your back-ups with
time would help you make that decision. In addition, you may be
interested in considering the cost tradeoffs of using higher priced
disks.
As an engineer with many years of experience in magnetic recording
I had never heard of the term "clipping Level" until it came up in a
discussion on KAY*FOG. In fact, I had never seen a specification sheet
in any box or bag (I buy the cheap stuff by mail order too!) of disks I
have purchased. However, I did spend a couple of years as a part of a
design team on a Winchester Drive for personal computers and one of my
tasks was the specification and testing of the disks used in those
drives.
CLIPPING LEVEL: Since magnetic media is pretty much the same
whether it is tape, diskettes, or hard disks (the major difference being
that the material to which the magnetic particles are bonded is mylar
for tape and diskettes, and aluminum for the hard disks), it wasn't
difficult for me to guess at what was meant by "clipping level." A
little looking through a parts catalog and I found a specification on a
chip designed for use in disk drives and they defined "clipping level"
(although in rather vague terms). It is unfortunate that these words
are used to describe a test performed on diskettes since they have a
different and more widely understood meaning throughout the general
electronics industry. In any case we will have to accept these words
since they are the ones used in the advertisements.
In simple terms, your drive uses a "head" to read the information
on the disk. You can think of this as being like the needle and pick-up
on your phonograph. The head reads the magnetic information previously
written on your disk and converts it into an electrical signal. This
signal is further processed and eventually takes on a form suitable for
transmission to your computer as bits, or bytes, which represent the
data.
SIGNAL VARIATION: The size and shape of the electrical signal
developed by the head varies for many reasons. First of all, it varies
as a result of the information written on the disk, and this variation
itself represents that information. However, there are other variations
which take place due to imperfections in the head, the mechanical
characteristics of the drive, or imperfections in the diskette. These
variations, if large enough, will lead to the electronics in the drive
not being able to correctly decode the information, and your computer
will indicate by means of some error message that it cannot read the
disk. It is therefore important to keep these variations (those not
part of the data) at a minimum.
COATING THICKNESS: Magnetic diskettes or tapes are manufactured by
bonding magnetic particles to a flexible mylar backing material.
Characteristics which affect the performance of the final product
include, but are not limited to, the magnetic characteristics of the
particles, the size of the particles [Note: it is modification of these
two characteristics that make the difference between a 1.2M disk and a
360K disk.], the thickness of the coating, and, most important to the
subject of "clipping level," the uniformity of the coating. If a tiny
part of the disk, the size of a pinhole, does not get coated, the signal
level recoverable from that spot is reduced. Thus, if there are a
number of these of sufficient size, the level of the signal will be
fairly uniform until that "pin-hole" passes under the head, at which
point it will drop. These are referred to as "drop-outs" in the
industry.
Furthermore, if the coating thickness varies over the surface of
the disk, the amplitude of the signal can vary in a relatively smooth
manner as the disk rotates. This is generally not a serious problem,
however.
Your drive can recover your data by separating these disk related
variations from the variations in signal due to the real data, provided
that disk related variations are not too large. Typically a drive might
be able to successfully ignore disk related variations which did not
reduce the amplitude of the real signal to less than 30% of the normal
output. This number, however, also depends on a wide variety of
factors, and varies from drive to drive, even the same model from the
same manufacturer.
Thus, anything one could do to assure that the level of these disk-
related variations are held within a specified range should reduce the
probability of errors. The key word is "probability," and more will be
said about this later. Therefore a disk which is tested to a "clipping
level" of 60% is tested to assure that the variations due to the disk
are small enough that the signal level never drops below 60%. That is,
the variations are held to a range between 60% and 100%. It follows
that the higher the "clipping level," the less variation in signal
output and the reduced probability of a disk error.
Now comes the tough part. How much extra money should you pay for
a disk tested to a 60% level as compared to one tested to a 40% level?
Would you pay 50% more? Twice as much? Ten times as much? The way I
look at it is this: There is a high probability that if I buy 25 or 50
brand X disks and they all work,whatever tests were run on them were
probably sufficient to assure me that brand X disks will always work. I
have no way of knowing what "clipping level" disks destined for my
drives should be tested at, nor, do I believe, do the manufacturers of
floppy disks.
A WORD ABOUT HARD DISKS: In the case of Winchester drives, the
situation is a little different. The manufacturers of the disks which
go in these drives are generally different companies than those that
manufacturer to drives. The drive manufacturer imposes specifications
on the disk manufacturer. Furthermore, the drive manufacturer
continually tests disks using sophisticated equipment to be sure that
the disk manufacturer meets these specifications. That is, people who
manufacture disks for use in hard drives do not sell them directly to
the end user (removable hard disks being the exception).
BOTTOM LINE $$: But, back to floppies. Assume I buy 100 diskettes
from each of two sources, SuperDisk and CheapDisk. Assume I pay 40
cents each for the CheapDisks and $2.00 each for the SuperDisks.
Finally, out of all the disks I bought, one SuperDisk won't format and
10 CheapDisks won't format. I have ended up paying slightly over
$2.00/disk for the good SuperDisks and about 45 cents each for the good
CheapDisks. I still think I got a better buy on the CheapDisks.
MORE USE -- BETTER PERFORMANCE: Now, what about disk failures in
the future? That is as I use these 90 CheapDisks are they more likely
to fail in the future that the 99 SuperDisks? Well, I suppose there are
those who would argue with me that in fact they would. But I really
don't believe it. The reason is that the first few times I use any
diskette its performance will improve. The surface of the disk is left
slightly rough (not on purpose) during the manufacturing process and
this process prevents good contact between the head and the disk. This
poor contact degrades performance of the disk. As the disk is used and
rotated past the head, the head knocks off some particles of the
coating, smoothing the surface and improving the contact and the
performance. In tape recording, in critical applications, new tape is
never used without running it through a machine at least once and
sometimes several times, just for this reason. Therefore, after I have
used my CheapDisks several times I feel more comfortable with them than
when they were brand new.
HOW LONG WILL THEY LAST? Finally, what about the really long term?
Will CheapDisks retain the information stored on them equally as well as
SuperDisks, say over a period of 100 years? Well, here we are dealing
with real unknowns. There are no disks around that are a hundred years
old. Magnetic recording using media of the type used in disks is only
about 40 years old. Archival data that has been around for long periods
of time has turned out to be a problem in a number of fields. Ask a
librarian about the problems facing the Library of Congress in
protecting many of its books.
There has been some experience with magnetic recording in general
that may be of interest. In tape, such as your audio or video
cassettes, or computer tape as used on large main frames, there is a
problem with long term storage known as "print through." The magnetic
pattern on the tape representing the information emanates a magnetic
field, just as the North and South Poles do. This field is very minute,
but still present, and any material susceptible to being magnetized will
do so in the presence of a magnetic field. This is true even for weak
fields if the material is held still within the field for long periods
of time. All tape is susceptible to being magnetized, that is it's prime
purpose in life.
When wound on a reel, each piece of tape is tightly pressed against
another one, and each piece emanates a field. If the tape is left
untouched in this form for several years, a little of the information
recorded on each piece is transferred to mix with the information on the
adjacent piece. In audio tapes one can hear this as a low level
background of the same music that played either a few seconds earlier or
a few seconds later, particularly where a loud passage is immediately
followed by a quiet one.
Normally disks have a jacket around them that is fairly thick.
Thus it is unlikely that print through would take place between disks.
On double sided disks however, the magnetic information on one side is
pretty close to that on the other side, the distance being in the same
range as that previously discussed in the case of tape on a reel. If I
were to make a guess at the first cause of long term failure, in the
sense of not being able to recover 100% of the material form a floppy, I
would guess that "print through" would be the cause.
RE-COPY YOUR FLOPPIES: Someone once raised the question of whether
it makes sense to re-copy masters or back-ups from time to time to make
new backups. My initial reaction was that I didn't think it was
worthwhile. Having given it some thought, however, it might not be a
bad idea. If there is a degradation that takes place with time on an
untouched back-up as it sits on the shelf, re-copying does in fact
restore the information to a more pristine state and thus acts as added
protection against the probability of losing your data. As to
SuperDisks being any better than CheapDisks in an archival sense, I can
think of no reason why there should be any difference, but perhaps we
won't know the answer to that for another 100 years.
HOW COME SO CHEAP? There are a lot of reasons SuperDisks sell for
more than CheapDisks. They spend more on advertising, and packaging,
and possibly corporate headquarters. They sell primarily to companies,
which avoid buying anything by mail order from some post office box
across the country. And they sell at the price they do because people
are willing to pay for it, whatever the reason. In fact, however, if
you look into it you will find that many of the people selling the
cheaper diskettes are buying their raw material from the same source as
those selling the expensive versions.
The whole thing about mass produced products, whether it is disks,
drives, computers, or light bulbs, is that they are produced on a
statistical basis. That is, costs are reduced to the point where the
probability of a bad one getting to the user is acceptably low. This is
simply good business. No company can stay in business if it strives for
perfection in a commercial product line. Only governments can afford
products which have been tested to the level of a space shuttle, and as
we found out even they are not perfect.
Personally I have always bought the least expensive disks I could
find. Furthermore, I buy single sided, single density diskettes and use
them in double sided, double density drives, with no problem. On one
occasion, I paid over $25.00 for a box of 10 diskettes. It was a
Sunday, I needed them, and they were the only ones I could find. One of
the disks in that box proved to be the first bad disk I ever ran into!
One last comment on probabilities. If the probability of a given
disk failing is one in 1,000 under whatever circumstances, the
probability of two failing under the same circumstances is one in 1,000
times 1,000, or one in 1,000,000. Anyone for making two 45 cent back-
ups instead of one $2.00 back-up?
Note: I have taken some liberties in the preceding article in the
interests of keeping it from becoming overly technical but do not
believe these affect the substance of the arguments for purchasing lower
cost disks. Also, I was not able to find detailed information on the
testing of diskettes in the literature and much of the above is based on
extending my experience from tape and hard disks to diskettes.
I would appreciate it if anyone having more information on the
subject, or finding inaccuracies within the article contact me either
through KAY*FOG PCBBS (415)285-2687 or by mail: P.O. Box 324, Redwood
City, CA 94062.