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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.