      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. 