FLOPPY DISKS
By Grant Kwai
Copyright (c) 1990 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
What makes a good floppy disk?
Your average 5.25" double sided disk is made out of a PVC film which is coated with iron oxide (rust!), and then inserted into its vinyl jacket. The iron oxide is used to magnetically store the bits of information we save onto each disk. What makes some disks better than others depends on how evenly the manufacturer coats each disk.
These disks are certified by the manufacturer to test that each disk is ok. They do this by recording a signal on every track of the disk. The certifier will then read each track bit by bit and record the average strength of the original signal.
In Australia, there is a standard which states that 40% or more of the average strength of each disk must be able to be retrieved. If any bit falls below this standard then it is defined as a 'missing bit'. A missing bit is caused by a gap in the magnetic coating of the disk. Any disks falling below 40% clipping level are rejected. You will note, though, some of the higher priced disks offer a clipping level of 75% or higher, which is why they charge such a high price. Also note though that a 100% 'error free' does not mean that it has a 100% clipping level. It simply implies that it has passed the Australian standard 40% clipping level.
Another problem with disks is an 'extra bit'. This is caused when the certifier erases the disk being certified then re-reads each track. If it picks up any extra signals, then it is called an 'extra bit'. This may cause a problem in that it may prevent data from being read and therefore will fail its certification. This is mostly caused by a buildup of magnetic particles on the coating.
A high clipping level can be vital if you are using two different disks drives. That is, one is used to write the data and another to read it. For example, your computer at home and at work. Unless your drives are in perfect alignment, the signal strength which one drive may write to the disk, might not be able to be picked up by another computer's drive. That is why sometimes a program will work on your computer and not on a friends and vice versa.

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