Re: High and Double density floppies

[scott@image.Kodak.COM (Kevin C Scott)]

There are several common types of 5 1/4" floppies:

Single Density - in the TRS-80 and CP/M days, these held 80K to 90K each
		 using "MFM" encoding onto a 40 track disk

		 Apple II used a technique called "Group Coding" to pack
		 143K onto a single density disk.  Apple decided to
		 only use 35 tracks rather than 40 (being cheap? who knows?)

Double Density - the very first IBM PCs used MFM encoding, but doubled the data
		 rate that the bits were written and read, so that they were
		 packed twice as dense along each track, so that they could
		 put 160K or 180K onto a 40 track disk (160K was done by
		 putting 8 sectors on each track, later they decided to put
		 9 sectors per track to get 180K)

Other techniques to increase the data capacity:

Double sided   - IBM PCs quickly switched over to double sided disks, which
		 held 320K or 360K (twice 160K or 180K)
80 track / Quad density - not very common.  These would use 80 tracks
		 rather than 40, and possibly again double the data rate
		 from that used by the IBM PC.  Long ago, Micro-Sci made an
		 80-track drive for Apple IIs that plugged into the standard
		 disk II controller; by writing 80 tracks instead of 35, it
		 stored 320K on a disk.  (they hardly sold very many at all,
		 though...BTW, does anyone reading this have these drives?)

All of the above techniques use basically the same magnetic material in a disk;
the different density levels that a disk manufacturer would put on the label
are determined by the quality/consistency/lack-of-defects/etc. that the disk
had so that it could reliably record and play back bits at that density.
You can use any disks labeled with any of the above terms in an Apple II 5 1/4"
drive; it is quite happy to work with a lowly single-density disk.

THEN...
we have High-density.  These disks use 80 tracks, and the magnetic material was
changed.  High-density disks require a stronger magnetic field to write them.
Sort of like high-bias (or metal) audio tape.  Since high-density disks
require a stronger field to write to, you can't use them in a
drive (such as on an Apple II) that is not high-density.

Kevin Scott
scott@disney.kodak.com