Re: High and Double density floppies

[jdb8042@tamuts.tamu.edu (John Donald Baker)]

[description of "single-density", double-density, etc. delete]

For the record:

Single-density diskettes on the old CP/M and TRS-80 (among other) machines
were recorded with "FM" encoding, _not_ MFM.

Double-density diskettes all use "MFM" encoding.

Both single-density (FM) and double-density (MFM) disks use an effective
data rate of 250Kb/s and spin at 300rpm. (note that this is for 5.25"/3.5"
disks only.  8" disks used 500Kb/s rate in FM and MFM mode and spin at 360
rpm.)

The "High-density" 5.25" formats also use MFM encoding, but use 500Kb/s
data rate, spinning at 360rpm (like the old 8" disks).  3.5" HD still spins
at 300rpm.

The old iron-oxide media that worked just fine for single- and double-density
formats simply can't handle the bit density of the "high-density" format.
The diskettes intended for the HD format use a cobalt-based compound instead
of iron (or an iron/cobalt combination).  This material requires a higher
magnetic flux to align the domains--which the HD drives can generate across
the record gap of the head.

Drives intended for SD or DD format can't generate the required magnetic
flux.  This includes the Disk ][ and it's successors.

The upshot of all this is that you can't format an HD disk as DD and
you can't format a DD disk as HD.

Although there those who HAVE successfully formatted a given disk with the
"opposite" format, I've seen too many failures to do it myself.  In short,
use HD disks for HD format only.  Use DD disks for SD/DD/GCR formats only.

I personally can attest that HD disks cannot be formatted DD.  I run a 
pair of TEAC FD55GFR high-density drives on my SVA ZVX4 8" disk controller.
Attempting to format DD causes the drives to use the lower magnetic flux,
which will not 'make an impression' on the media.


John D. Baker  ->A TransWarp'802'd Apple //e CardZ180 Z-System nut//
Internet:  jdb8042@tamuts.tamu.edu, @blkbox.com, jdbaker@taronga.com
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