5 1/4 diskettes available

Fred Cisin cisin at xenosoft.com
Thu Jul 11 00:40:04 CDT 2019

>> "360K" - 300 Oersted. vs 1.2M 600 Oersted.

On Thu, 11 Jul 2019, Guy Dunphy wrote:
> Can you believe that is the first time I have ever heard the actual figures for the coatings?
> I knew the 1.2M type had a higher coercivity, thus the incompatibility. Ditto with 3.5" 720K
> vs 1.44M.

Then add to the useless information:
"720K" diskettes ("micro-diskettes") are 600 Oersted.
"1.4M" diskettes are 720 - 750 Oersted.  Notice that it is not as much 
difference between them as there is between the 5.25 variants.  Therefore, 
using a "720K" diskette for "1.4M" or vice-versa, while less reliable, is 
something that you might "GET AWAY WITH".  There were punches being 
peddled for doing so!  You could take a good quality "720K" diskette and 
turn it into a mediocre/flaky "1.4M"

BTW, if you multiply out the space on it (2 sides * 80 tracks per side * 
18 sectors per track * 512 bytes per sector), 1474560, the only way that 
you can get 1.44 as the number of MBs is if you creatively redefine MB to 
be 1,024,000 (1000K, 1000* 1024, 2^10 * 10^3)
A MebiByte is 1048576 or 2^20.  A "1.4M" disk is 1.40625 MebiBytes.

BTW, ANOTHER incompatability between "360K" and "1.2M" was 48 tracks per 
inch V 96 tracks per inch.  ROUNDING THE NUMBERS (researching the correct 
numbers,and redoing the calculations ACCURATELY is "left as an exercise 
for the reader"),
a "360K" has tracks that are about 1/2mm spacing, that are about 1/3mm width.
96tpi is about 1/4mm spacing, with width about 1/6mm.

A 1.2M drive can read 360K by double-stepping.  No problem.
If it also adjusts the write current, then it can format a virgin 360K 
disk.  The tracks will be sub-standard width, but will work.  (1/6mm wide 
tracks with 1/2mm spacing)

HOWEVER, if you take a track that has been written to on a 360K drive, the 
1.2M drive, when it RE-writes that track will not erase the full width of 
the old 360K track.  You will have a 1/6mm track down the middle 
of a 1/3mm track.
The result is a "360K" track that still reads just fine on 
the 1.2M drive, but is not readable [reliably] on a 360K drive.
Visualize a car tiretrack that has been followed by a motorcycle.  Two 
motorcycles can leave a pair of tracks that resemble car tracks if they 
are the only tracks.  But, if there is already a car's tracks, the 
motorcycles won't fully replace the car's tracks.

Grumpy Ol' Fred     		cisin at xenosoft.com

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