Date   : Fri, 03 Nov 2006 19:27:23 -0000
From   : dl.harper@... (David Harper)
Subject: DFS on 3.5inch - Is this supposed to work?

Chris Johns wrote:

>> Now my question is, is this working perfectly, or is there some sort of
>> half-way format that I've created where single-density data is being
>> recorded in a double-density mode or some such silliness? I.e. if I put
>> these discs into a PC 3.5in drive connected to my model B (being set up
>> soon), is the humble 8271 going to suggest orifices to which the idea my
>> be suitably inserted?
>
> Isn't it an encoding difference, rather than a physical difference in the
> drive? IIRC DFS uses FM, ADFS uses MFM.

Yes, it's a matter of how the magnetic fields (i.e. the pattern of NS and SN 
swaps) represents binary 0's and 1's. So at the very lowest level (at the 
drive head) the difference between SD and DD could be described as a 
software difference. If the drive will do one, then it will do the other. 
(The same is not true, of course, of drive controllers.)

HD is another matter, and for this the disks are magnetized in a different 
way.

David Harper