Re: Write Signal on Disk II Question

[ferdimh@gmx.de]

> Because of bit shifting and disk speed variations the ratio can't
> get so large and accurately detect which bit cell a transition
> belongs in.
There is another problem: Due to the design of the MC3470 (the read
preamp and decoder used in the Disk ][) it will detect false ones
after some time. I detects a zero crossing of the differentiated read
signal. If the read signal is zero (i.e. no field transistion for some
time) the differentiated signal will also be zero, and a little noise
can be seen as a transition then.
New floppy drives have the same problem, but to a much greater extent:
These can handle ONE zero at the 500kb/s bit rate and are unable to
read the Apple bit stream.

> I think it is usually a 2MHz clock for an MFM DD controller so it can
do precompensation.
The 250kb/s MFM encoding really is a 500kb/s enconding where directly
following transitions are not allowed. So it can be read/written with
the same drive (I doesn't need to handle more transitions) but stores
more data.
With the same trick could be done to increase space at least by 16% or
even by 33% with some tricks.
The term "double density" ist in comparison to the very inferior "FM"
encoding which just puts a one between every data bit, which is also
used in the Adress field of the Disk II.

       Ferdinand