Re: 9V AC needed for C64? and more......

From: Andre Fachat (a.fachat_at_physik.tu-chemnitz.de)
Date: 1998-09-04 10:49:59

Ojala Pasi 'Albert' wrote:
> > >Read: MFM uses two disk bits for each data bit.
> > >See http://www.cs.tut.fi/~albert/Dev/ -> C1581
> > 
> > Isn't that inefficient compared to GCR (five disk bits for four data bits)?
> > Or is it more complex than that?
> 
> Yes and no. With MFM you never have two consequtive 1-bits and thus
> you can pack the bits closer together.

Ok, MFM means "Modified Frequency Modulation". So what is FM?
Basically you have cells. Each Cell contains one data bit, but two 
positions for a magnetization flip. The first one is always done as
a clock. 4 bits are encoded like:

FM:
     cell
     |   |   |   |   | 
      1 0 1 1 1 0 1 1
        ^   ^   ^   ^
        data bits

This comes to a certain frequency for all-0: 1 0 1 0 1 0 1 0 
and the double frequency for all-1: 1 1 1 1 1 1 , thus Frequency
modulation.

MFM is pretty similar. Only you leave some bits out so that you
end up with less magnetization changes per data -> better packing 
-> more capacity. 
Unfortunately I cannot remember how it was done exactly...

I think it worked this way:

a) remove all the clock bits.

     |   |   |   |   |
      0 0 0 1 0 0 0 1

But when you encounter more then one 0 data bits then add a clock bit:

data    1   0   0   1
     |   |   |   |   |
      0 1 0 0 0 0 0 1
becomes 
      0 1 0 0 1 0 0 1 

I hope that's right, please correct me if possible.
With MFM you get a lot of less 1 per data, approx. half of them.
Therefore you can double the write frequency. But the position
of the transition of the magnetization must be detected
better then with FM.

Andre

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Andre Fachat, Institute of physics, Technische Universitšt Chemnitz, FRG
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