Re: assistance with disk ][ and modern 1.2M floppies

[dempson@actrix.gen.nz (David Empson)]

Patrick Schaefer <pa.schaefer@fz-juelich.de> wrote:

> FM uses 1 clock bit and one data bit for each bit of information. For 1
> kB of data you will get 2048 bits on disk. MFM doesn't use clock bits,
> on the same disk area you can store 2kB. 

MFM does use clock bits.  It just does use as many of them.

MFM is effectively FM with twice as many bit cells in the same length,
except that clock bits are not written in most cases.

To be precise: clock bits are only written between a pair of zero data
bits.

An example: consider the bit sequence 110011

Using FM, this is written as follows.  I've used "c" to represent clock
bits (all of which are "1"), and the appropriate value for data bits.

    1   c   1   c   0   c   0   c   1   c   1

Using MFM, the pattern is written as follows:

    1   1   0 c 0   1   1

The only clock bit is between the adjacent zero data bits, and all the
data bits are twice as close together compared to FM.

The interesting point is that the one bits written to disk (either data
or clock) are no closer than under FM, but twice the resolution is
required for placement of the bits.

Assuming 8 microseconds between data bits with FM (clock or data bit
every 4 microseconds), the minimum distance between "1" bits is 4
microseconds, and the maximum is 8 microseconds.

With MFM, data bits are 4 microseconds apart, and the minimum distance
between "1" bits is still 4 microseconds.  The maximum distance is 6
microseconds.

(There are probably special patterns for synchronization which break
these rules, but I'm only talking about normal data here.)

"1" bits (either data or clock) are flux reversals on the media.

> From the disk's point of view 2048 flux transitions are 2048 flux
> transitions. The disk cannot tell data bits from clock bits. So what is
> the difference between single density and double density media?

Beats me.

> BTW: Apple's Disk II uses a GCR recording scheme. The data density is
> the same as for MFM, but it is easier to decode.

GCR's density (as implemented on the Apple II) is actually somewhere
between FM and MFM.  I posted a detailed article about this a couple of
months ago which analysed the three schemes in some detail.

FM: 16 disk bits required for 8 data bits (ratio of 2.0).
GCR: 32 disk bits required for 24 data bits (ratio of 1.33).
MFM: equivalent of 8 FM disk bits for 8 data bits (ratio of 1.0).

GCR uses the same resolution for bit cells as FM (4 microseconds per bit
on 5.25" disks), and the disk controller doesn't have to worry about
clock bits (data bits are used to keep the PLL in sync).  The 6-and-2
pattern encoding and decoding is done in software, so the disk
controller hardware is very cheap.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand