Re: 6 & 2 / 5 & 3 Encoding

[christer@cs.umu.se (Christer Ericson)]

In <D3H63n.Ly9@actrix.gen.nz> dempson@atlantis.actrix.gen.nz (David Empson) writes:
>[...]
>It is a general characteristic of ALL disk drives that they cannot
>reliably read a continuous sequence of zero bits, because of timing
>differences between the drive used to read and write the disk, and
>speed variations.
>
>A one bit is recorded as a reversal in the magnetic flux.  A zero bit
>is no change in the flux.  The receive circuitry can cope with
>variations in speed as long as the one bits are close enough together.

Differences in speed only explain why synchronization would be
lost when "one bits" are far apart. It doesn't explain why
the Disk II can write, but cannot read back a long sequence of
zero bits.

My source have it that it is the MC3470 that gradually increases
its amplification when reading a sequence of "zero bits" and this
will eventually (rather soon) cause spurious one bits to be read.


>Older disk drives (pre Apple II) used a technique called FM (frequency
>modulation) in which the data bits were written to the disk with
>interleaved "clock bits" (one).  The Apple II uses this method for the
>address field of each sector (4 and 4 encoding).
>
>The Apple II gets quite a lot more storage out of a single density
>disk by eliminating the clock bits and allowing zero bits to be
>written more often.  (The technique used by the Apple is called "GCR"
>- Group Code Recording.)
>
>The PC uses a technique called MFM (modified frequency modulation)
>which allows most of the clock bits to be eliminated.  MFM is about as
>efficient as GCR for disk usage.  The PC also uses higher density
>(less time per bit) and also uses both sides and more tracks to get
>more data onto the disk.

MFM is twice as efficient as FM. Due to the way the encoding works
(a one bit is written as 01, a zero bit following a one bit is 00,
and a zero bit following a zero bit is 01) pulses are guaranteed to
be separated by two zero bits. Since the disk is primarily limited
in the number of FCI (flux changes per inch) it can handle, this
allows us to fit twice the amount of data by writing data at double
speed. This also makes MFM slightly more efficient than GCR.


Christer Ericson ---- tel: +46-90-16 67 94 ---- fax: +46-90-16 61 26
Internet: christer@cs.umu.se ------- http://www.cs.umu.se/~christer/
Dept. of Computing Science, University of Umea, S-90187 UMEA, SWEDEN