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From: "Tauno Voipio"
Subject: Re: Looking for MFM endec description
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Tue, 08 Oct 2002 08:23:52 GMT
NNTP-Posting-Date: Tue, 08 Oct 2002 11:23:52 EET DST
Organization: Sonera corp Internet services
wrote in message
> I can't seem to find a definitive description of an MFM
> I'm looking at the bits on the edge connector of a drive, and I'm just
> not "getting it".
> Any pointers would be greatly appreciated. All the google references
> I've found so far are either too general (vague) or descriptions of
> the outside of some chip or IP.
It's pretty difficult without a PLL to sync in the lead-in sequence.
In FM there is a clock pulse at the edge of a bit cell. In cell with '1'
data there is a pulse at the middle also. The dense spacing of clocks and
data limit the packing density, so in MFM the clocks are left off if there
are ones on either side of the clock. A stream of zeroes and a stream of
ones look excatly like each other - the only difference is the phase of the
pulses referred to the lead-in sequence.
The old Shugart Associates documents of 8 inch floppies (about 20 years ago)
contain a pretty extensive treatment of the coding schemes.
tauno voipio @ iki fi
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