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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Looking for MFM endec description
References: <3DA2D6FC.B68FEBC@yahoo.com> <3DA2EF5A.E07DAA01@webaccess.net> <3DA3057C.9C4AE87D@yahoo.com> <3DA33E44.FE8D2642@yahoo.com>
Date: Wed, 09 Oct 2002 12:51:24 GMT
NNTP-Posting-Date: Wed, 09 Oct 2002 05:51:24 PDT
Tauno Voipio wrote:
> "CBFalconer" wrote in message
> > Grant Edwards wrote:
> > >
> > > In article <3DA3057C.9C4AE87D@yahoo.com>, CBFalconer wrote:
> > >
> > > >> Contemplate some fun codes now. How about an RLL code that is
> > > >> also DC free?
> > > >
> > > > Obviously impossible on a bit by bit basis. But it should be
> > > > possible to encode each of a particular N bit sequence into
> > > > some M > N bit pattern that would be DC free, and table drive.
> > > > How to create a DC free sync pattern? The result would be
> > > > useful, especially for long line communications.
> > >
> > > Manchester encoding has been used for that for a long time (N=1
> > > and M=2).
> > But that doesn't get the reduced bandwidth/transitions of a longer
> > code. The DC doesn't usually matter for such things as local disk
> > drives, but the transitions do. Both are of interest for long
> > distance transmissions.
> Please note that the pulse stream is not fed directly to the write head in
> the disk drive. The pulses feed a divide-by-two flip-flop whose output is
> fed to the write head, so each pulse creates a transition in the
> magnetisation state. This signal is nearly DC-free.
This is conversion to NRZ which is not guaranteed to be DC free.
> When reading, the transition pulses are full-wave recitfied and
> stretched/cut to standard width, so the information is in the timing of the
> leading edges of the pulses.
When inductive heads were used, the head signal was peak detected.
Essentially the zero crossings of the derivative were used to generate a
pulse at the peak of each head pulse. For sensors like MR where the
actual field is detected, zero crossings are detected after DC
restoration. The two are equivalent because the original NRZ is
reconstructed. The effect of DC in the code is determined by the lower
cutoff of the filter and the time constant of the DC restorer.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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