Tim's own version of the Catweasel/Compaticard/whatever

From: CLASSICCMP_at_trailing-edge.com <(CLASSICCMP_at_trailing-edge.com)>
Date: Wed Jul 5 21:37:52 2000

>>some controllers, many of which use a less-than-ideal timebase to define
>>the precompensation offsets imposed on the data stream.

>This is true, or worse used oneshots. generally the time base for the
>bit encoding was always a crystal with not worse than 200ppm error
>and less than 50ppm drift. The typical system was usually within
>50ppm of exact and drifted less than 25ppm over temperature extremes.
>Often the actual data rate was far lower than that reference(usually 1/4
>or 1/8th).

IMHO it's very misleading - or even worse, foolish - to worry about errors in
the ppm range when the instantaneous rotation speed of the floppy can
vary a few percent.

Most data recovery circuits - even the most simplistic one-shot FM decoder -
can take a 25% change in data rate in stride. Sure, they don't have as
much error margin if you stray that far, but my point is that we're not
dealing with National Bureau of Standards platinum-iridium references

>I'd suggest some factor less than .5, flux shift errors on floppies
>rarely move a great amount unless the spindle bearings are rattling
>loose. Actually based on media and expected recording rate it's
>possible to plug in a set of expected timing windows and add/subtract
>a "precompenstation" window amount based on adjacent bits. For
>example adjacent ones or zeros (especially more than two bits)
>tend to spread or compress over patterns like alternating ones
>and zeros.

Look up "PRML" or "Partial Response Maximum Likelihood". It's a
branch of signal decoding for use in situations where adjacent pulses
most definitely have measureable effects on each other.

>Further with all the "timing image" in a memory it should be possible
>to look at longer strings of transistions and do simple predictive
>forcasting (software PLL). Add to that the encoding form (FM,
>MFM, M2FM, RLL or GCR), and previous bits history it should be
>straightforward enough to predict the likely next transistion(s)
>be they one or zero.

Most uses of PRML look both forward *and* backwards.

 Tim Shoppa                        Email: shoppa_at_trailing-edge.com
 Trailing Edge Technology          WWW:   http://www.trailing-edge.com/
 7328 Bradley Blvd		   Voice: 301-767-5917
 Bethesda, MD, USA 20817           Fax:   301-767-5927
Received on Wed Jul 05 2000 - 21:37:52 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:32:56 BST