Difference between "weak bits" and "fuzzy bits"

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NRS
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Joined: Mon May 06, 2013 8:32 pm

Difference between "weak bits" and "fuzzy bits"

Post by NRS » Sun Apr 05, 2015 8:55 pm

On this page, it distinguishes between "weak bits" (no flux transitions returning random data) and "fuzzy bits" (flux transitions between two MFM clocks (e.g. t+0.5) that may occur at clock position t+0 in one revolution and t+1 in another revolution because of minimal changes in drive speed.

I've got a PC disk that Kryoflux GUI displays as having errors in what I know (from the keydisk code) is a weak/fuzzy bits track when reading combined preservation+MFM formats. Excluding the affected sector removes the read errors, so I know the errors must come from that sector. Is there any way I can tell from DTC or the GUI's messages whether I have the weak bits or the fuzzy bits variant? Also, when I use the PFI utility from the PCE-PC emulator to produce the (undecoded) MFM data, I see four zero bytes in a row. Am I correct in concluding then that this must be a weak bits, rather than a fuzzy bits, protection scheme?

Here is the track in case you want to take a look at it:
http://symphoniae.com/nrs/track10.0.raw (It's sector 10).

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IFW
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Joined: Mon Nov 08, 2010 2:42 pm

Re: Difference between "weak bits" and "fuzzy bits"

Post by IFW » Tue Apr 07, 2015 11:57 am

The only known software that was ever released with a protection that changes the flux reversal positions and moves them around in the data window (as opposed to having them in the middle) is the games released by FTL, namely Dungeon Master, Chaos Strikes Back and Oids, making the weak bits deterministic - it's always the same bitcells changing their value.
I can't recall any protection that relies on having a borderline position for the flux reversal in order to create weak bits - being referred there as fuzzy bits.
Weak bits are very common, and yes, the track you posted contains weak bits.
You can easily see that in the GUI by activating the Scatter Window (View > Scatter Window) and pressing A (animate) to view the multiple revolutions sampled once the cursor is above this track.
If there was ever something referred as "fuzzy bits" you'd see part of a band having a line between two of the expected timing line positions/values.

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