Re: EDD + CARD

[mjmahon@aol.com (Michael J. Mahon)]

Dick Sisco replied:

>On Fri, 26 Jul 2002 15:33:17 -0000, russotto@grace.speakeasy.net
>(Matthew Russotto) wrote:
>
>>In article <md61ku4rts0okshsusisv15ifgeeuv9g2r@4ax.com>,
>>Dick Sisco  <dsisco11111@mchsi.com> wrote:
>>>
>>>But that is exactly why it works as a protection scheme.  After about
>>>8-10 0 bits the standard Apple (or IBM) controller begins returning
>>>random bits.
>>
>>Why does it do that?  A 0-bit is a lack of transition on the disk; while
>>the controller may not be able to figure out how many non-transitions
>>occurred, I don't see why it should start returning 1s. 
>
>As it was explained to me, the circuitry in the disk controller turns
>up its aplification(?) if it doesn't see transitions. It eventually
>senses a transition when there really isn't one.   I can't swear that
>that's an accurate description, but that was the way the instructor at
>a disk duplicaton seminar explained it to me.  And it did seem to
>work.

Exactly.

Since the head amplifier must accomodate wide variations in signal
levels, it incorporates an automatic gain control (AGC) which
continuously adjusts the degree of amplification of the head signal
prior to the thresholding operation that interprets it as a 1 or 0.
In the absence of transitions, the AGC turns up the gain, so that
within 8-10 bit periods, noise is being converted to "signal", which
results in about half the bit cells being interpreted as 1's.

One potential problem with this scheme, is that if the disk has ever
been recorded before, the noise that the read head picks up may
not actually be random, but may be either residual magnetization
of the medium or "fringe field" data--data outside the nominal track
but close enough to force a deterministic read pattern when the
gain is turned up.  Either effect could make the "zero nibbles"
be read deterministically, which would make the code that does
the "original verification" think that it was seeing a copy.

I speculate that this scheme was used only on virgin or
bulk-erased diskettes to avoid these problems.

-michael

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