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From: Pete Turnbull <pete_at_dunnington.u-net.com>

Date: Fri Jul 7 01:44:02 2000

On Jul 6, 15:54, Dwight Elvey wrote:

*> mann_at_pa.dec.com (Tim Mann) wrote:
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*> >
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*> > Another neat trick might be to notice when there is a CRC error and/or
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*> > a clock violation, and in that case backtrack to a recent past decision
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*> > where the second most likely alternative was close to the most likely,
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*> > try it the other way, and see if the result looks better. Obviously
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one

*> > can't overdo that or you'll just generate random data with a CRC that
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*> > matches by chance, but since the CRC is 16 bits, I'd think it should be
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*> > OK to try a few different likely guesses to get it to match.
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*> CRC's are quite good at fixing a single small burst.
*

Dwight, I think you're confusing CRC (Cyclic Redundancy Check) with ECC

(Error Correction Code). CRC is very good at detecting errors, including

bursts of errors that might slip by simpler checks, but AFAIK tells you

next to nothing about where they occurred. ECC tells you enough to correct

small errors. I've not heard of anyone using CRCs for correction (not

directly, anyway).

*> As I recall,
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*> CRC32 can fix a single error burst up to 12 bits long. The
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*> error correcting method is based on the cycle length of the original
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*> polynomial relative to the length of the data block. What this
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*> means is that if you have a burst longer than 12 bits, it is
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*> more likely that the errors will appear to be outside the data
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*> block than within the data block.
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Although disks use the V41 polynomial (X^16 + X^12 + X^5 + 1) not CRC32.

*> All errors that happen
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*> within a 12 bit window are 100% correctable.
*

Depends how large the data covered by the check is. For amounts of data

larger than a certain size (dependant on the number of check bits and the

algorithm used) there are several errors that will produce the same change

in the ECC or CRC. So the window size is meaningless unless you also

specify the data size and number of check bits.

Date: Fri Jul 7 01:44:02 2000

On Jul 6, 15:54, Dwight Elvey wrote:

one

Dwight, I think you're confusing CRC (Cyclic Redundancy Check) with ECC

(Error Correction Code). CRC is very good at detecting errors, including

bursts of errors that might slip by simpler checks, but AFAIK tells you

next to nothing about where they occurred. ECC tells you enough to correct

small errors. I've not heard of anyone using CRCs for correction (not

directly, anyway).

Although disks use the V41 polynomial (X^16 + X^12 + X^5 + 1) not CRC32.

Depends how large the data covered by the check is. For amounts of data

larger than a certain size (dependant on the number of check bits and the

algorithm used) there are several errors that will produce the same change

in the ECC or CRC. So the window size is meaningless unless you also

specify the data size and number of check bits.

-- Pete Peter Turnbull Dept. of Computer Science University of YorkReceived on Fri Jul 07 2000 - 01:44:02 BST

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