6 Jul
2000
6 Jul
'00
2:25 p.m.
No, Dwight, there is a difference between CRC and ECC. CRC will detect
errors, but ECC can correct errors, too.
For single bit correction, the number of extra bits required is M+1 (M is
the log2(# of bits)); so if you have a 16-bit memory, 21 bits will correct
single-bit errors (and will detect most multi-bit errors).
There are also games that you can do to interleave your bits, so that a
burst error (that you describe, 12 bits in a row) can in fact be corrected.
Coding Theory is a deep subject with many advanced types of codes possible
for particular error probability density functions. And I'm certainly no
expert at it!.
-Mike Cheponis
On Thu, 6 Jul 2000, Dwight Elvey wrote:
mann(a)pa.dec.com (Tim Mann) wrote:
Another neat trick might be to notice when there is a CRC error and/or
a clock violation, and in that case backtrack to a recent past decision
where the second most likely alternative was close to the most likely,
try it the other way, and see if the result looks better. Obviously one
can't overdo that or you'll just generate random data with a CRC that
matches by chance, but since the CRC is 16 bits, I'd think it should be
OK to try a few different likely guesses to get it to match.
Hi
CRC's are quite good at fixing a single small burst. As I recall,
CRC32 can fix a single error burst up to 12 bits long. The
error correcting method is based on the cycle length of the original
polynomial relative to the length of the data block. What this
means is that if you have a burst longer than 12 bits, it is
more likely that the errors will appear to be outside the data
block than within the data block. In this case, you would have
what is called an uncorrectable error. No amount of fiddling
will give you a correction. If there is an error in the CRC
data, you have the same ratio. As an example, using CRC32 and
a data block of 512, the probability of an error looking correctable
that was not is 512/(2^32 -1) or 1.2X10^-7 of errors greater
than 12 bits will seem correctable. All errors that happen
within a 12 bit window are 100% correctable.
Does this make any sense?
Dwight