CRC errors - test-drb@ccmp.vtda.org

6 Jul 2000

From: Tim Mann &lt;mann(a)pa.dec.com&gt;

...
 Of course, with floppy disks we have sectors of 128 to
1024 *bytes*, not
bits, and the CRC is only 16 bits, not 32, so I don't think we can do
much correction.  With a 1024 byte sector, it already takes 13 bits of
information to say where a 1-bit error is.  So if we use a CRC16 to correct
...
 it, we have about a 2^(-3) = 1/8 probability that if
more than one bit
is in error, we'll make a spurious correction. 

Therein lies the difference, the use of CRC vs ECC, floppies have a
fairly
high soft error rate compared to hard errors so detecting an error and
rereading is the strategy.  It is also an economy of design that comes
...
 from the characteristic otherwise you can bet there
would be ECC. 
Allison