Re: backup archive format saved to disk

hendrik@xxxxxxxxxxxxxx wrote:
On Tue, Dec 05, 2006 at 06:58:35PM -0600, Mike McCarty wrote:

Speaking pedantically, if the probability of error is greater than 50%, you can complement every bit and gte a probability less than 50%.

No, not so. Because on the channels we are discussing, the bits
have three states: 0, 1, and "unable to read". I don't know how
to complement a bit which is in the state "unable to read". The
best I can do is assign it an arbitrary value of 0 or 1.

In some circumstances, this can actually work, and is one of the
reasons that "missing" bits are easier to correct. What one can
do is go through all possible assignments of 0 and 1 to each bit
which is missing. Along the way, one may encounter a combination
which is correctable to a code word (or may actually *be* a code
word). If that happens, one may actually be able to correct. This
can be time consuming, however, as you may guess.

It's true that I spoke simply, because I didn't want to get into
more technical depth than we have already. This whole discussion
is getting pretty far afield from Debian.

Another reason this is not true, is that one has to know
*in advance* that the channel flips bits (not erases them)
in order to do that sort of error correction. In that case, one
has a mis-designed channel. The only channels which I've seen
considered which cause errors on a very high proportion of
bits create missing bits. Anything else would be, well,
crazy.

Mike
--
p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
This message made from 100% recycled bits.
You have found the bank of Larn.
I can explain it for you, but I can't understand it for you.
I speak only for myself, and I am unanimous in that!


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