[Scummvm-devel] Fwd: from MAME forum

[Max Horn]

Am 20.08.2005 um 11:02 schrieb Marcus Comstedt:

>
> Max Horn <max at quendi.de> writes:
>
>
>>  BTW, and slightly offtopic (the following is not meant pro
>> nor contra the whole idea): Bits rarely flip on modern media. There
>> are all kinds of damage that can occur, but bit flips on CDs or
>> modern HDs are about as likely as being hit by a meteor (mind you I
>> am not saying you can't loose data, but it'll manifest quite
>> differently :-). Also I always wondered: Once you found out using MD5
>> that "a bit flipped".... what exactly do you do then? :-)
>>
>
> If a bit flips on a CD, you'll never know about it.  The CD-ROM data
> standard ("yellow book") includes an error correction code which is
> able to automatically correct single bit and double bit errors.  If
> three bits flip (in the same frame (24 bytes)), it can still be
> detected, but not corrected.  In that case you'll get a read error
> from the drive, not faulty data.

Yeah. Theoretically. However, the chance that 3 bits flip, as opposed  
to a whole sector becoming unreadable, is extremly slim. In fact I  
haven't heard or read about this ever happening, did you? I am  
seriously interested in a (serious) source for future reference.  
Usually, CDs are eitehr destroyed by scratches, or at least heavily  
damaged; or they do not last long because the reflecting material  
detaches from the plastic body of the CD.

The problem libraries and archives have with using CDs as long-term  
storage is caused precisely by the latter (as well as some other  
physical problems, like CDs deforming, which makes it kind of  
dangerous playing them with a 48x CD player, unless you like evading  
shrapnels <>), and not by random bit flipping due to cosmic rays :-)


Cheers,
Max