Re: VMs: Odd Thoughts

[elvogt@xxxxxxxxxxx]

Zitat von steve ekwall <ekwall2@xxxxxxxx>:

> Thx Elmar :-) 
> 
> Reminds me of even older CRC? (cyclic Redundicies checksums etc..)
> 

Uhm... not quite. I think you may be confusing "lossy transformation" 
with "fault tolerance".

Checksums are appended to a message to let you verify whether the transmission 
worked well. In its most trivial form, for the transmission of a digital data 
stream, you just add up all the bytes you transferred, divide them by 256 and 
send the remainder of the division as checksum.

Of course, in this manner you will not be able to detect missing or 
superfluous "0" bytes in your message. Likewise, you can't detect a byte swap 
("444123444" will render the same checksum as "444321444"") this way.

CRC is a more sophisticated algorithm which will also take these possibilities 
into account.

In any case, I don't think the VM employed any kind of "checksum" -- the prime 
use of a checksum is to make you demand a re-transmit from the sender, if the 
checksum doesn't fit. It's not quite straightforward how one would demand a re-
transmit from the author of the VM. ;-)

(Please also note that even a lossy transformation may not be fault-tolerant: 
Corrupting a single byte in a Jpeg or Mpeg might corrupt the whole file.)

Cheers,

   Elmar


> for my 0.02 worth VMS is NOT "Lossy".. 
> 
> but, hey, you give me a buck & i'll honestly give you 0.98 change :-)
> 
> best to you & yours
> -=se=-
> steve (STARS with a (center) DOT are MALE births/NOT STARS) ekwall
>  zip < crc ? ok ok...
> 

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