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Matth · 22-04-2005, 12:10

Many programs can unpack RAR - even some of the free ones, but nothing else can create this closed, proprietary format.

The OPEN alternative to RAR, is 7-zip's "7Z" format, becoming a standard among the free ones, though only the current 7-zip betas can do multipart in it.

Just say no to proprietary formats!

PAR has two forms, the older and easir to understand one - the basic concept is similar to that used in an old tape backup - the "XOR Group".

In that, an additional block of data that is the bitwise exclusive-or of all the grouped blocks is recorded. If one block is missing, all the remaining ones can be xor'ed back out of the XOr group, leaving the missing one.

That's still an oversimplification, as extending it to cover more than one lost block in a group is difficult.

The use of PAR2, which allows partial blocks to be used, is even harder to explain, especially as it can also use data blocks that have been "cut & shut" where segments are missing.

22-04-2005, 12:10	#15
Matth Inactive Join Date: Mar 2004 Services: BB:M, TV:XL, Phone:M, Loyalty Posts: 2,516	Re: What is a RAR file? Many programs can unpack RAR - even some of the free ones, but nothing else can create this closed, proprietary format. The OPEN alternative to RAR, is 7-zip's "7Z" format, becoming a standard among the free ones, though only the current 7-zip betas can do multipart in it. Just say no to proprietary formats! PAR has two forms, the older and easir to understand one - the basic concept is similar to that used in an old tape backup - the "XOR Group". In that, an additional block of data that is the bitwise exclusive-or of all the grouped blocks is recorded. If one block is missing, all the remaining ones can be xor'ed back out of the XOr group, leaving the missing one. That's still an oversimplification, as extending it to cover more than one lost block in a group is difficult. The use of PAR2, which allows partial blocks to be used, is even harder to explain, especially as it can also use data blocks that have been "cut & shut" where segments are missing.