File compression.

I've recently been noticing that I've downloaded some rared files with really good compression - I'm talking like a 250 meg rar with a 500 meg iso in it. I'd like to find something that can do that kind of compression to make it easier to back up large files to CD. I use the free WinRar, and even at the highest compression rates I can't get that. Recommendations?

I think it depends on the files you're trying to compress.

That's right, compression algorithms depend on the content of the files. Most compression algorithms look for some kind of pattern in the data of the file, so a plaintext file for example, which has a very limited number of combinations of data (limited to ASCII usually), can be compressed a great deal. Whereas binary files, being much more random, usually can't be compressed nearly that much, at least not losslessly. Algorithms also have their own strengths and weaknesses, in terms of how much they can compress a file, and how fast they can compress it, and whether they work well on binary files or not, etc.

I've seen 700 MB PSX discs compressed into under 70 MB :p Yeah, it varies from game to game too :p

Tweaking how the compression algorithm works too can also produce different results. For example with rar, you can select how strongly you want it compressed (traded off with time) in WinRAR. You can select Solid Compression too to allow it to compress across files rather than the conventional way of compressing files individually and putting them together in an archive - this has disadvantages when decompressing though if you want a single file, as you'll then have to scan the entire archive from the start. You also can manually increase the amount of memory it uses to allow it to match a wider range of patterns and so compress more. Don't give it too much memory though - if the PC can't handle it, you're gonna see a really nasty increase in time as the computer starts using swap file space instead of physical memory. Shame there's an upper limit of memory you can assign though (maybe hard coded into Winrar rather than being algorithm dependant, I don't know) - I'd like to give it 900 MB or so combined with Solid compression to allow compressing across several ISOs that contain some areas of similar content so they compress even more :p