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For example if you used WinRAR or Winzip on .mp3 or .wmv files would it save disk space? I thought the answer was no because the compression was already built into their encoding. But then I red this wikipedia article and it says

7z's LZMA algorithm has a higher compression ratio than RAR, except for "multimedia" files like .wav and .bmp files, for which RAR uses specialized routines that outperform LZMA.

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Helps for what? I think you should revise your question to be more specific for your problem, and to fit better on superuser. –  amotzg Aug 14 '12 at 8:13
    
help save space –  Celeritas Aug 14 '12 at 8:21
    
mp3 and wmv are compressed files, but bmp files are not compressed and wav files could be lossless if it contains PCM. So it's normal that compressing mp3s and wmvs doesn't save space, but with bmp and wav you could. :) –  epingle Aug 14 '12 at 8:25
    
Sorry, didn't read well, still I think it is a too general question. But I'll try to write an answer. –  amotzg Aug 14 '12 at 8:26
    
Putting media files in archives saves space if two things are the case. First, the media files have to be compressible. Second, the archiver has to know how to exploit the compressibility of those files. –  David Schwartz Aug 14 '12 at 9:40

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WAV and BMP are not compressed as part of their formats, so it will usually reduce the file size considerably to compress it with a general compression tool. Here there can be an advantage to an algorithm that takes the file type into account, or that is more suitable to a specific file type.
MP3 on the other hand is compressed as part of saving the file format. Compressing a MP3 file will usually reduce very little from the file size and sometimes will increase the total archive size do to archive format overhead.
For a WMV container format things are more complicated, this is because the data compression can be different depending on the implementation.

In some cases, even when compressing MP3 files or files with other built-in compression (e.g. jpeg), you can achieve more compression when compressing more than one file into one archive. Also, when dealing with many files, only packing the files into a single file can save some space filesystem wise.
But those are much little differences and you should consider the time and overhead and archive size overhead depending on the reason you like to compress the files. For instance, when copying files, one large file will generally copy faster then many small ones with the same size.

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