Compressed Files

Downloading large files or software from the Web, you’ll find that these files usually arrive zipped, that’s saved in a special compressed format that your Mac has to decompress before you can do anything with the contents. This kind of compression serves two purposes. First it reduces the overall size of files in order to make them quicker to transfer by email or over the Web, and to make them more space efficient on a hard drive or other media. Second, this method allows you to bundle any number of separate files into a single archive, again, making them much more convenient to transfer over the Internet.

Compressed Files

Opening Compressed Formats

There are many different compressed file formats, and your Mac can open most of the files you’ll come across without using any extra software. Double-click the file, and a new unzipped version will appear next to the original. However at some stage you’ll inevitably be sent a Stuffit file, in which case you’ll need a free application called StuffIt Expander to open it up.

Creating Compressed Files

If you’re sending files by email or trying to save disk space by far the easiest way to compress them is to make a ZIP file: select one or more files and folders, right-click, then choose Compress from the context menu.

Images, Music & Video Compression

The type of file compression discussed so far is known as lossless compression, meaning that the data that comes out when the file is unzipped or unstuffed will be identical to what you put in. This action is distinct from the lossy compression widely used for shrinking sound, image and video files.

With lossy compression, the computer applies ingenious methods of picjing out which bits of data we notice least, and discards them. Unlike lossless compression, this is a one-way process, and you can’t take a compressed music or image file and turn it back into the uncompressed original. Sound, video and image files can withstand degradation through file compression, but you could never perform the same techniques on a text or a number document. Indeed the words and numbers are types of code, so a minor change can completely alter the meaning.

StuffIt vs ZIP

StuffIt was once the most widely used compression format on Macs. StuffIt Expander was included in OS X up to version 10.3. Compared with ZIP files, StuffIt archives are better at squashing data to the smallest possible size, but OS X can now handle many compressed formats itself.

How Lossless Compression Works

When a file is zipped up or stuffed, the computer looks for repeating patterns that it can represent more concisely. For example, you save a document containing this text : sell now, sell used electronics, sell Apple products, sell used Apple computers. The compression program would notice that the word “sell” occurs more than once, so would assign each of these collections of letters a more concise symbol. In reality, of course, the situation is much more complex, but the computer would use numbers rather than symbols. But the basic principle is the same.


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