Compression

May 28, 2010

Compression is a process by which a piece of data is encoded in such a way that it will take up less space when stored. Once compressed, it must be uncompressed before that piece of data can be used.

Let me give you a very simplified example. You already know that a computer stores information in binary format. That is ones and zeros. So let’s say you had the following information:

11100000101000100000000111110111111100100000100000

The data above consists of 50 bits or digits. Is there a shorthand way to write this that takes up less space? Yes there is. Anytime a particular digit is repeated more than two times in a row, you could write it in a different way. For example, since the only valid digits are ones and zeros, any other digit could specify the number of times to repeat the next digit. So if you had six zeros (000000), you could write it 60 meaning six zeros. Writing it that way takes 2 bits instead of 6. If you used this convention to write the above number, you would get:

11150101301805107100150150

That only takes up 26 bits. That’s a savings up 24 bits. And that’s compression.

That is a very simple example of compression. The compression used on computers is more complex than that, but you get the idea.

Compression is pretty common place in the computer world. It’s used to reduce the size of something so that it can be transferred over the internet (or, God forbid, over a phone line) faster and also so that it takes up less space when stored. It’s also a form of encryption, although its goal is not security.

Years ago when hard drives weren’t all that big and they were expensive, you could compress your whole hard drive to save space. But there was a price to pay. Every time read from the hard drive, that data has to be decompressed and every time you write to the hard drive, that data has to be compressed. That slows things down. But with today’s large, inexpensive hard drives, there’s no reason to even consider compressing a hard drive.

Some files cannot be compressed because they are, in effect, already compressed or the format of the file is just not condusive to compression. One example is the jpg format, often pronounced jpeg. This is a method for storing images. Most digital cameras product pictures in the jpeg format. One of the great things about jpeg files is that their native storage format is already compressed. You can try to compress it further, but you want get much, if any savings in size. In fact, due to the overhead of compression, you’ll probably make it bigger.

Zip files are compressed, but they have the additional advantage that you can group more than one file into the compressed file. Windows can handle zip files, but it calls them compressed folders. If you have multiple files you want to store together, you can put them in a zip file (compressed folder). You can then store them somewhere and since they are compressed, they will take up less space. You can transfer or transport them as well. When you are ready to use them, you must decompress them.

If you want to create a zip file in Windows, just select the file, or files, you want to put in the zip file, then right-click on them and choose “Send To” and then choose “Compressed (zipped) folder”. It will create the file and then let you choose a name. To open a zip file, just double click it.

By the way, Zip files (compressed folders) are not the only way to compress data on a computer. But Zip files are the only ones supported by Windows. If you want to use one of the other ones, then you will need additional software.

Some of the more popular formats used today include RAR and 7z. If you want to create RAR files, check out WinRAR (http://www.rarlab.com/). WinRAR also works on zip files, but it’s not free.

For the 7z format, check out 7-Zip (http://www.7-zip.org/). 7-Zip not only works on 7z files, but also works on Zip and RAR files. This is the utility I use most. It works great and the best part is, it’s free!

Most residential computer uses, however, don’t need anything more than what’s built-in to Windows.

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