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92. Magical shrinking files

By Andrew D. Wright

Whether you call it a jpeg, an mp3 or a DVD, what you are talking about is a magic trick that works.

Data compression shrinks the size of information, saving money on its storage and the time needed to transfer it. Less is more.

There are two kinds of data compression, lossless and lossy.

In lossless data compression the original file information can be extracted intact from a smaller file. A ZIP file would be an example: the file you unzip is identical to the original file that was compressed.

Lossless data compression uses a number of different techniques to shrink information size. The most basic one is to remove redundant information. Let's say this column you're now reading was being compressed. The words "data compression" could be replaced by something smaller: a symbol or a reference to where those characters first appeared in the text.

Most lossless compression programs use variations on this theme, keeping indexes of the information that has been compressed then removing the redundant information to compress the indexes in turn. The end user just needs to be able to interpret the particular index format in order to restore the original data.

While ZIP files are the most popular lossless format, others such as RAR, CAB, ACE, and JAR files are also in wide use. There are many programs available that use one or more of these formats to create reduced-size archives of multiple files and folders such as hard drive backups.

How much the data can be compressed, how long will it take to compress and uncompress the data and how well a program can handle different kinds of data vary from program to program.

For the last two years the most efficient and fastest lossless compression software has been a Windows-based shareware program from New Zealand called WinRK using its own proprietary .rk file extension. Unfortunately unlike the other lossless compression formats mentioned above, there is no free program to open the .rk compressed files. The shareware program is free to try for thirty days.

Lossy data compression selectively deletes some information to make files smaller. The popular mp3 audio format removes sound the human ear is unlikely to miss so a lot less data is needed to make a reasonably good copy of the original sound.

Lossy data compression is everywhere around us. The MPEG-2 format used to encode satellite TV and the video on DVDs uses statistical analysis of the building blocks of the image to remove redundant information. Near duplicate video frames can be merged, visual detail can be blurred and substantially less bandwidth is needed to send a close copy of the original video signal.

Psychoacoustics is the science of understanding how humans hear sound and is used to quantify the kind of audio information that can be removed from a sound file without producing a discernible difference to most people. A sound file can be made ten to twelve times smaller than its original size after lossy compression. Your dog and cat will notice the difference right away but you probably won't.

WinRK (shareware):

The Mousepad runs every two weeks. It's a service of Chebucto Community Net, a community-owned Internet provider. If you have a question about computing, email or click here. If we use your question in a column, we'll send you a free mousepad.


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Originally published 10 September 2006


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