Compression
Ever since people started using personal computers, space has always been a problem.
For many years, it was hard drive space. Like closets in your home, they fill up too quickly. People soon found it necessary to be creative in how they stored data. They could back it up on floppies or CDs, or delete files no longer needed. Or, they could make the data smaller somehow.
Today, storage has expanded greatly, and is less of a problem. Although we now keep much larger files (high-resolution digital photos and movies eat up most of the space), storing data is not as difficult as it was before.
However, now that the Internet is here, we have problems sending data; too many Internet users still have slow connections (Japan is very advanced in terms of Internet connection speeds), and so if we want to make data available, we have to try to make it smaller.
A common solution for both past and present is to make files smaller. The question is, how?
Real-World Compression
Consider putting a piece of letter-sized paper in your pocket. A piece of paper that size is too big. So, what do we do? Naturally, we fold the paper, usually three or even four times--and then it's small enough to fit in our pockets. There is a problem, however: when we open up the paper again, it now has fold lines--in other words, it has lost quality.
In some cases, however, you can make things smaller without losing quality. Think about a large amount of string, which has been unrolled and lays in an unorganized pile. If you try to put it in your pocket, it won't fit. However, most of that is just empty air; if you roll the string around a few fingers and then bind it with a rubber band, it is now compact enough to fit in your pocket. And because string bends naturally, there is no loss in quality.
Computer Compression
This works almost exactly the same way on computers. Instead of folding, however, we use something called CompressionTo "compress" means to make something smaller. Compression is commonly used in many file types--most of your music, images, and movies are already compressed..
Compression works by finding ways to make the data take up less space. One method is called "run-length encoding," and works by finding long strings of repeated data and reducing them to one string with a notation for how many times it repeats; for example, 111110000000 could be expressed as 1[5]0[7]. This is partly how a GIF file works--GIF images work best when there are many pixels of the exact same color, allowing for more "run-length encoding."
However, most compression is more complex, requiring mathematical formulas and special tricks to take a large amount of data and reduce it to a smaller amount. Two basic ways to do this are called lossy (methods where some quality is lost), and lossless (methods where no quality is lost). The example of folding a piece of paper is a "lossy" method; the example of binding up the string is an example of "lossless" compression.
Where Compression Is Used
Typically, most files which take up a large amount of space--photos, audio, and video--are compressed. This is done by saving the data in special file formats with compression built-in.
Take music files, for example. When you buy a music CD, the songs are saved as AIFF files--an older format which is not very compressed. An AIFF audio file containing one song might be 50 MB in size--allowing for about a dozen songs to fit on a CD. This is OK for that format, but if you want to save music on your computer--or more importantly, a small DAP (digital audio player) device, 50 MB per song is much too big. So when you transfer the song from the CD to your computer, it is translated into an MP3 file--a format with strong compression. The 50 MB AIFF file could be compressed into a 5 MB MP3. Now the music is small enough to fit on the DAP.
MP3 is a "lossy" format, meaning that you lose quality the more you compress it. Partly this is done by simply removing some of the data. Fortunately, most audio has more information than people can actually hear, so taking away some of the music data will not really affect how most people hear the music. Other tricks are also used to make the file smaller--and a great reduction in space is achieved.
The problem is when you try to compress something too much, and the quality become terrible as a result. Take the MP3 audio file below, a few seconds from a popular song. The first half (about 5 seconds) is at normal compression, and then the same bit repeats at extreme compression.
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