Lossless versus lossy compression

Summary

It is often necessary to compress a file to make it small enough to be used - for example making a music file small enough so that enough can be stored on an iPod. There are two main possibilities:

Lossless
- These are used to make a file a smaller size but without losing any of the information. Using this method you can always get back to the original file
Lossy
- Sometimes some loss of quality is acceptable. For example the human ear cannot hear all frequencies, so a file format that throws away parts that people can't hear may end up with a smaller file, but it is not possible to get back to how exactly the original music sounded.

Which format to choose?

The only real reason for choosing a lossy format is because the file would be too big if you used a lossless one. For example, a lossless picture may be too big to download in a sensible amount of time, or you could store many less tracks on an iPod if you used a lossless format instead of AAC or MP3. Another factor to choose lossy or lossless compression may be the computational power needed (and the time).