There are two types of compression: lossy and lossless.
Lossy compression attempts to exploit the inadequacies of the human senses to remove information which we won't miss. In the visual realm this includes JPEG images, and MPEG video, in the aural there are the MP3, WMA, Vorbis, AAC. (Not by coincidence nearly all those formats both for the eyes or ears are based upon Fourier transforms which substitute actual waveforms for slices of cosines which can easily be quantized and examined for what can be discarded.)
Lossless compression attempts to exploit redundancy in the actual data patterns. At it's simplest this would be akin to looking for strings of the same number and only recording that number once along with how many times it occurred in that run (this is called RLE-Run Length Encoding). To reverse that when you come to the encoded number and it's run, you simply pay out the correct count of that number and you have exactly the same data as it was originally recorded. Now, that doesn't get one very good compression, but more complicated techniques have been found which get about 50% compression on typical audio data. This is no were near the 90% which MP3 gains, but it does let you store 28 albums in 8 GB instead of the 14 of raw, uncompressed waves. Think about at Zip file, the data which goes into it is exactly what comes out, but it took less space to store in it's packed form. Blu-ray discs are now making use of two new lossless, but compressed audio formats known as Dolby TrueHD and DTS-HD MasterAudio, these take up less space on the disc than the raw multi-channel PCM, freeing up room for better quality video (BTW, the video is still stored in a lossy format on Blu-ray).
Last edited by ClubNeon; 03/16/09 10:12 PM.
