MP3 is a way of compressing digital audio files. Using MP3 compression, sound files are often compressed to about one-tenth of their original size (16 bits, 44.1 kHz).
This is done in a fairly complex way, known as perceptual encoding since it uses a model of how our ear actually hears to figure out whats important to store and whats not. What MP3 does is break up a sound into frequency bands and throw out information thats soft enough in each band to be masked by louder information in that band (thats also what our ear does). This is very different from compressing bitstreams: its a sophisticated way of modeling the data so that the compressed version is still convincing to the human ear, though actually lots of numbers are left out.
As of this writing, the standard for compression is MP3, which means MPEG Audio Layer III format. This format will likely be improved in the near future. MPEG is an acronym for Moving Pictures Experts Group, which worked under the direction of the ISO (the International Organization for Standardization). MP3 is part of the MPEG standards, which include standards for digital versatile disc (DVD) and DirecTV.
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