MPEG (The Moving Picture Experts Group) has developed compression systems used for video data. For example, DVD movies, HDTV broadcasts and DSS satellite systems use MPEG compression to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound, called MPEG Audio Layer-3. We know it by its abbreviation, MP3.
MP3 can compress a song by a factor of 10 or 12 and still retain something close to CD quality. So a 30-megabyte sound file from a CD reduces to 3 megabytes or so in MP3. When you download the MP3 file and play it, it sounds almost as good as the original file. If you wanted to, you could download an MP3 file, expand it back to its original size and then record it on a writable CD so you can play it in a CD player. All that you are doing is converting back and forth between different formats to make downloading easier.
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