text: ATRAC is a Sony proprietory digital music compression much like .mp3 or AAC.  Problem is it is only playable on Sony stuff.  Sony almost choked on it when they introduced it in the digital players meant to compete with iPod because it would play ONLY atrac and no mp3.  Hundreds of mp3's on your computer?  Sorry you gotta convert them all to atrac for your Sony player.  Consumers screamed, Sony listened...sorta.  How many codecs do we need?