http://www.vorbis.com/

One of many compression formats for audio. Unlike mp3, Vorbis' ogg format allows you to write  your own encoder without having to pay the Fraunhofer Institute for the mp3 patent.

Testing suggests that Vorbis has slightly better quality/bitrate than mp3. There is however a lot of variety in audio encoders, some of them are quite bad. There are some, but not many, hardware players that understand ogg, this might still be an interesting DSP project.

The SansaClip series of players can decode ogg (and flac), but Wikipedia sez that decoding Ogg consumes ~twice the battery power as decoding mp3
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CategoryCompression