Perceptual Audio Coding
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Perceptual Audio Coding (PAC) is an algorithm, like MPEG's MP3 standard, used to compress digital audio by removing extraneous information not perceived by most people. It is used by Sirius Satellite Radio for their DARS service, and iBiquity has been testing it for their IBOC digital radio upgrade for FM and AM.
It was originally developed by Elemedia, a subsidiary of Bell Labs. A known software implementation of this codec is CelestialTech's AudioLib (http://www.rjamorim.com/rrw/audiolib.html). Later, it was considerably improved and renamed to ePAC (enhanced Perceptual Audio Coder) by Lucent. It's available in the AudioVeda (http://www.rjamorim.com/rrw/audioveda.html) music library manager.
The term is sometimes used non-capitalized, to describe all lossy compression formats for audio.