The Mp3PRO reference article from the English Wikipedia on 24-Apr-2004
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Mp3PRO

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mp3PRO is an audio compression algorithm (a.k.a. codec) capable of greatly reducing the amount of data required to reproduce audio, while sounding like a faithful reproduction of the original uncompressed audio to the listener.

More information about this audio format can be found at mp3PRO.com and mp3PROzone.com.