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2.1. A "Perceptual" Codec

Well-encoded MP3 files can sound pretty darn good, considering how small they are. As mentioned in Chapter 1, your typical MP3 file is around one-tenth the size of the corresponding uncompressed audio source. How is this accomplished? That's a somewhat complex topic, so we've devoted this entire chapter to explaining the process.

2.1.1. MPEG Audio Compression in a Nutshell

Uncompressed audio, such as that found on CDs, stores more data than your brain can actually process. For example, if two notes are very similar and very close together, your brain may perceive only one of them. If two sounds are very different but one is much louder than the other, your brain may never perceive the quieter signal. And of course your ears are more sensitive to some frequencies than others. The study of these auditory phenomena is called psychoacoustics, and quite a lot is known about the process; so much so that it can be quite accurately described in tables and charts, and in mathematical models representing human hearing patterns.


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