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Inside the MP3 Codec
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Inside the MP3 Codec - Page 2
featured with permission from MP3: The Definitive Guide by Scot Hacker

Waveforms and Psychoacoustics

Everything is vibration. The universe is made of waves, and all waves oscillate at different lengths (a wavelength is defined as the distance between the peak of one wave and the peak of the next). Waves vibrating at different frequencies manifest themselves differently, all the way from the astronomically slow pulsations of the universe itself to the inconceivably fast vibration of matter (and beyond). Somewhere in between these extremes are wavelengths that are perceptible to human beings as light and sound. Just beyond the realms of light and sound are sub- and ultrasonic vibration, the infrared and ultraviolet light spectra, and zillions of other frequencies imperceptible to humans (such as radio and microwave). Our sense organs are tuned only to very narrow bandwidths of vibration in the overall picture. In fact, even our own musical instruments create many vibrational frequencies that are imperceptible to our ears. Frequencies are typically described in units called Hertz (Hz), which translates simply as "cycles per second." In general, humans cannot hear frequencies below 20Hz (20 cycles per second), nor above 20kHz (20,000 cycles per second), as shown in Figure 2-1.[1] While hearing capacities vary from one individual to the next, it's generally true that humans perceive midrange frequencies more strongly than high and low frequencies,[2] and that sensitivity to higher frequencies diminishes with age and prolonged exposure to loud volumes. In fact, by the time we're adults, most of us can't hear much of anything above 16kHz (although women tend to preserve the ability to hear higher frequencies later into life than do men). The most sensitive range of hearing for most people hovers between 2kHz to 4kHz, a level probably evolutionarily related to the normal range of the human voice, which runs roughly from 500Hz to 2kHz.


Figure 2-1: While vibratory frequencies extend both above and below, human hearing is pretty much limited to the range between 20Hz and 20kHz

These are simple and well-established empirical observations on the human hearing mechanism. However, there's a second piece to this puzzle, which involves the mind itself. Some have postulated[3] that the sane mind functions as a sort of "reducing valve," systematically bringing important information to the fore and sublimating or ignoring superfluous or irrelevant data.[4] In fact, it's been estimated that we really only process a billionth of the data available to our five senses at any given time. Clearly, one of the most important functions of the mind is to function as a sieve, sifting the most important information out of the incoming signal, leaving the conscious self to focus on the stuff that matters.

The basic principle of any perceptual codec is that there's little point in storing information that can't be perceived by humans anyway. As obvious as this may sound, you may be surprised to learn that a good recording stores a tremendous amount of audio data that you never even hear, because recording equipment (microphones, guitar pickups, and so on) is sensitive to a broader range of sounds and audio resolutions than is the human ear. After getting an overview of how perceptual codecs work in general, we'll take a closer look at exactly how the MP3 codec does its thing.


The word " codec" is a foreshortening of the words "compress" and "decompress," and refers to any of a class of processes that allow for the systematic compression and decompression of data. While various codecs are fundamental to many file formats and transmission methods (for instance image and video compression formats have their own codecs, some of which are perceptual as well), it's the MP3 codec that concerns us here.


Next:  Breaking it Down



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