How does the MP3 format accomplish its radical feats of compression and decompression, while still managing to maintain an acceptable level of fidelity to the original source material? The entire MP3 phenomenon is made possible by the confluence of several distinct but interrelated elements: A few simple insights into the nature of human psychoacoustics, a whole lot of number crunching, and conformance to a tightly specified format for encoding and decoding audio into compact bitstreams.

Well-encoded MP3 files can sound pretty  good, considering how small they are. 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.

MPEG Audio Compression

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.

MP3 encoding tools analyze incoming source signal, break it down into mathematical patterns, and compare these patterns to psycho-acoustic models stored in the encoder itself. The encoder can then discard most of the data that doesn't match the stored models, keeping that which does. The person doing the encoding can specify how many bits should be allotted to storing each second of music, which in effect sets a " tolerance" level-the lower the data storage allotment, the more data will be discarded, and the worse the resulting music will sound. The process is actually quite a bit more complex than that. This kind of compression is called lossy, because data is lost in the process. However, a second compression run is also made, which shrinks the remaining data even more via more traditional means (similar to the familiar "zip" compression process).

MP3 files are composed of a series of very short frames, one after another, much like a filmstrip. Each frame of data is preceded by a header that contains extra information about the data to come. In some encodings, these frames may interact with one another. For example, if one frame has leftover storage space and the next frame doesn't have enough, they may team up for optimal results.

At the beginning or end of an MP3 file, extra information about the file itself, such as the name of the artist, the track title, the name of the album from which the track came, the recording year, genre, and personal comments may be stored. This is called " ID3" data, and will become increasingly useful as your collection grows. We'll look at the structure of MP3 files and their ID3 tags, and the process of creating and using ID3 tags.

Sound and codecs  

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 billions 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). 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, 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. 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.

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 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. 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.

Codec- compression/decompression

What is MPEG -The Moving Picture Experts Group ?

The Moving Picture Experts Group is a working group of ISO/IEC in charge of the development of international standards for compression, decompression, processing, and coded representation of moving pictures, audio and their combination.

The Moving Picture Coding Experts Group (MPEG) was established in January 1988 with the aim to develop standards for coded representation of moving pictures, audio and their combination. It operates in the framework of the bd>Joint ISO/IEC Technical Committee (JTC 1) on Information Technology and is formally WG11 of SC29.

Starting from its first meeting in May 1988 when 25 experts participated, MPEG has grown to an unusually large committee. Usually some 350 experts from some 200 companies and organizations from about 20 countries take part in MPEG meetings. As a rule, MPEG meets three times a year (in March, July and November) but meets more frequently when the workload so demands.

To date MPEG has produced:
MPEG-1, the standard for storage and retrieval of moving pictures and audio on storage media (approved Nov. 1992).
MPEG-2, the standard for digital television (approved Nov. 1994).
bd>MPEG-4, the standard for multimedia applications - version 1 was approved Oct. 1998 - version 2 was approved Dec. 1999.

MPEG is now developing:
MPEG-4 versions 3, 4 and 5
MPEG-7, the content representation standard for multimedia information search, filtering, management and processing.
MPEG-21, the multimedia framework.

MP3 Players