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Inside the MP3 Codec
Breaking It Down
Lossy Compression
Masking Effects
Freedom of Implementation
Who Defines Imperceptible?
The Huffman Coding
MP3 Decoding
Anatomy of an MP3 File
ID3 Space
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Inside the MP3 Codec - Page 8
featured with permission from MP3: The Definitive Guide by Scot Hacker

Who Defines "Imperceptible?"

Before moving away from the topic of perceptual codecs, there's an important point to be made about the category as a whole: They all make baseline assumptions about the limitations of human perception, and about how closely the end result will be listened to. The fact of the matter is that all that stuff being stripped out adds up to something. While no recording format, whether it be vinyl, reel-to-reel, compact disk, or wax cylinder, can capture all of the overtones and subtle nuances of a live performance, nor can any playback equipment on the face of the earth reproduce the quality of a live performance. All compression formats-especially perceptual codecs are capable of robbing the signal of subtleties. While certain frequencies may not be distinctly perceptible, their cumulative effect contributes to the overall "presence" and ambience of recorded music. Once a signal has been encoded, some of the "magic" of the original signal has been stripped away, and cannot be retrieved no matter how hard you listen or how good your playback equipment. As a result, MP3 files are sometimes described as sounding "hollow" in comparison to their uncompressed cousins. Of course, the higher the quality of the encoding, the less magic lost. You have to strike your own compromises.

Many feel that the current digital audio standard offers less resolution than the best analog recording, which is why many audiophile purists still swear by vinyl LPs. Digital audio introduced a host of distortions never before encountered with analog, but hasn't had analog's 50+ years of research and development to eradicate them. Compressing and further modifying "CD quality" audio with a lossy perceptual codec like MP3, some might say, adds insult to injury.

But then there's reality, and the reality right now is that the vast majority of us do not listen to music with the trained ears of a true audiophile, nor do most of us possess magnificent playback equipment. Most of us use middle-ground sound cards and PC speakers, most of us have limits to the amount of data we can store conveniently, and most of us connect to the Internet with relatively low-bandwidth modems. Reality dictates that we make compromises. Fortunately, the reality of our sound cards and speakers, the quality of which lags far behind the quality of decent home audio systems, also means that most of these compromises won't be perceived most of the time.

The bottom line is that the perceptual codec represents a "good enough" opportunity for us to have our cake and eat it too. As things stand now, it all comes down to a matter of file size if we want to store and transfer audio files with anything approaching a level of convenience. In a perfect world, we would all have unlimited storage and unlimited bandwidth. In such a world, the MP3 format may never have come to exist-it would have had no reason to. If necessity is the mother of invention, the invention would never have happened. Compression techniques and the perceptual codec represent a compromise we can live with until storage and bandwidth limitations vanish for good.

Next:  The Huffman Coding


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