Audio Encoding Implementation

The MP3 specification, also known as ISO 11172-3, does not explain the specifics as to how MP3 encoding is accomplished. Rather, it outlines techniques and details a level of conformance. In other words, it informs developers that their resulting MP3 files must meet certain structural criteria. This is necessary for the same reason that any standard exists; to allow for the proliferation of MP3 encoders and players by various vendors and developers. The specification only serves to guarantee a baseline consensus in the community regarding how certain things operate. An encoder developed according to the MP3 specification is then capable of outputting a "compliant bitstream" that can be played with any MP3-compliant decoder, just as you can render any standard W3C-compliant HTML document in any browser on any operating system.

It is important to maintain the distinction between the primary developers of the codec itself, The Fraunhofer Institute, and the committee that codified the work of Fraunhofer into the MPEG-I Layer 3 specification, or the International Standards Organization (ISO). Standards are often created this way; a company produces a technology, other companies apply to become a part of the standards-creation process, and together they lay down the laws of implementation so that all vendors can compete around the same technology. However, just because MP3 has been standardized by ISO does not mean that Fraunhofer and their partners Thomson Multimedia don't still hold the patent on the technology itself. At the current time Fraunhofer's patent is being aggressively exercised making it difficult for small-time developers to affordably implement the ISO standard. Documentation on the MP3 specification can be ordered for around $150 from the main ISO web page, http://www.iso.ch/.

While the standard specifies exactly how decoding is to be accomplished, it only provides sample implementations (one simple and one complex) for encoding. As a result, a certain degree of headroom is available for developers to make up some of the rules as they go along. In general, encoder developers work toward two goals: speed and quality. While there is some difference in the quality of audio files output by various encoders, there are vast differences in the speed at which encoders operate. Sometimes, encoding speed comes at a distinct disadvantage to the quality of the resulting bitstream.

A good example of the kind of freedom left to developers is the fact that the MP3 standard does not specify exactly how to treat the upper end of the spectrum, or the range above 16kHz. Since human auditory perception begins to diminish between 16kHz and 20kHz, some developers have historically chosen to simply chop off frequencies above 16kHz. This can be beneficial at low bitrates, as it leaves more bits available for encoding more audible frequencies. Xing, for example, did this with the first versions of their very fast codec. Later, they rewrote their codec to handle frequencies up to 20kHz perhaps to satisfy the audiophile MP3 community.