Talking about MPEG Audio, I hear about phases, layers and levels. What does that all mean?
There are two different matters that have to be distinguished. First, MPEG works in phases. These phases are normally denoted by Arabic numbers (MPEG-1, MPEG-2, MPEG-4). The audio activities of the first phase, MPEG-1, were finalised in 1992 and resulted in the International Standard ISO/IEC 11172-3 which was published in 1993. Part of the audio activities in the second phase, MPEG-2, have been finalised in 1994 and resulted in the International Standard ISO/IEC 13818-3 which was published in 1995. Further work relating to MPEG-2 will be finalised in 1997 and published in International Standard 13818-7. Another phase, currently under way, is called MPEG-4, and is planned to be finalised in 1998. Both in MPEG-1 and in MPEG-2, three different layers are defined, sometimes incorrectly called ‘levels’. These layers represent a family of coding algorithms. The layers are preferably denoted by roman figures, i.e. Layer I, Layer II and Layer III.
