First of all what's the difference between phases, layers and levels?
With regard to audio, there currently exist three phases of MPEG:
- MPEG-1 -- Completed in October, 1992, this standard was designed for delivery of mono and stereo audio to consumer devices at single-speed CD-ROM data rates.
- MPEG-2 -- Completed in 11/94, this standard was designed for delivery of multichannel audio, as well as very low bitrates.
- MPEG-4 -- When completed, this standard will provide coding and composition of natural and synthetic audio objects at a very wide range of bit rates.
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.
Both MPEG-1 and MPEG-2 specify a family of three audio compression and coding schemes: Layer-1,-2,-3, with increasing encoder complexity and performance (sound quality per bitrate) from 1 to 3. The three schemes are compatible in a hierarchical way; that is, a Layer-3 decoder may accept Layer-1,-2 and -3, whereas a Layer-2 decoder may accept only Layer-1 and -2.
MPEG audio compression is achieved by taking advantage of the "masking" tendency inherent in the human ear. It removes the lower level audio information that is present "behind" louder audio at or near the same pitch, or frequency. It processes audio in this way for a number of frequency ranges, or bandwidths. Removing this "masked" digital data significantly reduces the size of the resulting files with little or no audible effect.
In terms of web-based MPEG audio, two types are common: .mp2, which is MPEG-1 layer 2; and .mp3, which is MPEG-1, layer 3.
Try playing the MPEG audio files below to determine whether your MPEG player or plugin can play the appropriate formats. If you don't see a control panel you either don't have an MPEG audio plug-in installed or it's improperly configured. If you do have an appropriate plug-in, clicking on the text link will cause the file to play in a new window using that plug-in.