MP3 and MP2 are two of three audio layers defined in the MPEG standard. These include layer 1, layer 2 (MP2), and layer 3 (MP3). These are a group of related compression schemes for audio within MPEG. The compression complexity and efficiency increases as you go up the layer hirearchy. However, all layers share a common structure, including the use of the same sampling rates. The layers are also called Layer N, and they allow backward compatiblity, (Layer 3 can play layer 1 and 2, layer 2 can play layer 1.) A decoder that can play all these layers is known as a full Layer N decoder.
The compression schemes were developed from two existing standards: MUSICAM and ASPEC. Each standard offered what the other did not. MUSICAM offered reduced data complexity but low performance, while ASPEC offered high performance with high complexity. The layers were created to take advantage of the best of both. Thus, each layer has its own merits, depending on the bit rate (the number of bits per second for audio) needed by an application.



Layer 1 is meant for high-bit-rate applications. It has a bit rate range from 32 kilobits per second to 448 kbps, with a compression ratio of about 1:4. It offers the lowest data complexity where the need for a low complexity encoder is needed.


Layer 2 is meant for midlevel bit rate applications. It has a bitrate range from 32 kbps to 384 kbps, with a compression ratio of about 1:6. It is a tradeoff between data complexity and bit rate. At lower rates it suffers from artifacting.


Layer 3 is meant for low-bit-rate applications. It has a bitrate range from 32 kbps to 320 kbps, with a compression ratio of about 1:10. It is meant for applications requiring high performance audio at low bit rates. This is why MP3 has become so popular among Internet audiophiles: it can maintain a high degree of sound quality in a realtively small package.




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