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MPEG-2 Layer 3 (MP3)

MPEG-2 Layer 3 is probably the most popular audio codec in the world, so let?s have a closer look.

The main enhancement to the basic encoder principle is the Huffman coding section. This process causes values that appear more frequently at the quantizer output to be encoded with shorter words, while values that appear only rarely are coded with longer words. This is similar to the common PC zip-style compression and results in an increase in coding efficiency with no degradation since it is a completely lossless process.

Another interesting and novel idea in Layer 3 is the Bit Reservoir buffering. Often, there are some critical parts in a piece of music that cannot be encoded at a given data rate without audible noise. These sequences require a higher data rate to avoid artifacts. On the other hand, some signals are easy to code. If a frame is easy, then the unused bits are put into a reservoir buffer. When a frame comes along that needs more than the average amount of bits, the reservoir is tapped for extra capacity. The bit reservoir buffer also offers an effective solution for the inclusion of such ancillary data as text or control signaling. The data is held in a separate buffer and gated onto the output bitstream using some of the bits allocated for the reservoir buffer when they are not required for audio.

There is a Joint Stereo mode that takes advantage of the redundancy in stereo program material. The encoder switches from discrete L/ R to a matrixed L+R/ L-R mode dynamically, depending upon the program material.  

The two-stage filterbank was used in order to have some measure of compatibility with Layer 2, which has only the first section.