MPEG defines 3 layers for
audio. Basic model is same, but codec complexity increases with each
layer.
Divides data into frames, each of them contains 384 samples,
12 samples from each of the 32 filtered subbands as shown below.
Figure: Grouping of Sub-band Samples for Layer 1, 2, and 3
Layer 1: DCT type filter with one frame and equal frequency
spread per band. Psychoacoustic model only uses frequency masking.
Layer 2: Use three frames in filter (before, current, next, a total
of 1152 samples). This models a little bit of the temporal masking.
Layer 3: Better critical band filter is used (non-equal frequencies),
psychoacoustic model includes temporal masking effects, takes into
account stereo redundancy, and uses Huffman coder.
