This part of the encoding comprises two parts. First,
a polyphase filterbank, then a Modified Discrete Cosine Transform (MDCT).
The basic idea is 'divide and conquer.' The encoder breaks up the signal
into parts and then eliminates (in later steps) the redundant information.

The polyphase filterbank divides
the signal into 32 equal bands .
However, these 32 equal
bands have some overlap. Incidently, MPEG Layer 1 and 2 both use
a similar polyphase filterbank,
but the next part, MDCT, is added in Layer 3.

The MDCT applies
a separate window, individually
controlled by the perceptual model, to blocks of data from each of
the incoming 32 bands from the polyphase filterbank. The analysis of
the perceptual model determines whether a long or a short window is
used. If a short window is used, then this band is further divided
into 6 frequencies (and the window length is 8 ms at a sampling rate
of 48 kHz). If a long window is used, then the band is divided into
18 frequencies (a window length of 24 ms at a sampling rate of 48 kHz).
Therefore, the total number of frequencies bands at the end of the
filters is either 192 (6 * 32) or 576 (18 * 32). The long window could
be considered the default window, while the short is used to handle
transients.

**click** for advanced topic on MDCT!