From Hydrogenaudio Knowledgebase
Filterbank as used in all mpeg1 layers encoding and MPC encoding (polyphase analysis filterbank), is a set of bandpass filter, which divide the audio signal into 32 equal-width frequency subband. So, with cd-audio's 22.05kHz bandwidth, the width of 1 subband is about 689Hz (32*689Hz=22.05kHz). Filterbank and its inverse are not lossless transformations. Even without quantization, the inverse transformation cannot perfectly recover the original signal. Filterbank is designed so, that the error introduced is small and inaudible.