In the encoder the 16 bit PCM format audio signal is windowed and
converted into spectral subband components via a QMF polyphase filterbank
consisting of 32 equally spaced bandpass filters (Fig. 
). Such filterbanks
perfectly cancel the aliasing of adjacent overlapping bands in the
absence of quantization errors;they are computationally very
efficient, since an FFT can be used in the filtering process;and they
are of moderate complexity and low delay compared to QMF binary three
structure (Fig. ). On the negative side, the filters are equally spaced, and
therefore the frequency bands do not correspond well to the critical
bands at low frequencies. The filters are designed for high side lobe
attenuation exceeding 96 dB that is necessary for sufficient
cancelation of aliasing distortion caused by quantization noise. The
shape of the filter impulse response supports temporal masking of
pre-echoes in case of an attack signal.
Figure: Performance comparison of QMF and polyphase networks filter
banks in terms of complexity and delay of synthesis filter.
The filtered bandpass output signals are critically subsampled. In the receiver, the sampling rate of each subband is increased to that of source signal by filling in the appropriate number of zero samples, and interpolated subband signals appear at the bandpass outputs of the synthesis filterbank.
In Layer 3 a higher frequency resolution closer to critical band partitions is achieved by subdividing the 32 subband signals further in frequency content by applying a 6-point or 18-point modified Discrete Cosine Transform(MDCT) with 50 percent overlap to each of the subbands. The maximum number of frequency components in Layer 3 is therefore 32*18=576, each representing a bandwidth of 24000/576=41.67Hz.