5.2 Analysis filterbank

26.4043GPPEnhanced aacPlus encoder Spectral Band Replication (SBR) partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS

Subband filtering of the input signal is done by a 64-subband QMF bank. The output from the filterbank, i.e. the subband samples, are complex-valued and thus oversampled by a factor two compared to a regular QMF bank. The flowchart of this operation is given in Figure 6. The filtering comprises the following steps, where an array x consisting of 640 time domain input samples are assumed. Higher indices into the array corresponds to older samples:

– Shift the samples in the array x by 64 positions. The oldest 64 samples are discarded and 64 new samples are stored in positions 0 to 63.

– Multiply the samples of array x by window c. The window coefficients are found in Figure 6.

– Sum the samples according to the formula in the flowchart to create the 128-element array u.

– Build two arrays, r and i, from u by the operations

– Calculate 64 new complex-valued subband samples, X = R + i I, where i is the imaginary unit, by DCT and DST type III transforming r and i according to

Every loop in the flowchart produces 64 complex-valued subband samples, each representing the output from one filterbank subband. For every SBR frame the filterbank will produce subband samples from every filterbank subband, corresponding to a time domain signal of length samples. In the flowchart X[k][l] corresponds to subband sample l in QMF subband k.

Figure 6: Flowchart of encoder analysis QMF bank