5.2 Analysis filterbank

26.4053GPPEncoder specification parametric stereo partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS

5.2.1 QMF analysis filterbank

This filterbank is identical to the 64 complex QMF analysis filterbank as defined in ISO/IEC 14496-3/AMD1:2003, sub clause 4.B.18.2 [1], [2]. However, in the equation for matrix M(k,n) and in Figure 4.B.20, the term "(2*n+1)" has to be substituted by "(2*n-1)". The input to the filterbank are blocks of 64 samples of the monaural synthesized signal M. For each block the filterbank outputs one slot of 64 QMF samples.

5.2.2 Low frequency filtering

The lower QMF subbands are further split in order to obtain a higher frequency resolution enabling a proper stereo analysis and synthesis for the lower frequencies. To achieve those, in total 77 frequency bands, a hybrid filterbank configurations have been defined. The filter used for this sub subband filtering, is defined according to:

where represents the prototype filters in QMF subband p. represents the number of sub-subbands in QMF subband p, q the sub-subband index in QMF channel p and n the time index. The prototype filters are all of length 13 and have a delay of 6 QMF samples. The prototype filters are listed in Table 1.

Table 1: Prototype filter coefficients for the filters that split the lower QMF subbands

0	0.00746082949812	-0.00305151927305
1	0.02270420949825	-0.00794862316203
2	0.04546865930473	0
3	0.07266113929591	0.04318924038756
4	0.09885108575264	0.12542448210445
5	0.11793710567217	0.21227807049160
6	0.125	0.25000000000000
7	0.11793710567217	0.21227807049160
8	0.09885108575264	0.12542448210445
9	0.07266113929591	0.04318924038756
10	0.04546865930473	0
11	0.02270420949825	-0.00794862316203
12	0.00746082949812	-0.00305151927305

Figure 2 and Figure 3 illustrate the hybrid analysis and synthesis filterbank for the 77 frequency bands configuration.

Figure 2: Hybrid QMF analysis filterbank providing 77 output bands. The three lower subbands of the 64 QMF (see dashed box) are further split to provide for increased resolution for the lower frequencies

Figure 3: Hybrid QMF synthesis filterbank using 77 input bands. The coefficients offering higher resolution for the lower QMF subbands are simply added prior to the synthesis with the 64 subbands QMF (see dashed box)

In order to time align all the samples originating from the hybrid filterbank, the remaining QMF subbands that have not been filtered are delay compensated. This delay amounts to 6 QMF subband samples. This means for k=3…63. In order to compensate for the overall delay of the hybrid analysis filterbank, the first 10 sets (6 from delay and 4 from QMF filter) of hybrid subbands are flushed and therefore not taken into account for processing.

The resultant of this operation is a slot of hybrid subband samples consisting of a LF (low frequency) sub QMF subband portion and HF (high frequency) QMF subband portion.