5.7 Synthesis filterbank
26.4053GPPEncoder specification parametric stereo partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS
The stereo to mono down-mixed hybrid subband signal 
is fed into the hybrid synthesis filterbank, that is implemented as an adder of sub QMF samples. This is illustrated in Figure 3.
The synthesis filtering and implicit down-sampling of the 64 subband signals is achieved using a 32-channel QMF bank. The output from the filterbank is real-valued time domain samples. The process is given by the flowchart in Figure 4. The synthesis filtering comprises the following steps, where an array v consisting of 640 samples is assumed:
– Shift the samples in the array v by 64 positions. The oldest 64 samples are discarded.
– The array of 32 complex-valued subband samples Z is separated into the real and imaginary components as Z = R + i I. The components are scaled and DCT and DST type IV transformed as
– The arrays r and i are combined and stored in the positions 0 to 63 in array v as
– Extract samples from v according to the flowchart in Figure 4 to create the 320-element array g.
– Multiply the samples of array g by every other coefficient of window w. The window coefficients of c can be found in Figure 4, and are the same as for the analysis filterbank.
– Calculate 32 new output samples by summation of samples from array w according to the last step in the flowchart of Figure 4
Every SBR frame produces an output of 
time domain samples. In the flowchart of Figure 4 X[k][l] corresponds to subband sample l in the QMF subband k, and every new loop produces 32 time domain samples as output.
Figure 4: Flowchart of encoder synthesis QMF bank
Annex A (informative):
Change history
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Change history |
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Date |
TSG SA# |
TSG Doc. |
CR |
Rev |
Subject/Comment |
Old |
New |
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2004-09 |
25 |
SP-040637 |
Approved at SA#25 |
2.0.0 |
6.0.0 |
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2005-03 |
27 |
SP-050095 |
001 |
1 |
Correction to written specification: wrong formula on ICC parameter extraction |
6.0.0 |
6.1.0 |
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2007-06 |
36 |
Version for Release 7 |
6.1.0 |
7.0.0 |
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2008-12 |
42 |
Version for Release 8 |
7.0.0 |
8.0.0 |
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2009-12 |
46 |
Version for Release 9 |
8.0.0 |
9.0.0 |
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2011-03 |
51 |
Version for Release 10 |
9.0.0 |
10.0.0 |
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2012-09 |
57 |
Version for Release 11 |
10.0.0 |
11.0.0 |
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2014-09 |
65 |
Version for Release 12 |
11.0.0 |
12.0.0 |
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2015-12 |
70 |
Version for Release 13 |
12.0.0 |
13.0.0 |
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Change history |
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Date |
Meeting |
TDoc |
CR |
Rev |
Cat |
Subject/Comment |
New version |
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2017-03 |
75 |
Version for Release 14 |
14.0.0 |
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2018-06 |
80 |
Version for Release 15 |
15.0.0 |
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2020-07 |
– |
– |
– |
– |
– |
Update to Rel-16 version (MCC) |
16.0.0 |
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2022-04 |
– |
– |
– |
– |
– |
Update to Rel-17 version (MCC) |
17.0.0 |