5.4 Stereo parameter extraction
26.4053GPPEncoder specification parametric stereo partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS
5.4.1 Parameter estimation
In order to estimate the stereo parameters the signals M, L and R are analyzed using the hybrid filterbank as in Figure 2 for providing the 77 frequency bands addressed by the index, 
. This results in the (sub-)subband domain signals, 
, 
and 
.
To estimate the parameters for the current frame the following is calculated:
where 
, 
and 
are the left channel excitation, the right channel excitation and the non-normalized cross-channel excitation between left and right channel for stereo bin 
respectively, L the segment length, 
a very small value preventing division by zero (
). The summation over 
is shown in Table 2 for 20 bands case. For the 10 bands case additional formulas below are used.
Table 2: Summation range in 71 sub subbands in case of 20 bands
|
Parameter index b |
Sub subband index k |
QMF channel |
|
0 |
0 |
0 |
|
1 |
1 |
0 |
|
2 |
2 |
0 |
|
3 |
3 |
0 |
|
4 |
10 |
1 |
|
5 |
11 |
1 |
|
6 |
12 |
2 |
|
7 |
13 |
2 |
|
8 |
16 |
3 |
|
9 |
17 |
4 |
|
10 |
18 |
5 |
|
11 |
19 |
6 |
|
12 |
20 |
7 |
|
13 |
21 |
8 |
|
14 |
22-23 |
9-10 |
|
15 |
24-26 |
11-13 |
|
16 |
27-30 |
14-17 |
|
17 |
31-35 |
18-22 |
|
18 |
36-47 |
23-34 |
|
19 |
48-76 |
35-63 |
For the 10 band case, summation will be use the same table as the 20 band case but with the additional summation:
Where 
and 
and 
should replace 
and 
and 
in the following formulas for the 10 band case.
The IID, denoted as iid(b) and the ICC, denoted as icc(b), for each stereo band 
are calculated as:
5.4.2 Quantization of IID and ICC parameters
The obtained values for IID and ICC are quantized to the nearest lower value given in Table 3 and Table 4, respectively, then coded into Huffman words according to Huffman tables found in [4] section 8.B.1 Huffman tables ps_data.
Table 3: Quantization grid for iid
|
Index |
-7 |
-6 |
-5 |
-4 |
-3 |
-2 |
-1 |
0 |
|
IID [dB] |
-21.5 |
-16 |
-12 |
-8.5 |
-5.5 |
-3 |
-1 |
0 |
|
Index |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
|
|
IID [dB] |
1 |
3 |
5.5 |
8.5 |
12 |
16 |
21.5 |
Table 1: Quantization grid for icc
|
index |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
|
Icc |
0 |
0.088900 |
0.229800 |
0.364250 |
0.504500 |
0.635100 |
0.799600f |
0.945650 |