Partially complex modulated filter bank
An apparatus for processing a plurality of real-valued subband signals using a first real-valued subband signal and a second real-valued subband signal to provide at least a complex-valued subband signal comprises a multiband filter for providing an intermediate real-valued subband signal and a calculator for providing the complex-valued subband signal by combining a real-valued subband signal from the plurality of real-valued subband signals and the intermediate subband signal.
1. Apparatus for processing a plurality of real-valued subband signals, the plurality of real-valued subband signals comprising a first real-valued subband signal and a second real-valued subband signal, to obtain a complex-valued subband signal, comprising:
a multiband filter for providing a real-valued intermediate subband signal based on filtering the first real-valued subband signal to obtain a first filtered subband signal and the second real-valued subband signal to obtain a second filtered subband signal and based on by combining the first filtered subband signal and the second filtered subband signal to obtain the real-valued intermediate subband signal; and
a calculator for providing the complex-valued subband signal by combining the real-valued subband signal from the plurality of real-valued subband signals as a real part of the complex-valued subband signal and a signal based on the intermediate subband signal as an imaginary part of the complex-valued subband signal,
wherein the apparatus is operative to assign to each real-valued subband signal according to a center frequency associated with the real-valued subband signal an index m, so that the real-valued subband signals are with an increasing index m, arranged according to the center frequency associated with the real-valued subband signals, wherein the plurality of real-valued subband signals comprises K real-valued subband signals, wherein K is a positive integer and m is an integer in the range from 0 to (K−1).
2. Apparatus (according to claim 1 , wherein the apparatus comprises a delayer for delaying the real-valued subband signal to provide the real-valued subband signal to the calculator in a delayed form.
3. Apparatus according to claim 1 , wherein the apparatus comprises a gain adjuster for receiving the complex-valued subband signal from the calculator and for adjusting a value of the complex-valued subband signal.
4. Apparatus according to claim 1 , wherein the plurality of real-valued subband signals is output by a real QMF analysis bank.
5. Apparatus according to claim 1 , wherein a multiband filter is operative to employ a low-pass filter characteristics, a high-pass filter characteristics or a bandpass filter characteristics for filtering the first real-valued subband signal and for filtering the second real-valued subband signal.
6. Apparatus according to claim 1 , wherein the multiband filter is operative to provide the real-valued intermediate subband signal with an index m, which corresponds to an index m, associated with the first real-valued subband signal.
7. Apparatus according to claim 6 , wherein the multiband filter is operative to use a real-valued subband signal from the plurality of real-valued subband signals, with which an index (m+1) or (m−1) is associated as the second real-valued subband signal.
8. Apparatus according to claim 6 , wherein the multiband filter is operative to provide a real-valued intermediate subband signal by further filtering a third real-valued subband signal to obtain a third filtered subband signal, and by combining the first filtered subband signal, the second filtered subband signal and the third filtered subband signal to obtain the real-valued intermediate subband signal, wherein either an index of the second real-valued subband signal (m−m′) and an index of the third real-valued subband signal is (m+m′) or the index of the second real-valued subband signal is (m+m′) and the index of the third real-valued subband signal is (m−m′), wherein m′ is a positive integer, and m is the index of the first real-valued subband signal.
9. Apparatus according to claim 8 , wherein the multiband filter is operative to provide a real-valued intermediate subband signal for each real-valued subband signal as the first real-valued subband signal from the plurality of real-valued subband signals with an index m−q(m) from, wherein the index of the second real-valued subband signal is m and the index of the third subband signal is (m+q(m)).
10. Apparatus as in one of claims 1 to 9 , wherein the multiband filter is operative to provide K intermediate real-valued subband signals having a value {circumflex over (x)} imag,k n,m , wherein n and m are positive integers, based on the equation
x
^
imag
,
k
n
,
m
=
∑
r
=
q
(
m
)
p
(
m
)
∑
v
=
0
10
f
m
,
r
[
v
]
x
^
real
,
k
n
-
v
,
m
+
r
,
m
=
0
,
1
,
…
,
K
-
1
for each of the K real-valued subband signals with the index m in the range of 0 to (K−1) and νis an integer in the range from 0 to 10, wherein
f
m
,
r
[
v
]
=
{
sin
[
π
2
[
-
(
2
m
+
1
)
(
v
-
5
)
]
]
a
0
[
v
]
+
(
-
1
)
m
a
1
[
v
]
,
if
(
m
,
r
)
∈
{
(
0
,
0
)
,
(
K
-
1
,
0
)
}
sin
[
π
2
[
-
r
-
(
2
m
+
1
+
r
)
(
v
-
5
)
]
]
a
r
[
v
]
,
else
wherein a 0 [ν] and a 1 [ν] are coefficients of a prototype filter, and wherein each coefficient of the prototype filter a 0 [ν] and a 1 [ν] obey the relations
0.003 ≦ a 0 [0] ≦ 0.004
| a 0 [1] | ≦ 0.001
−0.072 ≦ a 0 [2] ≦ −0.071
| a 0 [3] | ≦ 0.001
0.567 ≦ a 0 [4] ≦ 0.568
| a 0 [5] | ≦ 0.001
0.567 ≦ a 0 [6] ≦ 0.568
| a 0 [7] | ≦ 0.001
−0.072 ≦ a 0 [8] ≦ −0.071
| a 0 [9] | ≦ 0.001
0.003 ≦ a 0 [10] ≦ 0.004
and
0.0008 ≦ a 1 [0] ≦ 0.0009
0.0096 ≦ a 1 [1] ≦ 0.0097
0.0467 ≦ a 1 [2] ≦ 0.0468
0.1208 ≦ a 1 [3] ≦ 0.1209
0.2025 ≦ a 1 [4] ≦ 0.2026
0.2388 ≦ a 1 [5] ≦ 0.2389
0.2025 ≦ a 1 [6] ≦ 0.2026
0.1208 ≦ a 1 [7] ≦ 0.1209
0.0467 ≦ a 1 [8] ≦ 0.0468
0.0096 ≦ a 1 [9] ≦ 0.0097
0.0008 ≦ a 1 [10] ≦ 0.0009.
11. Apparatus according to claim 10 , wherein the multiband filter is designed so that the coefficients of the prototype filters a 0 [ν] and a 1 [ν] obey the relations
0.00375672984183 ≦ a 0 [0] ≦ 0.00375672984185
| a 0 [1] | ≦ 0.00000000000010
−0.07159908629243 ≦ a 0 [2] ≦ −0.07159908629241
| a 0 [3] | ≦ 0.00000000000010
0.56743883685216 ≦ a 0 [4] ≦ 0.56743883685218
| a 0 [5] | ≦ 0.00000000000010
0.5674388368521 6 ≦ a 0 [6] ≦ 0.56743883685218
| a 0 [7] | ≦ 0.00000000000010
−0.07159908629243 ≦ a 0 [8] ≦ −0.07159908629241
| a 0 [9] | ≦ 0.00000000000010
0.00375672984183 ≦ a 0 [10] ≦ 0.00375672984185
and
0.00087709635502 ≦ a 1 [0] ≦ 0.00087709635504
0.00968961250933 ≦ a 1 [1] ≦ 0.00968961250935
0.04670597747405 ≦ a 1 [2] ≦ 0.04670597747407
0.12080166385304 ≦ a 1 [3] ≦ 0.12080166385306
0.20257613284429 ≦ a 1 [4] ≦ 0.20257613284431
0.23887175675671 ≦ a 1 [5] ≦ 0.23887175675673
0.20257613284429 ≦ a 1 [6] ≦ 0.20257613284431
0.12080166385304 ≦ a 1 [7] ≦ 0.12080166385306
0.04670597747405 ≦ a 1 [8] ≦ 0.04670597747407
0.00968961250933 ≦ a 1 [9] ≦ 0.00968961250935
0.00087709635502 ≦ a 1 [10] ≦ 0.00087709635504.
12. Apparatus according to claim 1 , wherein the calculator is operative to provide K complex-valued subband signals with an index m and a value {circumflex over (x)} k n,m , wherein k, n, m are integers, wherein m is in the range from 0 to (K−1), based on the equation
x
^
k
n
,
m
=
1
2
(
x
^
real
,
k
n
-
5
,
m
-
ⅈ
x
^
imag
,
k
n
,
m
)
,
m
=
0
,
1
,
…
,
K
-
1
wherein {circumflex over (x)} real,k n,m represents a value of a real-valued subband signal and {circumflex over (x)} imag,k n,m represents a value of a real-valued intermediate subband signal and i represents the complex unit according to
i =√{square root over (−1)}.
13. Apparatus according to claim 1 , wherein the apparatus is operative to receive a further plurality of real-valued subband signals comprising (L−K) real-valued subband signals and to provide the further plurality of real-valued subband signals as real-valued subband signals, wherein L is a positive integer and wherein L is greater or equal to K.
14. Apparatus according to claim 13 , wherein the apparatus is designed so that the positive integer L equals 64.
15. Apparatus according to claim 13 , wherein the apparatus comprises a further delayer ( 202 ; 660 ) for delaying the real-valued subband signals of the further plurality of real-valued subband signals and wherein the apparatus is operative to provide the further plurality of real-valued band signals in a delayed form.
16. System comprising:
an analysis filter bank for processing an audio input signal into a plurality of real-valued subband signals;
an apparatus for processing the plurality of real-valued subband signals to obtain a complex-valued subband signal according to claim 1 ;
a modifier for receiving the complex-valued subband signal and for providing the complex-valued subband signal in a modified form;
an apparatus to obtain a real-valued subband signal; and
a synthesis filter bank for processing the real-valued subband signal into an audio output signal.
17. System according to claim 16 , wherein the analysis filterbank is designed such that the plurality of real-valued subband signals comprises L real-valued subband signals,
wherein L is a positive integer, wherein the apparatus for processing the plurality of real-valued subband signals is designed such that the apparatus provides a plurality of complex-valued subband signals and a further plurality of real-valued subband signals;
wherein the plurality of complex-valued subband signals comprises K complex-valued subband signals and the further plurality of real-valued subband signals comprise (L−K) real-valued subband signals;
wherein K is an integer in the range from 1 to L;
wherein the modifier is operative to modify the K complex-valued subband signals of the plurality of complex-valued subband signals to provide K complex-valued subband signals in a modified form;
wherein the system further comprises further modifier for modifying the further plurality of real-valued subband signals and for providing the further plurality of real-valued subband signals in a modified form;
wherein the apparatus is designed to process the plurality of complex-valued subband signals comprising K real-valued subband signals and the further plurality of real-valued subband signals comprising (L−K) real-valued subband signals to obtain a final plurality of real-valued subband signals,
wherein the final plurality of real-valued subband signals comprises L real-valued subband signals; and
wherein the synthesis filter band is designed such that the final plurality of real-valued subband signals is processed into the audio output signal.
18. Method for processing a plurality of real-valued subband signals, the plurality of real-valued subband signals comprising a first real-valued subband signal and a second real-valued subband signal to obtain a complex-valued subband signal, comprising:
filtering, by a filter, the first real-valued subband signal to obtain a first filtered subband signal, and
the second real-valued subband signal to obtain a second filtered subband signal;
combining, by a calculator, the first filtered subband signal and the second filtered subband signal when deriving a real-valued intermediate subband signal and
combining a real-valued subband signal from the plurality of real-valued subband signals as a real part of a complex-valued subband signal and a signal which is based on the intermediate subband signal as an imaginary part of the complex-valued subband signal,
wherein the method further comprises assigning an index m to each real-valued subband signal according to a center frequency associated with the real-valued subband signal, so that the real-valued subband signals are with an increasing index m, arranged according to the center frequency associated with the real-valued subband signals, wherein the plurality of real-valued subband signals comprises K is a positive integer and m, is an integer in the range from 0 to (K−1), and
wherein the filter or the calculator comprises a hardware implementation.
19. Non-transitory storage medium having stored thereon a computer program for performing, when running on a computer, a method in accordance with the methods of claim 18 .