Apparatus and method for downmixing or upmixing a multichannel signal using phase compensation
An apparatus for downmixing a multi-channel signal having at least two channels, has: a downmixer for calculating a downmix signal from the multi-channel signal, wherein the downmixer is configured to calculate the downmix using an absolute phase compensation, so that a channel having a lower energy among the at least two channels is only rotated or is rotated stronger than a channel having a greater energy in calculating the downmix signal; and an output interface for generating an output signal, the output signal having information on the downmix signal.
1. An apparatus for upmixing an encoded multi-channel signal, comprising:
an input interface for receiving the encoded multi-channel signal and for acquiring a downmix signal from the encoded multi-channel signal and for acquiring a side gain from the encoded multi-channel signal, the side gain indicating an energy relation between a first original channel and a second original channel; and
an upmixer for upmixing the downmix signal, wherein the upmixer is configured to calculate a reconstructed first channel and a reconstructed second channel using a phase compensation,
wherein the downmix signal is, in calculating the reconstructed first channel, only phase-rotated or is phase-rotated stronger than in calculating the reconstructed second channel depending on the side gain.
2. The apparatus of claim 1 ,
wherein the input interface is configured to acquire, from the encoded multichannel signal, inter-channel phase difference values, and
wherein the upmixer is configured to apply the inter-channel phase difference values in the phase compensation, when calculating the reconstructed first channel and the reconstructed second channel.
3. The apparatus of claim 2 , wherein the upmixer is configured
to calculate a phase rotation parameter from an inter-channel phase difference value and the side gain, and
to apply the phase rotation parameter in the phase compensation, when calculating the reconstructed first channel in a first manner, and
to apply the inter-channel phase difference value and the phase rotation parameter in the phase compensation, when calculating the reconstructed second channel in a second manner, wherein the first manner is different from the second manner.
4. The apparatus of claim 3 ,
wherein the upmixer is configured to calculate the phase rotation parameter so that the phase rotation parameter is within ±20% of a value determined based on the following equation:
ß
=
a
tan
(
sin
(
IPD
t
,
b
)
,
cos
(
IPD
t
,
b
)
+
A
1
+
g
t
,
b
1
-
g
t
,
b
)
,
wherein atan is an arctangent function, wherein β is the value, wherein IPD is the inter-channel phase difference, wherein t is a frame index, b is a sub-band index, and g t,b is the side gain for the frame t and the sub-band b, and wherein A is a value between 0.1 and 100 or between −0.1 and −100.
5. The apparatus of claim 4 , wherein the atan function comprises an atan2 function, the atan2(y,x) function being the two argument arctangent function whose value is an angle between the point (x,y) and a positive x-axis.
6. The apparatus of claim 4 ,
wherein the upmixer is configured to calculate the reconstructed first channel and the reconstructed second channel so that the reconstructed first channel and the reconstructed second channel comprise values that are in the range of ±20% with respect to values as determined based on the following equations:
L
~
t
,
k
=
e
i
ß
~
M
~
t
,
k
2
and
R
~
t
,
k
=
e
i
(
)
M
~
t
,
k
2
wherein {tilde over (M)} t,k is the downmix signal for the frame t and the frequency bin k, and wherein {tilde over (L)} t,k represents the values of the reconstructed first channel for the frame t and the frequency bin k, and wherein {tilde over (R)} t,k represents the values of the reconstructed second channel for the frame t and the frequency bin k, wherein IPD b is an inter-channel phase difference for the sub-band b comprising the frequency bin k, and wherein β is the phase rotation parameter.
7. The apparatus of claim 3 ,
wherein the upmixer is configured to calculate the reconstructed first channel and the reconstructed second channel so that the reconstructed first channel and the reconstructed second channel comprise values that are in the range of ±20% with respect to values as determined based on the following equations:
L
~
t
,
k
=
e
i
ß
~
(
M
~
t
,
k
(
1
+
g
~
t
,
b
)
+
r
~
t
,
b
g
norm
ρ
~
t
,
k
)
2
and
R
~
t
,
k
=
e
i
(
)
(
M
~
t
,
k
(
1
+
g
~
t
,
b
)
-
r
~
t
,
b
g
norm
ρ
~
t
,
k
)
2
wherein {tilde over (M)} t,k is the downmix signal for a frame t and a frequency bin k, wherein {tilde over (L)} t,k represents the values of the reconstructed first channel for the frame t and the frequency bin k, and wherein {tilde over (R)} t,k represents the values of the reconstructed second channel for the frame t and the frequency bin k, wherein IPD b is an inter-channel phase difference for the sub-band b comprising the frequency bin k, and wherein β is the phase rotation parameter, wherein {tilde over (g)} t,b is the side gain for the frame t and a subband b, wherein {tilde over (r)} t,b is a residual gain for the frame t and the subband b, wherein g norm is an energy normalization factor that can be there or not, and wherein {tilde over (ρ)} t,k is a raw signal for a residual signal for the frame t and the frequency bin k.
8. The apparatus of claim 1 ,
wherein the apparatus further comprises a residual signal synthesizer for synthesizing a residual signal using the residual gain;
wherein the upmixer is configured to perform a first weighting operation of the downmix signal using the side gain to acquire a first weighted downmix signal,
wherein the upmixer is configured to perform a second weighting operation using the side gain and the downmix signal to acquire a second weighted downmix signal,
wherein the first weighting operation is different from the second weighting operation, so that the first weighted downmix signal is different from the second weighted downmix signal, and
wherein the upmixer is configured to calculate the reconstructed first channel using a combination of the first weighted downmix signal and the residual signal and to calculate the reconstructed second channel using a second combination of the second weighted downmix signal and the residual signal.
9. The apparatus of claim 8 ,
wherein the upmixer is configured to combine the first weighted downmix signal and the residual signal using a first combination rule in calculating the reconstructed first channel, and
wherein the upmixer is configured to combine the second weighted downmix signal and the residual signal using a second combination rule in calculating the reconstructed second channel, wherein the first combination rule and the second combination rule are different from each other, or
wherein one of the first and the second combination rules is an adding operation and the other of the first and the second combination rules is a subtracting operation.
10. The apparatus of claim 8 ,
wherein the upmixer is configured to perform the first weighting operation comprising a weighting factor derived from a sum of the side gain and a first predetermined number, and
wherein the upmixer is configured to perform the second weighting operation comprising a weighting factor derived from a difference between a second predetermined number and the side gain, wherein the first predetermined number and the second predetermined number are equal to each other or are different from each other.
11. The apparatus of claim 8 , wherein the residual signal synthesizer is configured
to weight a downmix signal of a preceding frame using the residual gain for a current frame to acquire the residual signal for the current frame, or
to weight a decorrelated signal derived from the current frame or from one or more preceding frames using the residual gain for the current frame to acquire the residual signal for the current frame.
12. The apparatus of claim 8 ,
wherein the residual signal synthesizer is configured to calculate the residual signal so that an energy of the residual signal is equal to a signal energy indicated by the residual gain.
13. The apparatus of claim 8 ,
wherein the residual signal synthesizer is configured to calculate the residual signal so that values of the residual signal are in a range of ±20% of values determined based on the following equation:
res
t
,
k
=
r
~
t
,
b
g
norm
ρ
~
t
,
k
2
,
wherein res t,k is the residual signal for frame t and frequency bin k, wherein {tilde over (r)} t,b is the residual gain for the frame t and the sub-band b comprising the frequency bin k, and wherein {tilde over (ρ)} t,k is a raw signal for the residual signal, and wherein g norm is an energy normalization factor that can be present or not.
14. The apparatus of claim 13 ,
wherein g norm is the energy normalization factor comprising values in the range of ±20% of values determined based on the following equation:
E
M
~
,
t
,
b
E
ρ
~
,
t
,
b
,
wherein E {tilde over (M)},t,b is the energy of the downmix signal for the frame t and the sub-band b, and wherein E {tilde over (ρ)},t,b is the energy of the residual signal for the sub-band b and the frame t, or
wherein a raw residual signal is determined based on the following equation:
{tilde over (ρ)} t,k ={tilde over (M)} t−d b ,k ,
wherein {tilde over (ρ)} t,k is the raw residual signal,
wherein M t−d b ,k is the downmix signal for a frame t−t b and the frequency bin k, wherein d b is a frame delay greater than 0, or
wherein the upmixer is configured to calculate the reconstructed first channel and the reconstructed second channel so that the reconstructed first channel and the reconstructed second channel comprise values that are in the range of ±20% with respect to values as determined based on the following equations:
L
~
t
,
k
=
e
i
ß
~
(
M
~
t
,
k
(
1
+
g
~
t
,
b
)
+
r
~
t
,
b
g
norm
ρ
~
t
,
k
)
2
and
R
~
t
,
k
=
e
i
(
)
(
M
~
t
,
k
(
1
+
g
~
t
,
b
)
-
r
~
t
,
b
g
norm
ρ
~
t
,
k
)
2
wherein {tilde over (M)} t,k is the downmix signal for the frame t and the frequency bin k, wherein {tilde over (L)} t,k represents the values of the reconstructed first channel for the frame t and the frequency bin k, and wherein {tilde over (R)} t,k represents the values of the reconstructed second channel for the frame t and the frequency bin k, wherein IPD b is an inter-channel phase difference for the sub-band b comprising the frequency bin k, and wherein β is the phase rotation parameter, wherein {tilde over (g)} t,b is the side gain for the frame t and the subband b, wherein {tilde over (r)} t,b is the residual gain for the frame t and the subband b, wherein g norm is the energy normalization factor that can be there or not, and wherein {tilde over (ρ)} t,k is a raw signal for the residual signal for the frame t and the frequency bin k.
15. The apparatus of claim 1 ,
wherein the upmixer is configured to calculate the reconstructed first channel and the reconstructed second channel in a spectral domain,
wherein the apparatus further comprises a spectrum-time converter for converting the reconstructed first channel and the reconstructed second channel into a time domain,
wherein the upmixer is configured to rotate the channel comprising the lower energy more than the channel comprising the higher energy only when the energy difference between the channels is greater than a predefined threshold.
16. The apparatus of claim 15 ,
wherein the spectrum-time converter is configured to convert, for each one of the reconstructed first channel and the reconstructed second channel, subsequent frames into a time sequence of frames
to weight each time frame using a synthesis window; and
to overlap and add subsequent windowed time frames to acquire a time block of the reconstructed first channel and the time block of the reconstructed second channel.
17. A method of upmixing an encoded multi-channel signal, comprising:
receiving the encoded multi-channel signal and acquiring a downmix signal from the encoded multi-channel signal and acquiring a side gain from the encoded multi-channel signal, the side gain indicating an energy relation between a first original channel and a second original channel; and
upmixing the downmix signal, the upmixing comprising calculating a reconstructed first channel and a reconstructed second channel using a phase compensation, wherein the downmix signal is, in calculating the reconstructed first channel, only phase-rotated or is phase-rotated stronger than in calculating the reconstructed second channel depending on the side gain.
18. A non-transitory digital storage medium having stored thereon a computer program for performing, when said computer program is run by a computer, a method of upmixing an encoded multi-channel signal, comprising:
receiving the encoded multi-channel signal and acquiring a downmix signal from the encoded multi-channel signal and acquiring a side gain from the encoded multi-channel signal, the side gain indicating an energy relation between a first original channel and a second original channel;
upmixing the downmix signal, the upmixing comprising calculating a reconstructed first channel and a reconstructed second channel using a phase compensation,
wherein the downmix signal is, in calculating the reconstructed first channel, only phase-rotated or is phase-rotated stronger than in calculating the reconstructed second channel depending on the side gain.