Sound signal downmix method, sound signal coding method, sound signal downmix apparatus, sound signal coding apparatus, program
A sound signal downmixing method includes a step of obtaining, for each of two channels, a signal obtained by adding an input sound signal of one channel to a signal obtained by delaying an input sound signal of the other channel and multiplying the delayed input sound signal by a weight value as a delayed crosstalk-added signal of the one channel, a step of obtaining preceding channel information and a left-right correlation value, and step of obtaining a downmix signal by performing weighted addition on the input sound signals of the two channels based on the left-right correlation value and the preceding channel information such that more of a signal derived from an input sound signal of a preceding channel among the signals derived from the input sound signals of the two channels is included as the left-right correlation value becomes larger.
1 . A sound signal downmixing method for obtaining a downmix signal that is a monaural sound signal from input sound signals of two channels, the method comprising:
a delayed crosstalk addition step of obtaining, for each of the two channels, a signal obtained by adding an input sound signal of one channel to a signal obtained by delaying an input sound signal of the other channel and multiplying the delayed input sound signal by a weight value that is a predetermined value having an absolute value smaller than 1, as a delayed crosstalk-added signal of the one channel;
a left-right relationship information acquisition step of obtaining preceding channel information that is information indicating which of the delayed crosstalk-added signals of the two channels is preceding and a left-right correlation value that is a value indicating a magnitude of correlation between the delayed crosstalk-added signals of the two channels; and
a downmixing step of obtaining the downmix signal by performing weighted addition on the input sound signals of the two channels based on the left-right correlation value and the preceding channel information such that more of a signal derived from an input sound signal of a preceding channel among the signals derived from the input sound signals of the two channels is included as the left-right correlation value becomes larger.
2 . The sound signal downmixing method according to claim 1 , wherein, in the delayed crosstalk addition step,
when the input sound signals of the two channels are respectively a left channel input sound signal and a right channel input sound signal, the delayed crosstalk-added signals of the two channels are respectively a left channel delayed crosstalk-added signal and a right channel delayed crosstalk-added signal, a sample number is t, each sample of the left channel input sound signal is x L (t), each sample of the right channel input sound signal is x R (t), each sample of the left channel delayed crosstalk-added signal is y L (t), each sample of the right channel delayed crosstalk-added signal is y R (t), predetermined positive values are a 1 and a2, and predetermined values having an absolute value smaller than 1 are w 1 and w2,
each sample y L (t) of the left channel delayed crosstalk-added signal is obtained by the following expression, and
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Math
.
17
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y
L
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t
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=
x
L
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t
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+
w
1
×
x
R
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)
each sample y R (t) of the right channel delayed crosstalk-added signal is obtained by the following expression,
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Math
.
18
]
y
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=
x
R
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t
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+
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2
×
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.
3 . The sound signal downmixing method according to claim 1 , wherein, in the delayed crosstalk addition step,
when the input sound signals of the two channels are respectively a left channel input sound signal and a right channel input sound signal, the delayed crosstalk-added signals of the two channels are respectively a left channel delayed crosstalk-added signal and a right channel delayed crosstalk-added signal, a frequency number is k, each frequency spectrum sample of a frequency spectrum obtained by performing Fourier transform on the left channel input sound signal for each frame is X L (k), each frequency spectrum sample of a frequency spectrum obtained by performing Fourier transform on the right channel input sound signal for each frame is X R (k), each frequency spectrum sample of the left channel delayed crosstalk-added signal in a frequency domain for each frame is Y L (k), each frequency spectrum sample of the right channel delayed crosstalk-added signal in the frequency domain for each frame is Y R (k), predetermined positive values are a 1 and a 2 , and predetermined values having an absolute value smaller than 1 are w 1 and w 2 ,
each frequency spectrum sample Y L (k) of the left channel delayed crosstalk-added signal in the frequency domain for each frame is obtained by the following expression, and
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Math
.
19
]
Y
L
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k
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k
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each frequency spectrum sample Y R (k) of the right channel delayed crosstalk-added signal in the frequency domain for each frame is obtained by the following expression,
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Math
.
20
]
Y
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k
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k
.
4 . A sound signal encoding method comprising the sound signal downmixing method according to claim 1 as a sound signal downmixing step,
wherein the sound signal encoding method further comprises:
a monaural encoding step of encoding the downmix signal obtained in the downmixing step to obtain a monaural code; and
a stereo encoding step of encoding the input sound signals of the two channels to obtain a stereo code.
5 . A non-transitory computer readable medium that stores a program for causing a computer to execute processing of each step of the sound signal encoding method according to claim 4 .
6 . A non-transitory computer readable medium that stores a program for causing a computer to execute processing of each step of the sound signal downmixing method according to claim 1 .
7 . A sound signal downmixing apparatus for obtaining a downmix signal that is a monaural sound signal from input sound signals of two channels, the sound signal downmixing apparatus comprising processing circuitry configured to:
obtain, for each of the two channels, a signal obtained by adding an input sound signal of one channel to a signal obtained by delaying an input sound signal of the other channel and multiplying the delayed input sound signal by a weight value that is a predetermined value having an absolute value smaller than 1, as a delayed crosstalk-added signal of the one channel;
obtain preceding channel information that is information indicating which of the delayed crosstalk-added signals of the two channels is preceding and a left-right correlation value that is a value indicating a magnitude of correlation between the delayed crosstalk-added signals of the two channels; and
obtain the downmix signal by performing weighted addition on the input sound signals of the two channels based on the left-right correlation value and the preceding channel information such that more of a signal derived from an input sound signal of a preceding channel among the signals derived from the input sound signals of the two channels is included as the left-right correlation value becomes larger.
8 . The sound signal downmixing apparatus according to claim 7 , wherein, in the processing circuitry,
when the input sound signals of the two channels are respectively a left channel input sound signal and a right channel input sound signal, the delayed crosstalk-added signals of the two channels are respectively a left channel delayed crosstalk-added signal and a right channel delayed crosstalk-added signal, a sample number is t, each sample of the left channel input sound signal is x L (t), each sample of the right channel input sound signal is x R (t), each sample of the left channel delayed crosstalk-added signal is y L (t), each sample of the right channel delayed crosstalk-added signal is y R (t), predetermined positive values are a 1 and a 2 , and predetermined values having an absolute value smaller than 1 are w 1 and w 2 , each sample y L (t) of the left channel delayed crosstalk-added signal is obtained by the following expression, and
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Math
.
21
]
y
L
(
t
)
=
x
L
(
t
)
+
w
1
×
x
R
(
t
-
a
1
)
each sample y R (t) of the right channel delayed crosstalk-added signal is obtained by the following expression,
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Math
.
22
]
y
R
(
t
)
=
x
R
(
t
)
+
w
2
×
x
L
(
t
-
a
2
)
.
9 . The sound signal downmixing apparatus according to claim 7 , wherein, in the processing circuitry,
when the input sound signals of the two channels are respectively a left channel input sound signal and a right channel input sound signal, the delayed crosstalk-added signals of the two channels are respectively a left channel delayed crosstalk-added signal and a right channel delayed crosstalk-added signal, a frequency number is k, each frequency spectrum sample of a frequency spectrum obtained by performing Fourier transform on the left channel input sound signal for each frame is X L (k), each frequency spectrum sample of a frequency spectrum obtained by performing Fourier transform on the right channel input sound signal for each frame is X R (k), each frequency spectrum sample of the left channel delayed crosstalk-added signal in a frequency domain for each frame is Y L (k), each frequency spectrum sample of the right channel delayed crosstalk-added signal in the frequency domain for each frame is Y R (k), predetermined positive values are a 1 and a 2 , and predetermined values having an absolute value smaller than 1 are w 1 and w 2 ,
each frequency spectrum sample Y L (k) of the left channel delayed crosstalk-added signal in the frequency domain for each frame is obtained by the following expression, and
[
Math
.
23
]
Y
L
(
k
)
=
X
L
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k
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+
w
1
×
X
R
(
k
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×
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-
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1
π
T
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each frequency spectrum sample Y R (k) of the right channel delayed crosstalk-added signal in the frequency domain for each frame is obtained by the following expression,
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Math
.
24
]
Y
R
(
k
)
=
X
R
(
k
)
+
w
2
×
X
L
(
k
)
×
e
-
j
2
a
2
π
T
k
.
10 . A sound signal encoding apparatus comprising the sound signal downmixing apparatus according to claim 7 ,
wherein the sound signal encoding apparatus further comprises processing circuitry configured to:
encode the downmix signal obtained by the downmixing unit to obtain a monaural code; and
encode the input sound signals of the two channels to obtain a stereo code.