Sound signal downmixing method, sound signal coding method, sound signal downmixing apparatus, sound signal coding apparatus, program and recording medium
View Patent ↗A sound signal downmix method includes an inter-channel relationship information obtaining step of obtaining an inter-channel correlation value and preceding channel information of every pair of two channels included in N channels, the inter-channel correlation value being a value indicating a degree of a correlation between input sound signals of the two channels, the preceding channel information being information indicating which of the input sound signals of the two channels is preceding, and a downmix step of obtaining a downmix signal by weighting and adding the input sound signals of the N channels, the input sound signal of each channel being weighted based on the inter-channel correlation value and the preceding channel information such that the larger a correlation with an input sound signal of a preceding channel that precedes the channel, the smaller a weight, whereas the larger a correlation with an input sound signal of a succeeding channel that succeeds the channel, the larger the weight. The sum of the weights is normalized to 1.
1. A sound signal downmix method of obtaining a downmix signal that is a monaural sound signal from input sound signals of N channels, N being an integer of three or greater, the sound signal downmix method comprising:
obtaining an inter-channel correlation value and preceding channel information of every pair of two channels included in the N channels, the inter-channel correlation value being a value from 0 to 1 indicating a degree of a correlation between input sound signals of the two channels, the preceding channel information being information indicating which of the input sound signals of the two channels is preceding;
obtaining every sample x M (t) of the downmix signal, wherein
a sample number is denoted as t, every sample of the input sound signal of an ith channel whose i is from 1 to Nis denoted as x i (t), and every sample of the downmix signal is denoted as x M (t),
a set of channel numbers of channels preceding the ith channel is denoted as I Li ,
a set of channel numbers of channels succeeding the ith channel is denoted as I Fi ,
the inter-channel correlation value of a pair of the ith channel and every channel j preceding the ith channel is denoted as γ ij ,
the inter-channel correlation value of a pair of the ith channel and every channel k succeeding the ith channel is denoted as γ ik ,
a weight of the ith channel is denoted as w i , w i being expressed by
[
Math
22
]
w
i
=
1
N
∏
j
∈
I
Li
(
1
-
γ
ij
)
(
1
+
∑
k
∈
I
Fi
γ
ik
)
,
a normalized weight of the ith channel is denoted as w′ i , w′ i being expressed by
[
Math
23
]
w
i
′
=
w
i
∑
i
=
1
N
w
i
,
and
every sample x M (t) of the downmix signal is obtained by
[
Math
24
]
x
M
(
t
)
=
∑
i
=
1
N
w
i
′
×
x
i
(
t
)
;
encoding, based on the downmixed signal in embedded form focused on a monaural embedding, the plurality of sound signals of N channels.
2. A sound signal coding method comprising:
the sound signal downmix method according to claim 1 as a sound signal downmix step;
a monaural coding step of obtaining a monaural code by coding the downmix signal obtained in the downmix step; and
a stereo coding step of obtaining a stereo code by coding the input sound signals of the N channels.
3. A non-transitory computer-readable recording medium storing a program causing a computer to execute processing of each step of the sound signal coding method according to claim 2 .
4. A non-transitory computer-readable recording medium storing a program causing a computer to execute processing of each step of the sound signal downmix method according to claim 1 .
5. A sound signal downmix apparatus configured to obtain a downmix signal that is a monaural sound signal from input sound signals of N channels, N being an integer of three or greater, the sound signal downmix apparatus comprising:
an inter-channel relationship information obtaining unit configured to obtain an inter-channel correlation value and preceding channel information of every pair of two channels included in the N channels, the inter-channel correlation value being a value from 0 to 1 indicating a degree of a correlation between input sound signals of the two channels, the preceding channel information being information indicating which of the input sound signals of the two channels is preceding; and
a downmix unit configured to obtain every sample x M (t) of the downmix signal, wherein
a sample number is denoted as t, every sample of the input sound signal of an ith channel whose i is from 1 to N is denoted as x i (t), and every sample of the downmix signal is denoted as x M (t),
a set of channel numbers of channels preceding the ith channel is denoted as I Li ,
a set of channel numbers of channels succeeding the ith channel is denoted as I Fi ,
the inter-channel correlation value of a pair of the ith channel and every channel j preceding the ith channel is denoted as γ ij ,
the inter-channel correlation value of a pair of the ith channel and every channel k succeeding the ith channel is denoted as γ ik ,
a weight of the ith channel is denoted as w i , w i being expressed by
[
Math
25
]
w
i
=
1
N
∏
j
∈
I
Li
(
1
-
γ
ij
)
(
1
+
∑
k
∈
I
Fi
γ
ik
)
,
a normalized weight of the ith channel is denoted as w′i, w′i being expressed by
[
Math
26
]
w
i
′
=
w
i
∑
i
=
1
N
w
i
,
and
every sample x M (t) of the downmix signal is obtained by
[
Math
27
]
x
M
(
t
)
=
∑
i
=
1
N
w
i
′
×
x
i
(
t
)
;
encoding, based on the downmixed signal in embedded form focused on a monaural embedding, the plurality of sound signals of N channels.
6. A sound signal coding apparatus comprising:
the sound signal downmix apparatus according to claim 5 as a sound signal downmix unit;
a monaural coding unit configured to obtain a monaural code by coding the downmix signal obtained by the downmix unit; and
a stereo coding unit configured to obtain a stereo code by coding the input sound signals of the N channels.