Method and apparatus for decoding stereo loudspeaker signals from a higher-order ambisonics audio signal
Decoding of Ambisonics representations for a stereo loudspeaker setup is known for first-order Ambisonics audio signals. But such first-order Ambisonics approaches have either high negative side lobes or poor localization in the frontal region. The invention deals with the processing for stereo decoders for higher-order Ambisonics HOA. The desired panning functions can be derived from a panning law for placement of virtual sources between the loudspeakers. For each loudspeaker a desired panning function for all possible input directions at sampling points is defined. The panning functions are approximated by circular harmonic functions, and with increasing Ambisonics order the desired panning functions are matched with decreasing error. For the frontal region between the loudspeakers, a panning law like the tangent law or vector base amplitude panning (VBAP) are used. For the rear directions panning functions with a slight attenuation of sounds from these directions are defined.
1. Method for decoding stereo loudspeaker signals l(t) from a three-dimensional spatial higher-order Ambisonics audio signal a(t), with t designating time, from azimuth angle values φ L and φ R of left and right loudspeakers, and from S sampling points on a circle, said method including the steps:
receiving said audio signal a(t),
calculating by at least one processor, from azimuth angle values Φ of left and right loudspeakers and from the number S of virtual sampling points on a circle, a matrix G containing desired panning function values for all virtual sampling points,
wherein
G
=
[
g
L
(
ϕ
1
)
…
g
L
(
ϕ
S
)
g
R
(
ϕ
1
)
…
g
R
(
ϕ
S
)
]
and the g L (φ) and g R (φ) elements are the panning functions and g L (φ S ) and g R (φ S ) are the values at the S different sampling points corresponding respectively to values Φ 1 , Φ 2 . . . Φ S of said azimuth angle value Φ,
determining by said at least one processor the order N of said Ambisonics audio signal a(t);
calculating by said at least one processor from said number S and from said order N a mode matrix Ξ and the corresponding pseudo-inverse Ξ + of said mode matrix Ξ, wherein
Ξ=[y*(φ 1 ), y*(φ 2 ), . . . , y*(φ S )] and y*(φ)=[Y −N *(φ), . . . , Y 0 *(φ), . . . , Y N *(φ)] T is the complex conjugation of the circular harmonics vector
y(φ)=[Y −N (φ), . . . , Y 0 (φ), . . . , Y N (φ)] T of said Ambisonics audio signal a(t) and Y m (φ) are the circular harmonic functions, with m being an integer comprises between −N and N;
calculating by said from at least one processor from said matrices G and Ξ + a decoding matrix D=G Ξ + ;
calculating by said at least one processor the loudspeaker signals l(t)=Da(t), wherein a 3D-to-2D conversion of a(t) is carried out for this calculating,
outputting said loudspeaker signals l(t).
2. Method for determining a decoding matrix D that can be used for decoding stereo loudspeaker signals l(t)=Da(t) from a 2-D higher-order Ambisonics audio signal a(t), with t designating time said method including the steps:
receiving said audio signal a(t),
receiving the order N of said Ambisonics audio signal a(t);
calculating by at least one processor, from desired azimuth angle values Φ of left and right loudspeakers and from the number S of virtual sampling points on a circle, a matrix G containing desired panning function values for all virtual sampling points, wherein
G
=
[
g
L
(
ϕ
1
)
…
g
L
(
ϕ
S
)
g
R
(
ϕ
1
)
…
g
R
(
ϕ
S
)
]
and the g L (φ) and g R (φ) elements are the panning functions and g L (φ S ) and g R (φ S ) are the values at the S different sampling points corresponding respectively to values Φ 1 , Φ 2 , . . . Φ S of said azimuth value Φ,
calculating by said at least one processor from said number S and from said order N a mode matrix Ξ and the corresponding pseudo-inverse Ξ + of said mode matrix Ξ, wherein
Ξ=[y*(φ 1 ), y*(φ 2 ), y*(φ S )] and =[Y −N *(φ), . . . , Y 0 *(φ), . . . , Y N *(φ)] T is the complex conjugation of the circular harmonics vector
y(φ)=[Y −N (φ), . . . , Y 0 (φ), . . . , Y N (φ)] T of said Ambisonics audio signal a(t) and Y m (φ) are the circular harmonic functions, with m being an integer comprises between −N and N;
calculating by said at least one processor from said matrices G and Ξ + a decoding matrix D=G Ξ + ,
calculating by said at lease one processor the loudspeaker signals l(t)=Da(t), wherein a 3D-to-2D conversion of a(t) is carried out for this calculating,
outputting said loudspeaker signals l(t).
3. Method according to claim 1 , wherein a desired panning function is defined circle segment wise, and for said segments different panning functions are used.
4. Method according to claim 1 , wherein for the frontal region in-between the left and right loudspeakers the tangent law or vector base amplitude panning VBAP is used as desired panning functions.
5. Method according to claim 1 , wherein for the directions to the back, beyond the loudspeaker circle section positions, panning functions with an attenuation of sounds from these directions are used.
6. Method according to claim 1 , wherein more than two loudspeakers are placed on a segment of said circle.
7. Method according to claim 1 , wherein S=8N.
8. Method according to claim 1 , wherein in case of equally distributed virtual sampling points said decoding matrix D=G Ξ + is replaced by a decoding matrix D=α G Ξ H , wherein Ξ H is the adjoint of Ξ and a scaling factor α depends on the normalisation scheme of the circular harmonics and on S.
9. Apparatus for decoding stereo loudspeaker signals l(t) from a three-dimensional spatial higher-order Ambisonics audio signal a(t), with t designating time, from azimuth angle values φ L and φ R of left and right loudspeakers, and from S sampling points on a circle, said apparatus including:
at least one input adapted to receive said audio signal a(t),
means being adapted for calculating, from azimuth angle values of left and right loudspeakers and from the number S of virtual sampling points on a circle, a matrix G containing desired panning function values for all virtual sampling points, wherein
G
=
[
g
L
(
ϕ
1
)
…
g
L
(
ϕ
S
)
g
R
(
ϕ
1
)
…
g
R
(
ϕ
S
)
]
and the g L (φ) and g R (φ) elements are the panning functions and g L (φ S ) and g R (φ S ) are the values at the S different sampling points corresponding respectively to values Φ 1 , Φ 2 . . . Φ S of said azimuth angle value Φ,
means being adapted for determining the order N of said Ambisonics audio signal a(t);
means being adapted for calculating from said number S and from said order N a mode matrix Ξ and the corresponding pseudo-inverse Ξ + of said mode matrix Ξ, wherein Ξ=[y*(φ 1 ), y*(φ 2 ), . . . , y*(φ S )] and
y*(φ)=[Y −N *(φ), . . . , Y 0 *(φ), . . . , Y N *(φ)] T is the complex conjugation of the circular harmonics vector y(φ)=[Y −N (φ), . . . , Y 0 (φ), . . . , Y N (φ)] T of said Ambisonics audio signal a(t) and Y m (φ) are the circular harmonic functions, with m being an integer comprises between −N and N;
means being adapted for calculating from said matrices G and Ξ + a decoding matrix D=G Ξ + ;
means being adapted for calculating the loudspeaker signals l(t)=Da(t), wherein a 3D-to-2D conversion of a(t) is carried out for calculating l(t)=Da(t)
at least one output adapted to output said loudspeaker signals l(t).
10. Apparatus according to claim 9 , wherein a desired panning function is defined circle segment wise, and for said segments different panning functions are used.
11. Apparatus according to claim 9 , wherein for the frontal region in-between the left and right loudspeakers the tangent law or vector base amplitude panning VBAP is used as desired panning functions.
12. Apparatus according to claim 9 , wherein for the directions to the back, beyond the loudspeaker circle section positions, panning functions with an attenuation of sounds from these directions are used.
13. Apparatus according to claim 9 , wherein more than two loudspeakers are placed on a segment of said circle.
14. Apparatus according to claim 9 , wherein S=8N.
15. Apparatus according to claim 9 , wherein in case of equally distributed virtual sampling points said decoding matrix D=G Ξ + is replaced by a decoding matrix D=α G Ξ H , wherein Ξ H is the adjoint of Ξ and a scaling factor α depends on the normalisation scheme of the circular harmonics and on S.
16. Apparatus for decoding stereo loudspeaker signals l(t) from a three-dimensional spatial higher-order Ambisonics audio signal a(t), with t designating time, from azimuth angle values φ L and φ R of left and right loudspeakers, and from S sampling points on a circle, said apparatus including:
at least one input adapted to receive said audio signal a (t),
at least one processor configured for
calculating, from azimuth angle values of left and right loudspeakers and from the number S of virtual sampling points on a circle, a matrix G containing desired panning function values for all virtual sampling points,
wherein
G
=
[
g
L
(
ϕ
1
)
…
g
L
(
ϕ
S
)
g
R
(
ϕ
1
)
…
g
R
(
ϕ
S
)
]
and the g L (φ S ) and g R (φ S ) elements are the panning functions and g L (φ S ) and g R (φ S ) are the values at the S different sampling points corresponding respectively to values Φ 1 , Φ 2 . . . Φ S of said azimuth angle value Φ,
determining the order N of said Ambisonics audio signal a(t);
calculating from said number S and from said order N a mode matrix Ξ and the corresponding pseudo-inverse Ξ + of said mode matrix Ξ, wherein Ξ=[y*(φ 1 ), y*(φ 2 ), . . . , y*(φ S )] and y*(φ)=[Y −N *(φ), . . . , Y 0 *(φ), . . . , Y N *(φ)] T is the complex conjugation of the circular harmonics vector
y(φ)=[Y −N (φ), . . . , Y 0 (φ), . . . , Y N (φ)] T of said Ambisonics audio signal a(t) and Y m (φ) are the circular harmonic functions, with m being an integer comprises between −N and N;
calculating from said matrices G and Ξ + a decoding matrix D=G Ξ + ;
calculating the loudspeaker signals l(t)=Da(t), wherein a 3D-to-2D conversion of a(t) is carried out for calculating l(t)=Da(t)
at least one output adapted to output said loudspeaker signals l(t).