Method for and apparatus for decoding an ambisonics audio soundfield representation for audio playback using 2D setups
Sound scenes in 3D can be synthesized or captured as a natural sound field. For decoding, a decode matrix is required that is specific for a given loudspeaker setup and is generated using the known loudspeaker positions. However, some source directions are attenuated for 2D loudspeaker setups like e.g. 5.1 surround. An improved method for decoding an encoded audio signal in soundfield format for L loudspeakers at known positions comprises steps of adding ( 10 ) a position of at least one virtual loudspeaker to the positions of the L loudspeakers, generating ( 11 ) a 3D decode matrix (D′), wherein the positions (Formula I) of the L loudspeakers and the at least one virtual position (Formula II) are used, downmixing ( 12 ) the 3D decode matrix (D′), and decoding ( 14 ) the encoded audio signal (i 14 ) using the downscaled 3D decode matrix (Formula III). As a result, a plurality of decoded loudspeaker signals (q 14 ) is obtained.
1. A method for decoding an encoded audio signal in Ambisonics format for L loudspeakers at known positions, comprising:
adding at least one position of at least one virtual loudspeaker to the positions of the L loudspeakers;
generating a 3D decode matrix, wherein the positions of the L loudspeakers and the at least one virtual position are used and the 3D decode matrix has coefficients for said determined and virtual loudspeaker positions;
downmixing the 3D decode matrix, wherein the coefficients for the virtual loudspeaker positions are weighted and distributed to coefficients relating to the determined loudspeaker positions, and wherein a downscaled 3D decode matrix is obtained having coefficients for the determined loudspeaker positions; and
decoding the encoded audio signal using the downscaled 3D decode matrix, wherein a plurality of decoded loudspeaker signals is obtained,
wherein the coefficients for the virtual loudspeaker positions are weighted with a weighting factor g=1/√{square root over (L)},wherein L is number of loudspeakers.
2. The method according to claim 1 , further comprising:
determining positions of the L loudspeakers and an order N of coefficients of the soundfield signal;
determining from the positions that the L loudspeakers are substantially in a 2-dimensional plane; and
generating at least one virtual position of a virtual loudspeaker.
3. The method according to claim 1 , further comprising a step of separating the encoded audio signal into a plurality of frequency bands using band pass filters, wherein a plurality of separate 3D decode matrices are generated, one for each frequency band, and each 3D decode matrix is downmixed and optionally normalized separately, and wherein the step of decoding the encoded audio signal is performed for each frequency band separately.
4. The method according to claim 1 , wherein the known L loudspeaker positions are substantially within one 2-dimensional plane, with elevations of not more than 10°.
5. An apparatus for decoding an encoded audio signal in Ambisonics format for L loudspeakers at known positions, comprising:
an adder unit adapted for adding at least one position of at least one virtual loudspeaker to the positions of the L loudspeakers;
a decode matrix generator unit adapted for generating a 3D decode matrix, wherein the positions of the L loudspeakers and the at least one virtual position are used and the 3D decode matrix has coefficients for said determined and virtual loudspeaker positions;
a matrix downmixing unit adapted for downmixing the 3D decode matrix, wherein the coefficients for the virtual loudspeaker positions are weighted and distributed to coefficients relating to the determined loudspeaker positions, and wherein a downscaled 3D decode matrix is obtained having coefficients for the determined loudspeaker positions; and
a decoding unit for decoding the encoded audio signal using the downscaled 3D decode matrix, wherein a plurality of decoded loudspeaker signals is obtained
wherein the coefficient for the virtual loudspeaker positions are weighed with a weighting factor g=1/√{square root over (L)},wherein L is the number of loudspeakers.
6. The apparatus according to claim 5 , further comprising:
a first determining unit adapted for determining positions of the L loudspeakers and an order N of coefficients of the soundfield signal;
a second determining unit adapted for determining from the positions that the L loudspeakers are substantially in a 2-dimensional plane; and
a virtual loudspeaker position generating unit adapted for generating at least one virtual position of a virtual loudspeaker.
7. The apparatus according to claim 5 , further comprising a plurality of band pass filters adapted for separating the encoded audio signal into a plurality of frequency bands, wherein a plurality of separate 3D decode matrices are generated, one for each frequency band, and each 3D decode matrix is downmixed and optionally normalized separately, and wherein the decoding unit decodes each frequency band separately.
8. A non-transitory computer readable storage medium having stored thereon executable instructions to cause a computer to perform a method for decoding an encoded audio signal in Ambisonics format for L loudspeakers at known positions, the method comprising:
adding at least one position of at least one virtual loudspeaker to the positions of the L loudspeakers;
generating a 3D decode matrix, wherein the positions of the L loudspeakers and the at least one virtual position are used and the 3D decode matrix has coefficients for said determined and virtual loudspeaker positions;
downmixing the 3D decode matrix, wherein the coefficients for the virtual loudspeaker positions are weighted and distributed to coefficients relating to the determined loudspeaker positions, and wherein a downscaled 3D decode matrix is obtained having coefficients for the determined loudspeaker positions; and
decoding the encoded audio signal using the downscaled 3D decode matrix, wherein a plurality of decoded loudspeaker signals is obtained,
wherein the coefficents for the virtual loudspeaker positions are weighted with a weighting factor g=1/√{square root over (L)}, wherein L is the number of loudspeakers.
9. A method for decoding an encoded Ambisonics format audio signal for L loudspeakers, comprising:
adding at least a virtual position of at least a virtual loudspeaker to positions of the L loudspeakers;
determining a first matrix based on the positions of the L loudspeakers and the at least a virtual position, wherein the first matrix has coefficients for the determined and virtual loudspeaker positions;
determining a second matrix based on weighting and distributing of coefficients for the virtual loudspeaker positions of the first matrix, wherein the second matrix has coefficients for the determined loudspeaker positions; and
determining a third matrix based on a normalization of the second matrix,
wherein the coefficients for the virtual loudspeaker positions are weighted with a weighting factor
g
=
1
L
,
wherein L is the number of loudspeakers.
10. An apparatus for decoding an encoded Ambisonics format audio signal for L loudspeakers, comprising:
an adder unit for adding at least a virtual position of at least a virtual loudspeaker to positions of the L loudspeakers;
a first unit for determining a first matrix based on the positions of the L loudspeakers and the at least a virtual position, wherein the first matrix has coefficients for the determined and virtual loudspeaker positions;
a second unit for determining a second matrix based on weighting and distributing of coefficients for the virtual loudspeaker positions of the first matrix, wherein the second matrix has coefficients for the determined loudspeaker positions; and
a third unit for determining a third matrix based on a normalization of the second matrix,
wherein the coefficients for the virtual loudspeaker positions are weighted with a weighting factor
g
=
1
L
,
wherein L is the number of loudspeakers.