Vector quantization

1. A method performed by an apparatus comprising a codec, said method comprising:

quantizing an input vector using a first quantization stage resulting in a residual vector, wherein the first quantization stage comprises a codebook having a plurality of codevectors, and wherein the quantized input vector is represented as a codevector index;

splitting a plurality of vector components of the residual vector into said at least two groups of vector components;

swapping a vector component of a first group of the at least two groups of vector components for a vector component of a second group of the at least two groups of vector components until each group of the at least two groups of vector components fulfils a rule; and

quantizing at least one of the at least two groups of vector components with a second quantization stage comprising a plurality of lattice structures, wherein a lattice structure used for the quantization of the at least one of the at least two groups of vector components is selected from the plurality of lattice structures based on the codevector index from the first quantization stage.

2. The method according to claim 1 , wherein said rule is one of:

a rule based on energy values associated with the vector components; and

a rule based on a predefined norm associated with the vector components.

3. The method according to claim 2 , wherein said rule specifies that the vector components of each of the at least two groups of vector components must fulfil a predefined energy characteristic.

4. The method according to claim 3 , wherein said rule specifies that an energy value associated with a respective vector component of each vector component of the first group of the at least two groups of vector components is higher than an energy value associated with a respective vector component of each vector component of each group of the remaining groups of the at least two groups of vector components.

5. The method according to claim 1 , wherein the at least two groups of vector components represent n groups of vector components g x with x∈{1, 2, . . . n} with n≥2, wherein an xth group g x of the at least two groups of vector components comprises l x vector coefficients of a plurality of vector coefficients of a first quantized representation of the input vector, and wherein said rule specifies that the l x vector coefficients of the xth group g x represent the

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most energetic (or less energetic) vector coefficients of the plurality of vector coefficients.

6. The method according to claim 1 , further comprising determining information being configured to determine the input vector comprising the plurality of vector components based on the at least two groups of vector components.

7. An apparatus comprising a codec, the apparatus further comprising at least one processor; and at least one memory including computer program code, said at least one memory and said computer program code configured to, with said at least one processor, cause said apparatus at least to:

quantize an input vector using a first quantization stage resulting in a residual vector, wherein the first quantization stage comprises a codebook having a plurality of codevectors, and wherein the quantized input vector is represented as a codevector index;

split a plurality of vector components of the residual vector into said at least two groups of vector components;

swap a vector component of a first group of the at least two groups of vector components for a vector component of a second group of the at least two groups of vector components until each group of the at least two groups of vector components fulfils a rule; and

quantize at least one of the at least two groups of vector components with a second quantization stage comprising a plurality of lattice structures, wherein a lattice structure used for the quantization of the at least one of the at least two groups of vector components is selected from the plurality of lattice structures based on the codevector index from the first quantization stage.

8. The apparatus according to claim 7 , wherein said rule is one of:

a rule based on energy values associated with the vector components; and

a rule based on a predefined norm associated with the vector components.

9. The apparatus according to claim 8 , wherein said rule specifies that the vector components of each of the at least two groups of vector components must fulfil a predefined energy characteristic.

10. The apparatus according to claim 9 , wherein said rule specifies that an energy value associated with a respective vector component of each vector component of the first group of the at least two groups of vector components is higher than an energy value associated with a respective vector component of each vector component of each group of the remaining groups of the at least two groups of vector components.

11. The apparatus according to claim 7 , wherein the at least two groups of vector components represent n groups of vector components g x with x∈{1, 2, . . . n} with n≥2, wherein an xth group g x of the at least two groups of vector components comprises l x vector coefficients of a plurality of vector coefficients of a first quantized representation of the input vector, and wherein said rule specifies that the l x vector coefficients of the xth group g x represent the

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most energetic (or less energetic) vector coefficients of the plurality of vector coefficients.

12. The apparatus according to claim 7 , wherein said at least one memory and said computer program code are further configured to, with said at least one processor, further cause said apparatus to determine information being configured to determine the input vector comprising the plurality of vector components based on the at least two groups of vector components.