Audio signal encoder

1. An apparatus comprising at least one processor and at least one memory including computer program code for one or more programs, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:

determine for a first frame of at least one audio signal a set of first frame audio signal multi-channel parameters;

determine a coding bitrate for the first frame of the at least one audio signal;

select for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal; and

generate an encoded first frame audio signal multi-channel parameter based on the selected groups of elements of first frame audio signal multi-channel parameters.

2. The apparatus as claimed in claim 1 , wherein the apparatus caused to determine for a first frame of at least one audio signal a set of first frame audio signal multi-channel parameters is further caused to determine a set of differences between at least two channels of the at least one audio signal, wherein the set of differences comprises two or more difference values, where each difference value is associated with a sub-division of resources defining the first frame.

3. The apparatus as claimed in claim 2 , wherein the apparatus caused to determine a set of differences between at least two channels of the at least one audio signal is further caused to determine at least one of:

at least one interaural time difference; and

at least one interaural level difference.

4. The apparatus as claimed in claim 2 , wherein the sub-division of resources defining the first frame comprises at least one of:

sub-band frequencies; and

time periods.

5. The apparatus as claimed in claim 1 , wherein the apparatus caused to select for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal is further caused to:

determine a number of the elements within the set of first frame audio signal multichannel parameters;

determine a number of groups of elements to be selected; and

arrange the elements into the number of groups by grouping successively indexed elements such that in each group there are the rounded result of the number of the elements within the set divided by the number of groups of elements to be selected.

6. The apparatus as claimed in claim 1 , wherein the apparatus caused to select for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal is further caused to:

generate first groups of elements of the set of first frame audio signal multi-channel parameters, with a first number of elements per group; and

generate second groups of elements of the set of first frame audio signal multi-channel parameters, with a second number of elements per group.

7. The apparatus as claimed in claim 6 , wherein the apparatus caused to generate first groups of elements of the set of first frame audio signal multi-channel parameters, with a first number of elements per group is further caused to generate first groups of elements where the elements represent lower frequency first frame audio signal multi-channel parameters, and wherein the apparatus caused to generate second groups of elements of the set of first frame audio signal multi-channel parameters, with a second number of elements per group is further caused to generate second groups of elements where the elements represent higher frequency first frame audio signal multi-channel parameters.

8. The apparatus as claimed in claim 1 , wherein to the apparatus caused to generate the encoded first frame audio signal multi-channel parameter based on the selected groups of elements of the set of first frame audio signal multi-channel parameters is further caused to generate an encoded parameter for each of the groups of elements of the at least one first frame audio signal multi-channel parameter using vector or scalar quantization codebooks.

9. The apparatus as claimed in claim 8 , wherein the apparatus caused to generate the encoded parameter for each of the groups of elements of the at least one first frame audio signal multi-channel parameter using vector or scalar quantization codebooks is further caused to:

generate a first encoding mapping with an associated index for the at least one first frame audio signal multi-channel parameter dependent on a frequency distribution of mapping instances of the at least one group of elements of the first frame audio signal multi-channel parameter; and

encode the first encoding mapping dependent on the associated index.

10. The apparatus as claimed in claim 9 , wherein the apparatus caused to encode the first encoding mapping dependent on the associated index is further caused to apply a Golomb-Rice encoding to the first encoding mapping dependent on the associated index.

11. The apparatus as claimed in claim 1 , further caused to:

receive at least two audio signal channels;

determine a fewer number of channels audio signal from the at least two audio signal channels and the at least one first frame audio signal multi-channel parameter;

generate an encoded audio signal comprising the fewer number of channels;

combine the encoded audio signal and the encoded at least one first frame audio signal multi-channel parameter.

12. A method comprising:

determining for a first frame of at least one audio signal a set of first frame audio signal multi-channel parameters;

determining a coding bitrate for the first frame of the at least one audio signal;

selecting for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal; and

generating an encoded first frame audio signal multi-channel parameter based on the selected groups of elements of the set of first frame audio signal multi-channel parameters.

13. The method as claimed in claim 12 , wherein determining for a first frame of at least one audio signal a set of first frame audio signal multi-channel parameters comprises determining a set of differences between at least two channels of the at least one audio signal, wherein the set of differences comprises two or more difference values, where each difference value is associated with a sub-division of resources defining the first frame.

14. The method as claimed in claim 13 , wherein determining a set of differences between at least two channels of the at least one audio signal comprises determining at least one of:

at least one interaural time difference; and

at least one interaural level difference.

15. The method as claimed in claim 13 , wherein the sub-division of resources defining the first frame comprises at least one of:

sub-band frequencies; and

time periods.

16. The method as claimed in claim 12 , wherein selecting for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal comprises:

determining a number of the elements within the set of first frame audio signal multichannel parameters;

determining a number of groups of elements to be selected; and

arranging the elements into the number of groups by grouping successively indexed elements such that in each group there are the rounded result of the number of the elements within the set divided by the number of groups of elements to be selected.

17. The method as claimed in claim 12 , wherein selecting for the first frame groups of elements of the set of first frame audio signal multi-channel parameters based on the coding bitrate for the first frame of the at least one audio signal comprises:

generating first groups of elements of the set of first frame audio signal multi-channel parameters, with a first number of elements per group; and

generating second groups of elements of the set of first frame audio signal multi-channel parameters, with a second number of elements per group.

18. The method as claimed in claim 17 , wherein generating first groups of elements of the set of first frame audio signal multi-channel parameters, with a first number of elements per group comprises generating first groups of elements where the elements represent lower frequency first frame audio signal multi-channel parameters and wherein generating second groups of elements of the set of first frame audio signal multi-channel parameters, with a second number of elements per group comprises generating second groups of elements where the elements represent higher frequency first frame audio signal multi-channel parameters.

19. The method as claimed in claim 12 , wherein generating the encoded first frame audio signal multi-channel parameter based on the selected groups of elements of the set of first frame audio signal multi-channel parameters comprises generating an encoded parameter for each of the groups of elements of the at least one first frame audio signal multi-channel parameter using vector or scalar quantization codebooks.

20. The method as claimed in claim 19 , wherein generating the encoded parameter for each of the groups of elements of the at least one first frame audio signal multi-channel parameter using vector or scalar quantization codebooks comprises:

generating a first encoding mapping with an associated index for the at least one first frame audio signal multi-channel parameter dependent on a frequency distribution of mapping instances of the at least one group of elements of the first frame audio signal multi-channel parameter; and

applying a Golomb-Rice encoding to the first encoding mapping dependent on the associated index.