Apparatus, method and computer program for upmixing a downmix audio signal using a phase value smoothing

1. An apparatus for upmixing a downmix audio signal describing one or more downmix audio channels into an upmixed audio signal describing a plurality of upmixed audio channels, the apparatus comprising:

an upmixer configured to apply temporally variable upmix parameters to upmix the downmix audio signal, in order to acquire the upmixed audio signal, wherein the temporally variable upmix parameters comprise temporally variable smoothened phase values;

a parameter determinator, wherein the parameter determinator is configured to acquire one or more temporally smoothened upmix parameters for usage by the upmixer on the basis of a quantized upmix parameter input information,

wherein the parameter determinator is configured to combine a scaled version of a previous smoothened phase value with a scaled version of an input phase information using a phase change limitation algorithm, to determine a current smoothened phase value on the basis of the previous smoothened phase value and the input phase information.

2. The apparatus according to claim 1 , wherein the parameter determinator is configured to combine the scaled version of the previous smoothened phase value with the scaled version of the input phase information, such that the current smoothened phase value is in a smaller angle region out a first angle region and a second angle region, wherein the first angle region extends, in a mathematically positive direction, from a first start direction defined by the previous smoothened phase value to a first end direction defined by the input phase information, and wherein the second angle region extends, in a mathematically positive direction, from a second start direction defined by the input phase information to a second end direction defined by the previous smoothened phase value.

3. The apparatus according to claim 1 , wherein the parameter determinator is configured to select a combination rule out of a plurality of different combination rules in dependence on a difference between the input phase information and the previous smoothened phase value, and to determine the current smoothened phase value using the selected combination rule.

4. The apparatus according to claim 3 , wherein the parameter determinator is configured to select a basic phase combination rule, if the difference between the input phase information and the previous smoothened phase value is in a range between −π and +π, and to select one or more different phase adaptation combination rules otherwise;

wherein the basic phase combination rule defines a linear combination, without a constant summand, of the scaled version of the input phase information and the scaled version of the previous smoothened phase value; and

wherein the one or more phase adaptation combination rules define a linear combination, taking into account a constant phase adaptation summand, of the scaled version of the input phase information and the scaled version of the previous smoothened phase value.

5. The apparatus according to claim 1 , wherein the parameter determinator comprises a smoothing controller,

wherein the smoothing controller is configured to selectively disable a phase value smoothing functionality if a difference between a smoothened phase quantity and a corresponding input phase quantity is larger than a predetermined threshold value.

6. The apparatus according to claim 5 , wherein the smoothing controller is configured to evaluate, as the smoothened phase quantity, a difference between two smoothened phase values, and to evaluate, as the corresponding input phase quantity, a difference between two input phase values corresponding to the two smoothened phase values.

7. The apparatus according to claim 1 , wherein the upmixer is configured to apply, for a given time portion, different temporally smoothened phase rotations, which are defined by different smoothened phase values, to acquire signals of different of the upmixed audio channels comprising an inter-channel phase difference, if a smoothing function is enabled, and to apply temporally non-smoothened phase rotations, which are defined by different non-smoothened phase values, to acquire signals of different of the upmixed audio channels comprising an inter-channel phase difference, if the smoothing function is disabled;

wherein the parameter determinator comprises a smoothing controller; and

wherein the smoothing controller is configured to selectively disable a phase value smoothing function if a difference between the smoothened phase values applied to acquire the signals of the different upmixed audio channels differs from a non-smoothened inter-channel phase difference value, which is received by the apparatus or derived from a received information by the apparatus, is larger than a predetermined threshold value.

8. The apparatus according to claim 1 , wherein the parameter determinator is configured to adjust a filter time constant for determining a sequence of smoothened phase values in dependence on a current difference between a smoothened phase value and a corresponding input phase value.

9. The apparatus according to claim 1 , wherein the parameter determinator is configured to adjust a filter time constant for determining a sequence of smoothened phase values in dependence on a difference between a smoothened inter-channel phase difference which is defined by a difference between two smoothened phase values associated with different channels of the upmixed audio signal, and a non-smoothened inter-channel phase difference, which is defined by a non-smoothened inter-channel phase difference information.

10. The apparatus according to claim 1 , wherein the apparatus for upmixing is configured to selectively enable and disable a phase value smoothing function in dependence on an information extracted from an audio bitstream.

11. A method for upmixing a downmix audio signal describing one or more downmix audio channels into an upmixed audio signal describing a plurality of upmixed audio channels, the method comprising:

combining a scaled version of a previous smoothened phase value with a scaled version of a current phase input information using a phase change limitation algorithm, to determine a current temporally smoothened phase value on the basis of the previous smoothened phase value and the input phase information; and

applying temporally variable upmix parameters, to upmix a downmix audio signal in order to acquire an upmixed audio signal, wherein the temporally variable upmix parameters comprise temporally smoothened phase values.

12. A non-transitory computer readable medium including a computer program for performing the method for upmixing a downmix audio signal describing one or more downmix audio channels into an upmixed audio signal describing a plurality of upmixed audio channels when the computer program runs on a computer, the method comprising:

combining a scaled version of a previous smoothened phase value with a scaled version of a current phase input information using a phase change limitation algorithm, to determine a current temporally smoothened phase value on the basis of the previous smoothened phase value and the input phase information; and

applying temporally variable upmix parameters, to upmix a downmix audio signal in order to acquire an upmixed audio signal, wherein the temporally variable upmix parameters comprise temporally smoothened phase values.