Claims:1. An apparatus (100; 200) for upmixing a downmix audio signal (110; 210) describing one or more downmix audio channels into an upmixed audio signal (120; 214) describing a plurality of upmixed audio channels, the apparatus comprising:

an upmixer (130; 230) configured to apply temporally variable upmix parameters (150; 262; Hij) to upmix the downmix audio signal (110; 210) in order to obtain the upmixed audio signal (120; 214); and

a parameter interpolator (140; 260), wherein the parameter interpolator is configured to obtain one or more temporally interpolated upmix parameters (150; 262) to be used by the upmixer (130; 230) on the basis of an information describing a first complex-valued upmix parameter (142) and a subsequent second complex-valued upmix parameter (144),

wherein the parameter interpolator (140; 260) is configured to separately interpolate

(a) between a magnitude value (254) of the first complex- valued upmix parameter (142) and a magnitude value (254) of the second complex-valued upmix parameter (144), and

(b) between a phase value (256) of the first complex- valued upmix parameter (142) and a phase value (256) of the second complex-valued upmix parameter (144),

to obtain the one or more temporally interpolated complex-valued upmix parameters (150; 262);

wherein the upmixer (130; 230) is configured to perform a linear scaled superposition of complex-valued subband parameters of a plurality of upmixer audio input signals (x(k), q(k)), in dependence on the complex-valued temporally interpolated upmix parameters (150; 262) to obtain the upmixed audio signal (120; 214);

wherein the upmixer (130; 230) is configured to process sequences of complex-valued subband parameters representing subsequent audio samples of the upmixer audio input signals (x(k), q(k)); and

wherein the parameter interpolator (140; 260) is configured to receive a representation (142, 144; 254, 256) of subsequent complex-valued upmix parameters, which are temporally spaced by more than a duration of one of the audio samples, and

to update the interpolated upmix parameters (150; 262) more frequently.

2. A method for upmixing a downmix audio signal (110; 210) describing one or more downmix audio channels into an upmixed audio signal (120; 214) describing a plurality of upmixed audio channels, the method comprising:

applying temporally variable upmix parameters (150; 262; Hij) to upmix the downmix audio signal (110; 210) in order to obtain the upmixed audio signal (120; 214); and

obtaining one or more temporally interpolated upmix parameters (150; 262) to be used by the upmixer (130; 230) on the basis of an information describing a first complex-valued upmix parameter (142) and a subsequent second complex-valued upmix parameter (144),

wherein the method comprises separately interpolating

(a) between a magnitude value (254) of the first complex- valued upmix parameter (142) and a magnitude value (254) of the second complex-valued upmix parameter (144), and

(b) between a phase value (256) of the first complex- valued upmix parameter (142) and a phase value (256) of the second complex-valued upmix parameter (144),

to obtain the one or more temporally interpolated complex-valued upmix parameters (150; 262);

wherein the method comprises performing a linear scaled superposition of complex-valued subband parameters of a plurality of upmixer audio input signals (x(k), q(k)), in dependence on the complex-valued temporally interpolated upmix parameters (150; 262) to obtain the upmixed audio signal (120; 214);

wherein the method comprises processing sequences of complex-valued subband parameters representing subsequent audio samples of the upmixer audio input signals (x(k), q(k)); and

wherein the method comprises receiving a representation (142, 144; 254, 256) of subsequent complex-valued upmix parameters, which are temporally spaced by more than a duration of one of the audio samples, and

updating the interpolated upmix parameters (150; 262) more frequently.

3. A parameter determinator (140; 260) for determining temporally variable upmix parameters for upmixing a downmix audio signal (110; 210) describing one or more downmix audio channels into an upmixed audio signal (120; 214) describing a plurality of upmixed audio channels,

wherein the parameter determinator comprises a parameter interpolator,

wherein the parameter interpolator is configured to obtain one or more temporally interpolated upmix parameters (150; 262) to be used by an upmixer (130; 230) on the basis of an information describing a first complex-valued upmix parameter (142) and a subsequent second complex-valued upmix parameter (144),
wherein the parameter interpolator (140; 260) is configured to separately interpolate

(a) between a magnitude value (254) of the first complex- valued upmix parameter (142) and a magnitude value (254) of the second complex-valued upmix parameter (144), and

(b) between a phase value (256) of the first complex- valued upmix parameter (142) and a phase value (256) of the second complex-valued upmix parameter (144),

to obtain the one or more temporally interpolated complex-valued upmix parameters (150; 262);

wherein the parameter determinator is configured to provide upmix parameters for a linear scaled superposition of complex-valued subband parameters of a plurality of upmixer audio input signals (x(k), q(k)), in dependence on the complex-valued temporally interpolated upmix parameters (150; 262) to obtain the upmixed audio signal (120; 214); and

wherein the parameter interpolator (140; 260) is configured to receive a representation (142, 144; 254, 256) of subsequent complex-valued upmix parameters, which are temporally spaced by more than a duration of one of the audio samples, and

to update the interpolated upmix parameters (150; 262) more frequently.

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