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 comprises an upmixer configured to apply temporally variable upmixing parameters to upmix the downmix audio signal in order to obtain the upmixed audio signal. The apparatus also comprises a parameter interpolator, wherein the parameter interpolator is configured toobtain one or more temporally interpolated upmix parameters to be used by the upmixer onthe basis of a first complex-valued upmix parameter and a subsequent second complexvalued upmix parameter. The parameter interpolator is configured to separately interpolatebetween a magnitude value of the first complex-valued upmix parameter and a magnitude value of the second complex-valued upmix parameter, and between a phase value of the firstcomplex-valued upmix parameter and a phase value of the second complex-valued upmixparameter, to obtain the one or more temporally interpolated upmix parameters. A respectivemethod can be implemented, for example, as a computer program

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 comprises an upmixer configured to apply temporally variable upmixing parameters to upmix the downmix audio signal in order to obtain the upmixed audio signal. The apparatus also comprises a parameter interpolator, wherein the parameter interpolator is configured toobtain one or more temporally interpolated upmix parameters to be used by the upmixer onthe basis of a first complex-valued upmix parameter and a subsequent second complexvalued upmix parameter. The parameter interpolator is configured to separately interpolatebetween a magnitude value of the first complex-valued upmix parameter and a magnitude value of the second complex-valued upmix parameter, and between a phase value of the firstcomplex-valued upmix parameter and a phase value of the second complex-valued upmixparameter, to obtain the one or more temporally interpolated upmix parameters. A respectivemethod can be implemented, for example, as a computer program