1. Apparatus for synthesising an output signal comprising a first audio channel signal and a second audio channel signal, the apparatus comprising; a decorrelator stage for generating a decorrelated signal comprising a decorrelated single channel signal or a decorrelated first channel signal and a decorrelated second channel signal from a downmix signal, the downmix signal comprising a first audio object downmix signal and a second audio object downmix signal, the downmix signal representing a downmix of a plurality of audio object signals in accordance with downmix information; and a combiner for performing a weighted combination of the downmix signal and the decorrelated signal using weighting factors, wherein the combiner is operative to calculate the weighting factors for the weighted combination from the downmix information, from target rendering information indicating virtual positions of the audio objects in a virtual replay set-up, and parametric audio object information describing the audio objects, wherein the combiner is operative to calculate a mixing matrix C 0 for mixing the first audio object downmix signal and the second audio object downmix signal based on the following equation: 
 C 0 =AED *( DED *) −1 , wherein C 0 is the mixing matrix, wherein A is a target rendering matrix representing the target rendering information, wherein D is a downmix matrix representing the downmix information, wherein * represents a complex conjugate transpose operation, and wherein E is an audio object covariance matrix representing the parametric audio object information, and wherein at least one of the decorrelator stage or the combiner comprises a hardware implementation.