1. In an audio decoder, a computer-implemented method of decoding encoded multi-channel audio data, the method comprising: receiving channel extension coding data comprising: a combined audio channel; plural power ratios representing power of individual audio channels relative to the combined audio channel; and a complex parameter representing an imaginary-to-real ratio of cross-correlation between the individual audio channels; receiving frequency extension coding data comprising scale and shape parameters for representing extended-band coefficients as scaled versions of baseband coefficients; and reconstructing the individual audio channels using the channel extension coding data and the frequency extension coding data; wherein the reconstructing comprises performing a real portion of a forward channel extension transform followed by frequency extension processing, and wherein the reconstructing further comprises deriving an imaginary portion of the forward channel extension transform after the frequency extension processing.