1. In an audio encoder, a computer-implemented method comprising: receiving multi-channel audio data, the multi-channel audio data comprising a group of plural source channels; performing channel extension coding on the multi-channel audio data, the channel extension coding comprising: encoding a combined channel for the group; and determining plural parameters for representing individual source channels of the group as modified versions of the encoded combined channel, the plural parameters comprising a parameter representing a ratio of an imaginary part of cross-correlation between the individual source channels to a real part of cross-correlation between the individual source channels; and performing frequency extension coding on the multi-channel audio data.