1. A method for processing a multichannel audio signal to derive primary and ambient components of the signal, comprising: transforming at least a first and second channel of the audio signal to corresponding complex-valued time-frequency representations; and determining the primary component and ambient components by comparing frequency subband content using a complex-valued similarity metric, wherein one of the primary and ambient components is determined to be the residual after the other is identified and extracted using the complex-valued similarity metric.