In this paper we propose a series of upmixing techniques for generating multichannel audio from stereo recordings. The techniques use a common analysis framework based on the comparison between the Short-Time Fourier Transforms of the left and right stereo signals. An inter-channel coherence measure is used to identify time-frequency regions consisting mostly of ambience components, which can then be weighed via a non-linear mapping function, and extracted to synthesize ambience signals. A similarity measure is used to identify the panning coefficients of the various sources in the mix in the time-frequency plane, and different mapping functions are applied to unmix (extract) one or more sources, and/or to re-pan the signals into an arbitrary number of channels. We illustrate the application of the various techniques in the design of a two-to-five channel upmix system.
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