Maximal Coherence Rotation for stereo coding

This paper presents a linear operation called Maximal Coherence Rotation (MCR) on paired vectors for stereo audio coding. Intrigued by the idea that stronger coherence between paired channels will lead to higher stereo coding efficiency, we develop MCR to maximize the coherence restricted by being invertible and energy-conserving. It results in equal energy, minimized difference, and always non-negative coherence for the pair of channels processed. In binaural hearing, this can be viewed as turning a physical sound source at any azimuth to a virtual one on the median plane. A prototype MCR stereo coder shows significantly higher quality for some test sequences than that of AMR-WB+, one of the best low bitrate stereo coders in the public domain. And as a preprocessing tool for MPEG-4 Parametric Stereo (PS), MCR avoids out-of-phase inter-channel cancellation during 2-to-1 channel downmixing without any additional bandwidth requirement, thanks to the maximized coherence.