Parametric Stereo (PS) is the technique that lets HE-AAC v2 deliver a stereo image at very low bitrate. Instead of coding left and right separately, the encoder transmits a mono downmix plus a compact set of parameters describing the stereo cues — inter-channel level and phase differences, and a measure of how correlated the channels are. The decoder uses those to expand the mono back into a plausible stereo field. It is the stereo analogue of SBR's spectral trick: spend almost no bits on the second channel by describing it parametrically rather than coding it. Effective down to around 24 kbps, it is a clear compromise but far better than mono.
