HE-AAC v2 (also eAAC+) adds Parametric Stereo (PS) on top of HE-AAC, squeezing usable stereo into very low bitrates. Instead of coding two channels, the encoder codes a mono core and sends a tiny set of spatial parameters — level and phase differences — from which the decoder reconstructs a stereo image. Combined with the SBR bandwidth extension already in HE-AAC, this gives listenable stereo speech and music down to around 24-32 kbps, the realm of mobile streaming and digital radio on tight links. Quality is clearly a compromise at those rates, but it beats falling back to mono, and decoders are backward compatible with plain HE-AAC and LC.
