Spectral Band Replication
From Hydrogenaudio Knowledgebase
Spectral Band Replication (SBR) is a parametrical coding method developed by Coding Technologies to enhance audio quality at low bitrates.
Guessing the non-transmitted higher frequency range of a compressed audio file by using some helper bits (transmitted with the stream) in addition to the transmitted base band. SBR allows a restoration (not reconstruction) of the upper frequency range without lots of bits. It is used by MP3Pro, aacPlus (later standardized as HE-AAC) and Digital Radio Mondiale.