Spectral Band Replication
Spectral Band Replication or SBR is a lossy scheme developed by Coding Technologies (now part of Dolby) to efficiently reconstruct the high frequency data of an audio signal from the low frequency data. SBR is paired with a lossy audio codec (known as the core coder) at running half the target sample rate. Each audio frame is decoded like normal. The decoded samples are then pulled into the SBR decoder along with encoded SBR side information and the high frequency data is generated and recombined with the low frequency data. The full spectral data is then outputted at the target sample rate. Thanks to the standardization of of HE-AAC, technical details are available from ISO.