Spectral band replication
It can be combined with any audio compression codec: the codec itself transmits the lower and midfrequencies of the spectrum, while SBR replicates higher frequency content by transposing up harmonics from the lower and midfrequencies at the decoder. Some guidance information for reconstruction of the high-frequency spectral envelope is transmitted as side information.
When needed, it also reconstructs or adaptively mixes in noise-like information in selected frequency bands in order to faithfully replicate signals that originally contained no or fewer tonal components.
The SBR idea is based on the principle that the psychoacoustic part of the human brain tends to analyse higher frequencies with less accuracy; thus harmonic phenomena associated with the spectral band replication process needs only be accurate in a perceptual sense and not technically or mathematically exact.
Psychoacoustical codecs using SBR 
SBR has been combined with AAC LC to create MPEG-4 High Efficiency AAC (HE-AAC), with WMA 10 Professional to create WMA 10 Pro LBR, with MP3 to create mp3PRO, and with MPEG-1 Layer II (MP2). It was published for MPEG-4 Audio (MPEG-4 Part 3) in ISO/IEC 14496-3:2001/Amd 1:2003 and defined as one of the MPEG-4 Audio Object Types.
If the player is not capable of using the side information that has been transmitted alongside the "normal" compressed audio data, it may still be able to play the "baseband" data as usual, resulting in a dull (since the high frequencies are missing), but otherwise mostly acceptable sound. This is for example the case if an mp3PRO file is played back with mp3 software incapable of utilizing the side information.
See also 
- Novak, Clark. "Spectral Band Replication and aacPlus Coding - An Overview". Retrieved February 8, 2010.
- ISO (2003). "Bandwidth extension, ISO/IEC 14496-3:2001/Amd 1:2003". ISO. Retrieved 2009-10-13.
- "XM Radio - Fast Facts". Retrieved February 8, 2010.
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