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Spectral band replication
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Spectral band replication (SBR) is a technology to enhance audio or speech codecs, especially at low bit rates and is based on harmonic redundancy in the frequency domain.
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 none or less tonal components.
The beauty of 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 to create MPEG-4 High Efficiency AAC, with MP3 to create mp3PRO, and with MPEG-1 Layer II (MP2).
It is used in broadcast systems like Digital Radio Mondiale (DRM+, not DRM), HD Radio, and XM Satellite Radio.
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 a mp3PRO is played back with an mp3 software incapable of utilizing the side information.
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