Spectral band replication
Spectral band replication (SBR) is a technology to enhance audio or speech codecs, especially at low bit rates.
How it works
It can be combined with any audio compression codec: the codec itself transmits the lower frequencies of the spectrum, while SBR synthesizes associated higher frequency content based on the lower frequencies and transmitted side information.
When applicable, it involves reconstruction of a noise-like frequency spectrum by employing a noise generator with some statistical information (level, distribution, ranges), so the decoding result is not deterministic among multiple decoding processes of the same encoded data.
Both ideas are based on the principle that the human brain tends to consider high frequencies to be either harmonic phenomena associated with lower frequencies or noise, and is thus less sensitive to the exact content of high frequencies in audio signals.
Psychoacoustical codecs using SBR
SBR has been combined with AAC to create MPEG-4
High Efficiency AAC (
It is used in broadcast systems like Digital Radio Mondiale (DRM+, not DRM), HD Radio, and XM Satellite Radio. [citation needed]
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 this "normal" 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 a software uncapable of utilizing the side information.
See also
External links
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