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.
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[edit] 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.
[edit] Psychoacoustical codecs using SBR
SBR has been combined with AAC to create MPEG-4 High Efficiency AAC (HE AAC), with MP3 to create mp3PRO, and with MPEG-2 (Musicam).
It is used in broadcast systems like Digital Radio Mondiale, HD Radio, and XM Satellite Radio.
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