SBR (Spectral Band Replication) is the bandwidth-extension tool that makes HE-AAC efficient at low bitrates. Rather than spend bits coding the full spectrum, the encoder codes only the lower band with AAC-LC and adds a small stream of guidance — envelopes and noise/tonal parameters — telling the decoder how to rebuild the high frequencies by transposing and shaping the low band it already has. Because high frequencies are perceptually forgiving, this reconstruction is convincing at a fraction of the data. SBR is also used in mp3PRO and other codecs. Its weakness is that aggressive settings can make highs sound smeared or 'watery', a hallmark of very low-bitrate AAC+.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
SBR saves a lot of bitrate by rebuilding the top octaves at the decoder, and it is also the source of the watery sound some low-bitrate streams have. This volume explains SBR in depth within the AAC family, with HE-AAC, parametric stereo and xHE-AAC, so you can decide where SBR helps and where AAC-LC is safer. Written by Nikolay Sapunov, CEO at Fora Soft.
Hearing watery highs in low-bitrate streams and suspecting SBR artifacts?
Send us sample streams and target bitrates and we will compare SBR profiles with AAC-LC and recommend settings per rendition. 250+ video products shipped since 2005.
