Extensions and Improvements
Some extensions have been added to the first AAC standard (defined in MPEG-2 Part 7 in 1997):
- Perceptual Noise Substitution (PNS), added in MPEG-4 in 1999. It allows the coding of noise as pseudorandom data;
- Long Term Predictor (LTP), added in MPEG-4 in 1999. It is a forward predictor with lower computational complexity.
- Error Resilience (ER), added in MPEG-4 Audio version 2 in 2000, used for transport over error prone channels;
- AAC-LD (Low Delay), defined in 2000, used for real-time conversation applications;
- High Efficiency AAC (HE-AAC), a.k.a. aacPlus v1 or AAC+, the combination of SBR (Spectral Band Replication) and AAC LC; used for low bitrates; defined in 2003;
- HE-AAC v2, a.k.a. aacPlus v2 or eAAC+, the combination of Parametric Stereo (PS) and HE-AAC; used for even lower bitrates; defined in 2004 and 2006;
- MPEG-4 Scalable To Lossless (SLS), defined in 2006, can supplement an AAC stream to provide a lossless decoding option, such as in Fraunhofer IIS's "HD-AAC" product;
Read more about this topic: Advanced Audio Coding
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