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SLS is MPEG-4's hybrid lossless audio codec. It uses a standard MPEG-4 GA core layer. Valid cores include AAC-LC, AAC Scalable (without LTP), ER AAC LC, ER AAC Scalable, and ER BSAC. It can also operate without a core layer. The core layer is decoded with a fixed integer approximation. Then one of more Lossless enhancement layers may be added. Lossless enhancement layers are entropy coded with either Bit-Plane Golomb Codes (BPGC) or Context Based Arithmetic Codes (CBAC) and Low Energy Mode Codes (LEMC). SLS can also oversample the core coder by up to a factor of 2 or 4.

The pre-standardized name for SLS was Advanced Audio Zip (AAZ).


The HD-AAC Profile is a limited subset of MPEG-4 SLS in an effort to gain commercial adoption. HD-AAC limits SLS to an AAC-LC core and BPGC with LEMC entropy coding.