SLS (Scalable to Lossless) is an audio compression technology which extends MPEG-4 AAC (Advanced Audio Coding) from perceptually transparent playback to lossless audio quality in a scalable fashion. With SLS, a single compressed audio file can be played on a standard AAC decoder without any loss of quality compared to a standard AAC file, or can be played through an SLS decoder with results that are identical to the original source material.

Also known as "HD-AAC," SLS is particularly suited for use in broadcast and music production environments where the lossless representation allows multiple contributions and editing stages to be managed without signal loss, and production results archived in a format that faithfully reproduces the original uncompressed source file(s). SLS is implemented as an enhancement layer to AAC where the upward scalability (towards full lossless) is achieved by dedicating varying amounts of data to the SLS enhancement. At full lossless playback the average compression ratio of SLS is approximately 2:1 which is comparable to pure lossless audio compression schemes that don't provide backwards compatibility with AAC.

Consumer applications for SLS include an ability to manage a music library that simultaneously supports lossless playback on high quality audio equipment in the home, as well as playback on portable devices for on-the-go listening. SLS also serves as insurance against obsolescence. In the event that the use of a future audio coding technology is desired, re-encoding of material stored in a lossless format will guarantee optimum results.

A second version of SLS is defined in the standard. Known as SLS Non-Core, this version of the technology does not require AAC as a core codec and is not compatible with AAC decoders for base layer playback.

A copy of the MPEG-4 Audio standard, which incorporates the SLS specification, can be obtained from the ISO online store (search for "14496-3").