MPEG-4 SLS
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MPEG-4 SLS, also known as as MPEG-4 Scalable to Lossless as per ISO/IEC 14496-3:2005/Amd 3:2006 (Scalable Lossless Coding), is an extension to the MPEG-4 audio standard to allow lossless audio compression scalable to lossy AAC. It was developed jointly by Infocomm Research and Fraunhofer, which commercializes its implementation of the standard under the name of HD-AAC.
MPEG-4 SLS allows for having both a lossy layer and a lossless correction layer similar to Wavpack Hybrid or OptimFROG DualStream, allowing for backwards compatibility to MPEG AAC compliant bit streams. It can also work without AAC layer, in which case it will not be backwards compatible.
Using the AAC layer should allow for fast and easy ripping of lossless MPEG-4 SLS files to lower-quality AAC files which can be streamed to the Internet or played in devices with limited storage.
With DRM, ripping of the lossless data or playback on non DRM-enabled devices could be disabled.
MPEG-4 SLS is not related in any way to MPEG-4 ALS.
[edit] Background
The market demand for highly compressed, lossy audio compression technologies is slowly diminishing, as network and storage technologies continually improve their capabilities in delivering high sampling-rate and resolution audio.
New lossless audio coding technologies that need higher bandwidth and larger storage capacities, may now be appropriate for many applications and have been gaining attention in recent years. In addressing this need, MPEG issued a Call for Proposal (CfP) in October 2002 to solicit a technology that is able to address all these needs.
The CfP requested proposals for a lossless and scalable technology that was backward compatible with the existing MPEG AAC codec, and could operate efficiently at several different sampling rates and word length combinations. Institute for Infocomm Research (I2R) technologies were adopted for the scalable to lossless coding (14496-3/AMD5) architecture Reference Model 0.
I2R offers a fully scalable to lossless audio coding solution. On top of the core MPEG AAC codec, a scalable extension layer increases the signal-to-noise-ratio, reaching lossless quality at data rates comparable to that of current pure lossless audio codecs. That is, at average compression ratios of about 58%.
The scalability of the extension makes this combination a coding solution for production environments, where the result is to be transmitted to several recipients through channels of differing bandwidth. Furthermore, for private and professional use in music archives, this enables storing the original music data and transferring compressed copies (eg. to portable devices) without transcoding.
