INTERNATIONAL ORGANISATION FOR STANDARDISATION
ORGANISATION INTERNATIONALE
DE NORMALISATION
ISO/IEC JTC1/SC29/WG11
CODING OF MOVING PICTURES AND
AUDIO
ISO/IEC JTC1/SC29/WG11
N7707
MPEG2005/
October 2005, Nice, FR
Source: Audio Subgroup
Status: Approved
Title: MPEG Technologies: SLS
Author: Ralf Geiger, Rongshan Yu
MPEG-4 SLS provides a scalable lossless extension of AAC. The lossless enhancement is done in a fine-grain scalable way, allowing for signal representations from the quality of the AAC core to numerically lossless, including near-lossless signal representation. It also works as a standalone lossless coder when the AAC core is not present.
MPEG-4 SLS is motivated by applications that require audio quality beyond perceptual transparency such as archive applications which require lossless compression, or studio operation which requires additional coding margin for post-processing or tandem coding. In addition, it is also motivated by applications that require bit-rate adaptation at transmission.
For lossless compression, the total bit-rate of the compressed bit-stream is
time-varying, and signal dependent. Typically, the average bit-rate is
about 768 kbps for stereo audio at 48
The MPEG-4 SLS codec fills this gap by extending the quality of the its core coder to lossless in a fine-grain scalable way, thus it provides both lossless and intermediate signal representation. This is illustrated in Figure 1.
Figure 1. Bit rate range covered by MPEG-4 SLS
The SLS algorithm is a scalable transform-based coder, providing a gradual refinement of the description of the transform coefficients, starting with perceptually weighted reconstruction levels provided by the AAC core bitstream, up to the resolution of the original input signal. In order to achieve lossless reconstruction, the SLS decoder uses an inverse Integer MDCT (IntMDCT) transform, and deterministic versions of several tools used for AAC-core bitstream decoding (see Figure.2).
Figure 2. SLS decoder block diagram
MPEG-4 SLS can also be used as a stand-alone lossless codec without AAC core. The resulting structure is illustrated in Figure 3. In this mode, the SLS decoder has a very simple structure, mainly comprising the entropy decoder and the inverse transform. The AAC encoder and decoder are not required for this non-core mode operation. (Figure. 3)
Figure 3.SLS non-core decoder block diagram
Target applications include archiving with lossless signal representation and high-quality transmission with constant bit rate and robustness to tandem coding.
The following application scenarios motivate some of the foreseen applications:
Audio archiving
With its lossless compression capability, MPEG-4 SLS nicely fits into any applications that need lossless audio quality. Typical applications include music archival for either studio or personal users, and high-end audio applications.
The broadcast chain
In a broadcast environment, MPEG-4 SLS could be used in all stages comprising archiving, contribution/distribution and emission. This is illustrated in Figure 4. For archiving the codec can operate in a lossless way, for contribution/distribution a constant, high bit rate (e.g. 512 kbps) can be used, and finally the AAC core can be used for emission.
In this broadcast chain, one main feature of the MPEG-4 SLS architecture can be used: In every stage where no post-processing but only a lower bit rate is required, the bit stream is just truncated, and no re-encoding is therefore required.
Figure 4. MPEG-4 SLS in the broadcast chain
High quality transmission at constant bit-rate
In situations where a constant rate transmission
is required, SLS can provide a high near-lossless to lossless audio quality
at e.g. 768 kbps stereo for 48
Thus is a unique feature of SLS. While pure lossless codecs can provide an average compression of 2:1 for certain test material, this compression cannot be guaranteed, and thus a transmission at a reduced constant bitrate is not possible.
Digital music distribution
In combination with AAC, MPEG-4 SLS can be flexibly fitted into the full life cycle of digital music distribution. This is illustrated in Figure 5. In this application scenario, the users can obtain SLS files either by download from a service provider, or by ripping from their own music CDs. They can then play the music at the desirable bitrate, e.g., possibly lossless quality at their living room, and low bitrate copies for their portable audio devices.
Figure 5. MPEG-4 SLS for digital music distribution
