MPEG-4 SLS (SLS) is the international standard for scalable audio coding. Besides its function as an extension of MPEG-4 Advanced Audio Coding (AAC) perceptual audio coder, SLS has a "non-core mode" that is able to offer full scalability.
OraStream uses SLS "non-core mode" in its adaptive audio streaming applications. As the perceptual audio coder is absent in SLS non-core mode, scalability is achieved through pure bit-plane coding. Since there is no perceptual information adopted, OraStream uses an enhanced bit-plane coding method to improve perceptual efficiency of scalable audio at a wide range of intermediate bitrates.
In the latter, the entire frequency spectrum is divided into three domains which comprise a Low Frequency (LF), a Middle Frequency (MF) and a High Frequency (HF) region. For each frame, the scanning priorities of these frequency regions are assigned according to the energy distribution of the signal. This scanning scheme is adaptive to the signal features. With more efficient bit-plane coding for fully scalable audio, perceptual quality of output audio at intermediate bitrates is significantly improved.
As would be noted, perceptual optimisation is achieved by assigning priorities of bit-planes in different frequency regions across the entire frequency spectrum. Importantly, the method does not remove any audio data nor reduce bit-depth, thus maintaining bit-perfect lossless integrity. Consequently, downloads are delivered in bit-for-bit lossless files.
When channel bandwidth is insufficient for full-lossless streaming, audio bit-planes in the three frequency domains are streamed in the priorities assigned (for perceptual audio optimisation). When channel bandwidth allows, audio bit-planes across the entire frequency spectrum are delivered bit-for-bit lossless at native resolution.