BSAC - Bit Slice Arithmetic Coding

By: Christine Martz

Meaning of BSAC – “Bit Slice Arithmetic Coding”, is used in MPEG-4 Advanced Audio Coding (AAC) for enabling a scalable bitrate during the encoding or decoding process.

The BSAC tool is used in combination with the AAC coding tools. BSAC provides scalability in very small steps. 1 kbps for each audio channel, for example, 1 kbps for mono or 2 kbps for stereo. A base layer bitstream and many small enhancement layer bitstreams are used. The base layer contains the general side information, specific side information for the first layer and the audio data of the first layer. The enhancement streams contain only the specific side information and audio data for the corresponding layer. The benefits of such encoding is that different bitrates could be offered by content providers or online music stores and stored as only one file per song. A user could download files that are suited to the type of connection quality they have.

Such fine step scalability is obtained by using a bit-slicing scheme where quantized spectral values are grouped into frequency bands and then the bits of a group are processed according to their significance, in slices. Then the bit-slices are encoded using an arithmetic coding scheme so that entropy coding with minimal redundancy may be obtained.

Other Related Definitions:

“…Bit Slice Arithmetic Coding (BSAC) provides a very fine step bitrate scalability. At the top of the scalability range it has no penalty relative to single-rate AAC, however at the bottom of the scale it has a slight penalty relative to single-rate AAC.” [ISO/IEC Joint Committee]

“…In order to make the bitstream scalable, BSAC uses an alternative to AAC noiseless coding module, although the other coding modules are identical to AAC. A bitstream encoded by AAC can be transcoded to an BSAC bitstream noiselessly. BSAC is capable of generating a bitstream with a precise bit rate control in the range of 16kbps to 64kbps per channel. This bit rate enables the decoder to stop anywhere between 16kbps and the encoded bit rate with a 1kbps step size. Through use of this scalablity, the user can experience nearly transparent sound quality at 64kbps and graceful degradation at lower bit rates. BSAC is best performed in the range of 40kbps to 64kbps, though its operating range is 16kbps and 64kbps.” [ISO]

“…The concept of bitsliced arithmetic coding (BSAC) was introduced for audio coding in and is standardized as a part of MPEG-4. In this context, BSAC plays the role of an alternative lossless coding kernel for MPEG-4 AAC, utilizing the MDCT and applying a perceptually controlled bandwise quantization to the spectral values. The main difference between BSAC and the standard AAC lossless coding kernel is that the quantized values are not Huffman coded, but arithmetically coded in bitslices.” [Fraunhofer]

“…To support fine-grained scalability, MPEG4 v2 defined the bit-slice arithmetic coding (BSAC) profile. The AAC-BSAC codec is used in digital media broadcast (DMB) applications in Korea.” [Embedded.com]

Related Links:

Bit-Sliced Arithmetic Coding - A BSCA overview.
Audio compression gets better and more complex - An article describing the basics of audio coding in MPEG files.

Technical Resources:

MPEG-4 Audio Version 2
MPEG Audio FAQ MPEG-4
Research in Scalable Audio Coding

Products and Solutions:

Fraunhofer Audio & Multimedia Download

Blogs, News, Feeds, Discussion Lists:

MPEG Industry Forum News
Mp4-tech mailing list

Books:

The MPEG-4 Book - by Touradj Ebrahimi, Fernando Pereira
H.264 and MPEG-4 Video Compression: Video Coding for Next Generation Multimedia - by Iain E. G. Richardson
Digital Video Compression - by Peter Symes

See Also:

BSAC Resources

 

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