Audio Lossless Coding

MPEG-4 Audio Lossless Coding, also known as MPEG-4 ALS, is an extension to the MPEG-4 Part 3 audio standard to allow lossless audio compression. The extension was finalized in December 2005 and published as ISO/IEC 14496-3:2005/Amd 2:2006 in 2006.^[1] The latest description of MPEG-4 ALS was published as subpart 11 of the MPEG-4 Audio standard (ISO/IEC 14496-3:2009) (4th edition) in August 2009.^[2]

MPEG-4 ALS combines together a short-term predictor and a long term predictor. The short-term predictor is similar to FLAC in its operation - it is a quantized LPC predictor with a losslessly coded residual using Golomb Rice Coding or Block Gilbert Moore Coding (BGMC).^[3][4] The long term predictor is modeled by 5 long-term weighted residues, each with its own lag (delay). The lag can be hundreds of samples. This predictor improves the compression for sounds with rich harmonics (containing multiples of a single fundamental frequency, locked in phase) present in many musical instruments and human voice.

Features

• Support for PCM resolutions of up to 32-bit ^[2]
• Arbitrary sampling rates (tested up to 192 kHz, higher frequencies such as 384 kHz can be handled with the current specifications)^[5]
• Multi-channel / multi-track support (up to 65536 channels)
• Fast random access to any part of the encoded data.
• Tagging^{[citation needed]}
• Streaming^[5]
• Error correction mechanisms^[6]
• Optional storage in MP4 file format^[2]
• Can be multiplexed with video data using MP4 file format
• An MPEG-4 Audio profile "ALS Simple Profile".^[7]

Software support

As of 2006, there has not been wide acceptance of this format, possibly due to the lack of encoders and decoders available.^[6]

A reference implementation of MPEG-4 ALS encoder and decoder (mp4als - e.g. mp4alsRM22rev2) can be obtained at the MPEG-4 ALS homepage and it was also published as ISO/IEC 14496-5:2001/Amd 10:2007/Cor 3:2009.^[2]

There is a MPEG-4 ALS Decoder plugin for Winamp player.^[8]

On November 11, 2009, the FFmpeg open source project gained an MPEG-4 ALS decoder in its development version.^[9]

History

In July 2002, the Moving Picture Experts Group issued a call for proposals of lossless audio coding procedures to be sent in before December. Seven companies submitted their proposals which were examined taking into consideration compression efficiency, complexity and flexibility. By July 2003, Lossless Predictive Audio Compression (LPAC) was selected as the first draft for the future standard. The reference model was further developed under participation of the Technical University of Berlin (TUB), RealNetworks, and Nippon Telegraph and Telephone (NTT).

See also

• Lossless Predictive Audio Compression (predecessor)
• Lossless Transform Audio Compression (pre-predecessor)
• MPEG-4 SLS (MPEG-4 Scalable Lossless Coding)
• MPEG-4

References

External links

• MPEG-4 ALS homepage - Technical University Berlin
• MPEG-4 Audio Lossless Coding (ALS) - MultimediaWiki
• MPEG-4 ALS: Performance, Applications, and Related Standardization Activities - NTT (2007)
• Technical papers describing design of resdual coding and quantization algorithms in MPEG-4 ALS codec
• Announcement on ACN Newswire
• Technical specifications (ISO/IEC 14496-3:2005/Amd 2:2006) (not free)
• Official MPEG web site