MPEG-4 ALS

MPEG-4 Audio Lossless Coding (MPEG-4 ALS) is a lossless audio codec, developed to extend the MPEG-4 Part 3 audio standard to allow lossless audio compression. First released in 2006, it is standardized as ISO/IEC 14496-3. A reference encoder is available as an executable for Windows, with the most significant part of the codec also being released as source code compilable for Windows/Linux/Mac. Its primary developer was Tilman Liebchen, then at the Technische Universität Berlin, and it was based on his earlier LPAC codec.

The codec has not gained significant popularity and the reference encoder appears to be unmaintained since 2009. The current development team has however published several tests in (mainly) IEEE journals.

Features and performance

The format supports both integer and 32-bit floating-point PCM, arbitrary sample rates and multi-channel (up to 65536, higher than any widespread lossless codec). The reference encoder can enforce an MPEG-4 audio profile, but also provides for several parameters and two encoding modes. It can output the encoded stream in an MP4 container, or as .als files. By default it compresses files for full restore (not only lossless audio, but also recreating the original file bit by bit with the same metadata and structure) of WAVE and big-endian AIFF files.

In Martijn van Beurden's lossless performance test, the reference encoder (at default setting) did perform like other popular codecs (comparable to WavPack's -hx setting at CDDA). The higher compression settings could encode very slowly, but can make for certain size gains on non-CDDA sources. According to the README file, the "ALS reference software is not optimized, particularly not in terms of encoder speed."

Software support

The Windows executable can be integrated in e.g. Exact Audio Copy (instructions on the codec's website) or foobar2000. When used in an MP4 container, tagging is possible.

ffmpeg can decode MPEG-4 ALS files, fixed-point only, and only in the default encoding mode, which is the one documented with source. Unsupported are floating-point files, and the -z mode which was not published as source in the reference encoder/decoder.

There are currently no known hardware players supporting it.

Predecessors: LPAC and LTAC

MPEG-4 ALS was based on Liebchen's earlier Lossless Predictive Audio Compression (LPAC) codec, which in turn succeeded his LTAC codec. Thus, LPAC development was frozen in 2003 in favour of MPEG-4 ALS development.

LPAC was at some point relatively popular among lossless formats, as an encoder/decoder was available free of charge (for noncommercial purposes, and closed-source) for Windows, Linux and Solaris, along with a library that made integration of LPAC encoding and decoding into other applications relatively easy. Also for LPAC, the website offered instructions for Exact Audio Copy integration. Furthermore, LPAC was also much more efficient and featureful than the major lossless audio format of the 1990s, Shorten.

Links

• Wikipedia entry
• MPEG-4 ALS website with source/executable download
• LPAC website
• Further description of the LPAC algorithm
• LTAC/LPAC at ReallyRareWares
• Martijn van Beurden's lossless performance tests (revision 4 for comparison to WavPack -hx, revisions 5 and 6 for non-CDDA)
• HA Wiki's Lossless Codec Comparison originally by Rjamorim