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MPEG-4 Audio Lossless Coding (ALS)

The MPEG-4 Audio Lossless Coding (ALS) standard belongs to the family MPEG-4 audio coding standards pubished by ISO (www.iso.org). In contrast to lossy codecs such as MP3 and AAC, which merely strive to preserve the subjective audio quality, lossless coding preserves every single bit of the original audio data. The basic technology for MPEG-4 ALS was developed by the NUe Group (Fachgebiet Nachrichtenübertragung) at Technical University of Berlin. The first version of the MPEG-4 ALS standard was published in 2006, and the latest description is now available as part of the 4th (2009) edition of the comprehensive MPEG-4 audio standard (ISO/IEC 14496-3:2009).

A new version (RM23) of the MPEG-4 ALS Reference Software and codec is now available. More...

MPEG-4 ALS is supported by FFmpeg, MPlayer, VLC media player, and other applications. More...

Features of MPEG-4 ALS

MPEG-4 ALS defines efficient and fast lossless audio compression techniques for both professional and consumer applications. It offers many features not included in other lossless compression schemes.

  • General support for virtually any uncompressed digital audio format (including wav, aiff, au, bwf, raw).
  • Support for PCM resolutions of up to 32-bit at arbitrary sampling rate (including 16/44.1, 16/48, 24/48, 24/96, 24/192).
  • Multi-channel / multi-track support for up to 65536 channels (including 5.1 surround).
  • Support for 32-bit IEEE floating point audio data.
  • Fast random access to any part of the encoded data.
  • Optional storage in MP4 file format (allows multiplex with video).
  • High flexibility of codec parameters for various applications.

Besides these outstanding features, a global MPEG standard for lossless audio coding will facilitate interoperability between different hardware and software platforms, and will thus promote long-lasting multivendor support.

What is 'lossless' coding?

Picture
MPEG-4 ALS Encoder
Lupe
Picture
MPEG-4 ALS Decoder
Lupe

Lossless audio coding enables the compression of digital audio data without any loss in quality due to a perfect reconstruction of the original signal. It is a topic of high interest for both professional and customer applications. While modern lossy coding standards such as MP3 or AAC can achieve high compression ratios with transparent subjective quality, they do not preserve every single bit of the original audio data. Thus, lossy coding methods are not suited for editing or archiving applications, since multiple coding or post-processing can reveal originally masked distortions. Applying lossless entropy coding methods such as Lempel-Ziv, Huffman or arithmetic coding directly to the audio signal is not very efficient due to the long-time correlations and the high range of values. Therefore, conventional data compression tools such as Winzip or gzip fail in the case of digital audio data.

A decorrelation stage, which eliminates the statistical dependencies within the signal, leads to an almost uncorrelated source which is easier to code. A common method to achieve such decorrelation is linear prediction, where each sample of the original signal is predicted from previous samples. The difference between original and predicted version is called the residual. If prediction works well, the residual is a decorrelated signal and has smaller values than the original. The residual is usually coded using simple entropy coding methods such as Rice codes, which are a special case of Huffman codes.

The figures above show a simplified block diagram of a lossless encoder using linear predictive coding (LPC) followed by entropy coding, and the corresponding decoder. The LPC synthesis filter basically adds the dependencies previously removed by the analysis filter, i.e. it correlates the signal again, leading to a lossless reconstruction of the original. Although the combination of analysis and synthesis filter is not lossless in general, lossless processing can be achieved if some basic conditions are observed.

Applications

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MPEG-4 ALS Download
Lupe

Examples for the use of lossless audio coding in general and MPEG-4 ALS in particular include both professional and consumer applications:

  • Archival systems (broadcasting, studios, record labels, libraries)
  • Studio operations (storage, collaborative working, digital transfer)
  • High-resolution disc formats
  • Internet distribution of audio files
  • Online music stores (download)
  • Portable music players

In the case online music stores, downloads of the latest CD releases will no longer be restricted to lossy compressed formats such as MP3, AAC, or WMA. Instead, the consumer can purchase all tracks in full quality of the original CD, but still receive the corresponding files at reduced data rates (see picture below). Furthermore, MPEG-4 ALS is not restricted to audio signals, since it can also be used to compress many other types of signals, such as medical (ECG, EEG) or seismic data.

Reference Software

The latest official version of the MPEG-4 ALS reference software (RM23) is part of document N11858 (see WG11 and MPEG Documents). The most prominent changes compared to the previous version (RM22rev2) are:

  • Decoder indicates MP4 profiles and levels
  • Encoder can force compliance with ALS Simple Profile Level 1 ("-sp1" switch)
  • Support for sample rates of 288 kHz and above

For convenience, the latest ALS reference software archive is also provided in the following (please refer to the included readme.txt for a list of changes):

The archive contains everything to build the ALS reference codec, including source code, libraries, project files for Windows (MSVC/Visual Studio), and make files (GCC) for Linux/Mac.

The reference software archive does not contain any binary executables. A command-line codec for Windows (compiled with Visual Studio 2008) is available here:

Please type "mp4alsRM23 -h" for a comprehensive list of command-line parameters.

Conformance Bitstreams

MPEG-4 ALS conformance procedures and bitstreams were originally defined in ISO/IEC 14496-4:2004/Amd 19:2007, but all audio related conformance was recently moved from ISO/IEC 14496-4 to the new part ISO/IEC 14496-26 (Audio Conformance, document N10397, see WG11 and MPEG Documents).

All MPEG-4 Audio Conformance bitstreams are publicly available from ISO here. The ALS conformance bitstreams are in subdirectory "DVD1/mpeg4audio-conformance/compressedMp4/", while the correponding reference wave files are in "DVD2/mpeg4audio-conformance/referencesWav/". In both cases, the ALS related files begin with "als_".

MPEG members should make sure to download the latest ALS conformance bitstreams from ftp://sc29wg11@ftlabsrv.nue.tu-berlin.de/mp4lossless/conf/ (password of 78th meeting).

Software that supports MPEG-4 ALS

Download Test Material

In order to test ALS playback with one of the applications listed below, you can download a test file (MP4, 13,9 MB) with H.264 video (640 x 360 pixels, 1.5 Mbit/s) and ALS audio (16-bit stereo, 600 kbit/s). The file comprises the first 55 seconds of the animated movie "Big Buck Bunny" by the Peach open movie project. You can get the whole movie (though not with ALS audio) and additional information at www.bigbuckbunny.org.

FFmpeg

FFmpeg is a cross-platform, open-source audio/video solution and codec library. FFmpeg is used by many applications, especially in the open source world.

Since November 2009, FFmpeg supports decoding of MPEG-4 ALS. The decoder was added in revision 20517. The latest revision of the FFmpeg source code can be obtained from the FFmpeg download page. Within the project tree, the ALS decoder can be found in /libavcodec/alsdec.c.

Binary versions of the FFmpeg command line tool for Windows can be found here (ffmpeg.exe) and here (ffmpeg.exe and ffplay.exe).

Consequently, ALS playback, decoding and transcoding (e.g. into MP3) is also supported by several applications that use FFmpeg, some of which are described further below.

An MPEG-4 ALS encoder for FFmpeg is already under development and will be available soon.

MPlayer

MPlayer is an open-source media player which runs on all major operating systems and supports most media formats. Since revision 30687, MPlayer also supports MPEG-4 ALS (file extension must be .mp4). Binary versions of MPlayer can be found here (Windows) or here (MacOSX).

While MPlayer can be used as a command-line tool, there are several nice GUI frontends such as SMPlayer. However, these frontends sometimes come with an old copy of MPlayer that does not yet support MPEG-4 ALS. In that case, you can simply substitute the old MPlayer executable (i.e. mplayer.exe under Windows) in the frontend's installation directory by a recent one (r30867 or higher), or provide the path to an external MPlayer binary that should be used instead (e.g. under "Preferences" in SMPlayer).

VLC

VLC media player is a highly portable multimedia player and multimedia framework capable of reading most audio and video formats as well as DVDs, Audio CDs, VCDs, and various streaming protocols.

VLC officially supports MPEG-4 ALS playback (MP4 file format) since version 1.1.0, including display of embedded metadata such as title, artist and cover art. Self-installing VLC binaries are available for Windows and MacOSX. For Linux systems, please follow the descriptions on the VLC page. The latest developer versions for various operating systems can be obtained from the VLC "nightly builds" page.

XMedia Recode

XMedia Recode is a great free media conversion program that comes with lots of predefined profiles for mobile devices (including iPhone/iPod). It can be used (among many other things) to convert MPEG-4 ALS files into other formats (e.g. MP3, AAC, WMA).

Note: The website is in German only, but the program supports several languages, including English.

Winamp Plugin

The ALS plugin enables playback of ALS files (file extension .als) with the popular media player Winamp. In order to allow fast seek operations, files should be encoded using random access points (encoder option -r#, e.g. -r5 for 500ms, preferably together with -u1).

Simply unzip the archive and copy the file "in_mp4als.dll" into the Winamp Plugins directory (typically C:\Program Files\Winamp\Plugins, requires admin rights in Windows Vista).

Note: Version 1.1 only plays plain ALS encoded files (.als). A version that also supports the MP4 file format (.mp4) is under development.

SDK for Developers

NTT Japan offers an MPEG-4 ALS software development kit (SDK) that supports multiple software and hardware enviroments. Please check this developer overview for details and contact information.

Professional Archival Application Format

The MPEG-A Professional Archival Application Format (PA-AF) is defined in ISO/IEC 23000-6 (MPEG-A Part 6). It specifies an interoperable multimedia content archival format for the preservation of multimedia contents.

The PA-AF combines already tested and verified tools from the MPEG body of standards. It integrates MPEG-4 ALS as the default tool for lossless compression of audio content.

More information on the PA-AF, including reference software and conformance bitstreams, can be found here.

Using MPEG-4 ALS with other Software

The MPEG-4 ALS Reference Software can be used in conjunction with other software in order to allow e.g. batch encoding, CD ripping, or metadata editing.

A description of selected applications and corresponding explantions can be found here.

History

In July 2002, MPEG issued a call for proposals to initiate the submission of technology for lossless audio coding. This call basically supported two different approaches, either a hierarchical system consisting of a lossy core codec (e.g. MPEG-AAC) and a lossless enhancement layer, or a lossless-only codec.

By December 2002, seven companies submitted one or more codecs which met the basic requirements. In the following, those submissions were evaluated in terms of compression efficiency, complexity and flexibility. In March 2003, the audio subgroup decided to proceed at first with the standardization of a lossless-only codec, while further investigating hierarchical methods as well. The lossless-only codec of the Technical University of Berlin, which offered the highest compression among all submissions, was chosen as reference model. In the course of standardization, several improvements and extensions of the initial submission were added. The final specification of MPEG-4 ALS was issued in July 2005 and published as a standard (ISO/IEC 14496-3:2005/Amd 2:2006) in 2006. In the following, some corrigenda containing minor changes and clarifications were issued.

In August 2009, the final version of the 4th edition of the comprehensive MPEG-4 Audio standard (ISO/IEC 14496-3:2009) was issued, which now includes the latest description of MPEG-4 ALS as subpart 11.

Publications

Publications

2009

2006

2005

2004

2003

WG11 and MPEG Documents

Documents related to MPEG-4 ALS are available here
(Most documents are for MPEG members only - password required)

Contact

For questions or more information on MPEG-4 ALS please contact .

For more information on the NUe Group (Fachgebiet Nachrichtenübertragung) please contact Prof. Dr.-Ing. Thomas Sikora (Head of Group).

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