| Advanced Audio Coding | |
|---|---|
 | |
| Filename extension | .adif, .m4b, .mp4 |
| Internet media type |
audio/aac audio/aacp audio/3gpp audio/3gpp2 audio/mp4 audio/mp4a-latm audio/mpeg4-generic |
| Developed by | AT&T Labs, Dolby Laboratories, Fraunhofer Society, Sony |
| Initial release | December 1997;28 years ago (1997-12) |
| Latest release | ISO/IEC 14496-3:2019 December 2019;6 years ago (2019-12) |
| Type of format | Lossy audio |
| Contained by | MPEG-4 Part 14, 3GP and 3G2, ISO base media file format and Audio Data Interchange Format (ADIF) |
| Standard | ISO/IEC 13818-7, ISO/IEC 14496-3 |
| Open format? | No |
| Free format? | No |
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. It was developed by Dolby, AT&T, Fraunhofer and Sony, originally as part of the MPEG-2 specification but later improved under MPEG-4. AAC was designed to be the successor of the MP3 format (MPEG-2 Audio Layer III) and generally achieves higher sound quality than MP3 at the same bit rate. AAC encoded audio files are typically packaged in an MP4 container most commonly using the filename extension .m4a.
The basic profile of AAC (both MPEG-4 and MPEG-2) is called AAC-LC (Low Complexity). It is widely supported in the industry and has been adopted as the default or standard audio format on products including Apple's iTunes Store, Nintendo's Wii, DSi and 3DS and Sony's PlayStation 3. It is also further supported on various other devices and software such as iPhone, iPod, PlayStation Portable and Vita, PlayStation 5, Android and older cell phones, digital audio players like Sony Walkman and SanDisk Clip, media players such as VLC, Winamp and Windows Media Player, various in-dash car audio systems, and streaming services such as Spotify, Apple Music, YouTube and YouTube Music. AAC has been further extended into HE-AAC (High Efficiency, or AAC+), which improves efficiency over AAC-LC. Another variant is AAC-LD (Low Delay).
AAC supports inclusion of 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 16 low frequency effects (LFE, limited to 120 Hz) channels, up to 16 "coupling" or dialog channels, and up to 16 data streams. The quality for stereo is satisfactory to modest requirements at 96 kbit/s in joint stereo mode; however, hi-fi transparency demands data rates of at least 128 kbit/s (VBR). TestsAAC's improvements over MP3edit
Advanced Audio Coding is designed to be the successor of the MPEG-1 Audio Layer 3, known as MP3 format, which was specified by ISO/IEC in 11172-3 (MPEG-1 Audio) and 13818-3 (MPEG-2 Audio).
Improvements include:
- more sample rates (from 8 to 96 kHz) than MP3 (16 to 48 kHz);
- up to 48 channels (MP3 supports up to two channels in MPEG-1 mode and up to 5.1 channels in MPEG-2 mode);
- up to 24 audio bit depth;
- arbitrary bit rates and variable frame length. Standardized constant bit rate with bit reservoir;
- higher bit rate (up to 512 Kbps in two channels);
- nearly-lossless in 384 Kbps @ stereo, or higher bitrate;
- higher efficiency and simpler filter bank. AAC uses a pure MDCT (modified discrete cosine transform), rather than MP3's hybrid coding (which was part MDCT and part FFT);
- higher coding efficiency for stationary signals (AAC uses a blocksize of 1024 or 960 samples, allowing more efficient coding than MP3's 576 sample blocks);
- higher coding accuracy for transient signals (AAC uses a blocksize of 128 or 120 samples, allowing more accurate coding than MP3's 192 sample blocks);
- possibility to use Kaiser-Bessel derived window function to eliminate spectral leakage at the expense of widening the main lobe;
- much better handling of audio frequencies above 16 kHz;
- more flexible joint stereo (different methods can be used in different frequency ranges);
- additional modules (tools) added to increase compression efficiency: TNS, backwards prediction, perceptual noise substitution (PNS), etc. These modules can be combined to constitute different encoding profiles.
Overall, the AAC format allows developers more flexibility to design codecs than MP3 does, and corrects many of the design choices made in the original MPEG-1 audio specification. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression. This is especially true at very low bit rates where the superior stereo coding, pure MDCT, and better transform window sizes leave MP3 unable to compete.
Adoption
editWhile the MP3 format has near-universal hardware and software support, primarily because MP3 was the format of choice during the crucial first few years of widespread music file-sharing/distribution over the internet, AAC remained a strong contender due to some unwavering industry support. Due to MP3's dominance, adoption of AAC was initially slow. The first commercialization was in 1997 when AT&T Labs (a co-owner of AAC patents) launched a digital music store with songs encoded in MPEG-2 AAC. HomeBoy for Windows was one of the earliest available AAC encoders and decoders.
Dolby Laboratories came in charge of AAC licensing in 2000. A new licensing model was launched by Dolby in 2002, while Nokia became a fifth co-licenser of the format. Dolby itself also marketed its own coding format, Dolby AC-3.
Nokia started supporting AAC playback on devices as early as 2001, but it was the exclusive use of AAC by Apple Computer for their iTunes Store which accelerated attention to AAC. Soon the format was also supported by Sony for their PlayStation Portable (albeit Sony continued promoting its proprietary ATRAC), and music-oriented cell phones from Sony Ericsson, beginning with the Sony Ericsson W800. The Windows Media Audio (WMA) format, from Microsoft, was considered to be AAC's main competitor.
By 2017, AAC was considered to have become a de facto industry standard for lossy audio.
Functionality
editThe audio coding standards MPEG-4 Low Delay (AAC-LD), Enhanced Low Delay (AAC-ELD), and Enhanced Low Delay v2 (AAC-ELDv2) as defined in ISO/IEC 14496-3:2009 and ISO/IEC 14496-3:2009/Amd 3 are designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. They are closely derived from the MPEG-2 Advanced Audio Coding (AAC) format. AAC-ELD is recommended by GSMA as super-wideband voice codec in the IMS Profile for High Definition Video Conference (HDVC) Service.
Licensing and patents
editNo licenses or payments are required for a user to stream or distribute audio in AAC format. This reason alone might have made AAC a more attractive format to distribute audio than its predecessor MP3, particularly for streaming audio (such as Internet radio) depending on the use case.
However, a patent license is required for all manufacturers or developers of AAC "end-user" codecs. The terms (as disclosed to SEC) use per-unit pricing. In the case of software, each computer running the software is to be considered a separate "unit".
It used to be common for free and open source software implementations such as FFmpeg and FAAC to only be distributed in source code form so as to not "otherwise supply" an AAC codec. However, FFmpeg has since become more lenient on patent matters: the "gyan.dev" builds recommended by the official site now contain its AAC codec, with the FFmpeg legal page stating that patent law conformance is the user's responsibility. (See below under Products that support AAC, Software.) The Fedora Project, a community backed by Red Hat, imported the "Third-Party Modified Version of the Fraunhofer FDK AAC Codec Library for Android" to its repositories on September 25, 2018, and has enabled FFmpeg's native AAC encoder and decoder for its ffmpeg-free package on January 31, 2023.
The AAC patent holders include Bell Labs, Dolby, ETRI, Fraunhofer, JVC Kenwood, LG Electronics, Microsoft, NEC, NTT (and its subsidiary NTT Docomo), Panasonic, Philips and Sony. Based on the list of patents from the SEC terms, the last baseline AAC patent expires in 2028, and the last patent for all AAC extensions mentioned expires in 2031.
Extensions and improvements
editSome extensions have been added to the first AAC standard (defined in MPEG-2 Part 7 in 1997):
- Perceptual Noise Substitution (PNS), added in MPEG-4 in 1999. It allows the coding of noise as pseudorandom data.
- Long Term Predictor (LTP), added in MPEG-4 in 1999. It is a forward predictor with lower computational complexity.
- Error Resilience (ER), added in MPEG-4 Audio version 2 in 2000, used for transport over error prone channels
- AAC-LD (Low Delay), defined in 2000, used for real-time conversation applications
- High Efficiency AAC (HE-AAC), a.k.a. aacPlus v1 or AAC+, the combination of SBR (Spectral Band Replication) and AAC LC. Used for low bitrates. Defined in 2003.
- HE-AAC v2, a.k.a. aacPlus v2, eAAC+ or Enhanced aacPlus, the combination of Parametric Stereo (PS) and HE-AAC; used for even lower bitrates. Defined in 2004 and 2006.
- xHE-AAC, extends the operating range of the codec from 12 to 300 kbit/s.
- MPEG-4 Scalable to Lossless (SLS), Not yet published, can supplement an AAC stream to provide a lossless decoding option, such as in Fraunhofer IIS's "HD-AAC" product
- MPEG-4 Audio Lossless Coding (ALC)
Container formats
editFAAC and FAAD2 stand for Freeware Advanced Audio Coder and Decoder 2 respectively. FAAC supports audio object types LC, Main and LTP. FAAD2 supports audio object types LC, Main, LTP, SBR and PS. Although FAAD2 is free software, FAAC is not free software.
Fraunhofer FDK AAC
editAlmost all current computer media players include built-in decoders for AAC, or can utilize a library to decode it. On Microsoft Windows, DirectShow can be used this way with the corresponding filters to enable AAC playback in any DirectShow based player. Mac OS X supports AAC via the QuickTime libraries. Adobe Flash Player, since version 9 update 3, can also play back AAC streams. Since Flash Player is also a browser plugin, it can play AAC files through a browser as well.
The Rockbox open source firmware (available for multiple portable players) also offers support for AAC to varying degrees, depending on the model of player and the AAC profile.
Optional iPod support (playback of unprotected AAC files) for the Xbox 360 is available as a free download from Xbox Live.
The following is a non-comprehensive list of other software player applications:
- CorePlayer: also supports LC and HE AAC
- ffdshow: a free open source DirectShow filter for Microsoft Windows that uses FAAD2 to support AAC decoding
- foobar2000: a freeware audio player for Windows that supports LC and HE AAC
- KMPlayer
- MediaMonkey
- AIMP
- Media Player Classic Home Cinema
- mp3tag
- MPlayer or xine: often used as AAC decoders on Linux or Macintosh
- MusicBee: an advanced music manager and player that also supports encoding and ripping through a plugin
- RealPlayer: includes RealNetworks' RealAudio 10 AAC encoder
- Songbird: supports AAC on Windows, Linux and Mac OS X, including the DRM rights management encoding used for purchased music from the iTunes Store, with a plug-in
- Sony SonicStage
- VLC media player: supports playback and encoding of MP4 and raw AAC files
- Winamp for Windows: includes an AAC encoder that supports LC and HE AAC
- Windows Media Player 12: released with Windows 7, supports playback of AAC files natively
- Another Real: Rhapsody supports the RealAudio AAC codec, in addition to offering subscription tracks encoded with AAC
- XBMC: supports AAC (both LC and HE).
- XMMS: supports MP4 playback using a plugin provided by the faad2 library
Some of these players (e.g., foobar2000, Winamp, and VLC) also support the decoding of ADTS (Audio Data Transport Stream) using the SHOUTcast protocol. Plug-ins for Winamp and foobar2000 enable the creation of such streams.
Use in HDTV broadcasting
editJapanese ISDB-T
editIn December 2003, Japan started broadcasting terrestrial DTV ISDB-T standard that implements MPEG-2 video and MPEG-2 AAC audio. In April 2006 Japan started broadcasting the ISDB-T mobile sub-program, called 1seg, that was the first implementation of video H.264/AVC with audio HE-AAC in Terrestrial HDTV broadcasting service on the planet.
International ISDB-Tb
editIn December 2007, Brazil started broadcasting terrestrial DTV standard called International ISDB-Tb that implements video coding H.264/AVC with audio AAC-LC on main program (single or multi) and video H.264/AVC with audio HE-AACv2 in the 1seg mobile sub-program.
DVB
editThe ETSI, the standards governing body for the DVB suite, supports AAC, HE-AAC and HE-AAC v2 audio coding in DVB applications since at least 2004. DVB broadcasts which use the H.264 compression for video normally use HE-AAC for audio.
iTunes and iPod
editIn April 2003, Apple brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in MPEG-4 AAC format (via a firmware update for older iPods). Customers could download music in a closed-source digital rights management (DRM)-restricted form of 128 kbit/s AAC (see FairPlay) via the iTunes Store or create files without DRM from their own CDs using iTunes. In later years, Apple began offering music videos and movies, which also use AAC for audio encoding.
On May 29, 2007, Apple began selling songs and music videos from participating record labels at higher bitrate (256 kbit/s cVBR) and free of DRM, a format dubbed "iTunes Plus" . These files mostly adhere to the AAC standard and are playable on many non-Apple products but they do include custom iTunes information such as album artwork and a purchase receipt, so as to identify the customer in case the file is leaked out onto peer-to-peer networks. It is possible, however, to remove these custom tags to restore interoperability with players that conform strictly to the AAC specification. As of January 6, 2009, nearly all music on the USA regioned iTunes Store became DRM-free, with the remainder becoming DRM-free by the end of March 2009.
iTunes offers a "Variable Bit Rate" encoding option which encodes AAC tracks in the Constrained Variable Bitrate scheme (a less strict variant of ABR encoding); the underlying QuickTime API does offer a true VBR encoding profile however.
As of September 2009, Apple has added support for HE-AAC (which is fully part of the MP4 standard) only for radio streams, not file playback, and iTunes still lacks support for true VBR encoding.
Other portable players
edit- Archos
- Cowon (unofficially supported on some models)
- Creative Zen Portable
- Fiio (all current models)
- Nintendo 3DS
- Nintendo DSi
- Philips GoGear Muse
- PlayStation Portable (PSP) with firmware 2.0 or greater
- Samsung YEPP
- SanDisk Sansa (some models)
- Walkman
- Zune
- Any portable player that fully supports the Rockbox third party firmware
Mobile phones
editFor a number of years, many mobile phones from manufacturers such as Nokia, Motorola, Samsung, Sony Ericsson, BenQ-Siemens and Philips have supported AAC playback. The first such phone was the Nokia 5510 released in 2002 which also plays MP3s. However, this phone was a commercial failure
- ↑ only used on web player
