Advanced Audio Coding
Filename extension | MPEG/3GPP container
Apple container
ADTS stream
|
---|---|
Internet media type |
audio/aac audio/aacp audio/3gpp audio/3gpp2 audio/mp4 audio/mp4a-latm audio/mpeg4-generic |
Developed by | ISO/IEC 14496-3 |
Open format? | Yes |
Free format? | No[3] |
Advanced Audio Coding (AAC) is an
AAC has been standardized by
.AAC supports inclusion of 48 full-
AAC is the default or standard audio format for
History
Background
The
AAC was developed with the cooperation and contributions of companies including
Standardization
In 1997, AAC was first introduced as MPEG-2 Part 7, formally known as
MPEG-2 Part 7 defined three profiles: Low-Complexity profile (AAC-LC / LC-AAC), Main profile (AAC Main) and Scalable Sampling Rate profile (AAC-SSR). AAC-LC profile consists of a base format very much like AT&T's Perceptual Audio Coding (PAC) coding format,[23][24][25] with the addition of temporal noise shaping (TNS),[26] the Kaiser window (described below), a nonuniform quantizer, and a reworking of the bitstream format to handle up to 16 stereo channels, 16 mono channels, 16 low-frequency effect (LFE) channels and 16 commentary channels in one bitstream. The Main profile adds a set of recursive predictors that are calculated on each tap of the filterbank. The SSR uses a 4-band PQMF filterbank, with four shorter filterbanks following, in order to allow for scalable sampling rates.
In 1999, MPEG-2 Part 7 was updated and included in the MPEG-4 family of standards and became known as
The reference software for MPEG-4 Part 3 is specified in MPEG-4 Part 5 and the conformance bit-streams are specified in MPEG-4 Part 4. MPEG-4 Audio remains
The MPEG-4 Audio Version 2 (ISO/IEC 14496-3:1999/Amd 1:2000) defined new audio object types: the low delay AAC (AAC-LD) object type, bit-sliced arithmetic coding (BSAC) object type, parametric audio coding using harmonic and individual line plus noise and error resilient (ER) versions of object types.[30][31][32] It also defined four new audio profiles: High Quality Audio Profile, Low Delay Audio Profile, Natural Audio Profile and Mobile Audio Internetworking Profile.[33]
The
The current version of the AAC standard is defined in ISO/IEC 14496-3:2009.[41]
AAC+ v2 is also standardized by
The MPEG-4 Part 3 standard also contains other ways of compressing sound. These include lossless compression formats, synthetic audio and low bit-rate compression formats generally used for speech.
AAC's improvements over MP3
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).
Blind tests in the late 1990s showed that AAC demonstrated greater sound quality and transparency than MP3 for files coded at the same bit rate.[4]
Improvements include:
- more 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);
- arbitrary bit rates and variable frame length. Standardized constant bit rate with bit reservoir;
- 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.
While 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
Functionality
AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio:
- Signal components that are perceptually irrelevant are discarded.
- Redundancies in the coded audio signal are eliminated.
The actual encoding process consists of the following steps:
- The signal is converted from time-domain to frequency-domain using forward modified discrete cosine transform (MDCT). This is done by using filter banks that take an appropriate number of time samples and convert them to frequency samples.
- The frequency domain signal is quantized based on a psychoacoustic model and encoded.
- Internal error correction codes are added.
- The signal is stored or transmitted.
- In order to prevent corrupt samples, a modern implementation of the Luhn mod N algorithm is applied to each frame.[43]
The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bit rate speech coding to high-quality audio coding and music synthesis.
- The MPEG-4 audio coding algorithm family spans the range from low bit rate speech encoding (down to 2 kbit/s) to high-quality audio coding (at 64 kbit/s per channel and higher).
- AAC offers sampling frequencies between 8 kHz and 96 kHz and any number of channels between 1 and 48.
- In contrast to MP3's hybrid filter bank, AAC uses the modified discrete cosine transform (MDCT) together with the increased window lengths of 1024 or 960 points.
AAC encoders can switch dynamically between a single MDCT block of length 1024 points or 8 blocks of 128 points (or between 960 points and 120 points, respectively).
- If a signal change or a transient occurs, 8 shorter windows of 128/120 points each are chosen for their better temporal resolution.
- By default, the longer 1024-point/960-point window is otherwise used because the increased frequency resolution allows for a more sophisticated psychoacoustic model, resulting in improved coding efficiency.
Modular encoding
AAC takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want to use for a particular application.
The MPEG-2 Part 7 standard (Advanced Audio Coding) was first published in 1997 and offers three default profiles:[2][44]
- Low Complexity (LC) – the simplest and most widely used and supported
- Main Profile (Main) – like the LC profile, with the addition of backwards prediction
- Scalable Sample Rate (SSR)a.k.a. Sample-Rate Scalable (SRS)
The MPEG-4 Part 3 standard (MPEG-4 Audio) defined various new compression tools (a.k.a. Audio Object Types) and their usage in brand new profiles. AAC is not used in some of the MPEG-4 Audio profiles. The MPEG-2 Part 7 AAC LC profile, AAC Main profile and AAC SSR profile are combined with Perceptual Noise Substitution and defined in the MPEG-4 Audio standard as Audio Object Types (under the name AAC LC, AAC Main and AAC SSR). These are combined with other Object Types in MPEG-4 Audio profiles.[27] Here is a list of some audio profiles defined in the MPEG-4 standard:[35][45]
- Main Audio Profile – defined in 1999, uses most of the MPEG-4 Audio Object Types (AAC Main, AAC-LC, AAC-SSR, AAC-LTP, AAC Scalable, TwinVQ, CELP, HVXC, TTSI, Main synthesis)
- Scalable Audio Profile – defined in 1999, uses AAC-LC, AAC-LTP, AAC Scalable, TwinVQ, CELP, HVXC, TTSI
- Speech Audio Profile – defined in 1999, uses CELP, HVXC, TTSI
- Synthetic Audio Profile – defined in 1999, TTSI, Main synthesis
- High Quality Audio Profile – defined in 2000, uses AAC-LC, AAC-LTP, AAC Scalable, CELP, ER-AAC-LC, ER-AAC-LTP, ER-AAC Scalable, ER-CELP
- Low Delay Audio Profile – defined in 2000, uses CELP, HVXC, TTSI, ER-AAC-LD, ER-CELP, ER-HVXC
- Low Delay AAC v2 - defined in 2012, uses AAC-LD, AAC-ELD and AAC-ELDv2[46]
- Mobile Audio Internetworking Profile – defined in 2000, uses ER-AAC-LC, ER-AAC-Scalable, ER-TwinVQ, ER-BSAC, ER-AAC-LD
- AAC Profile – defined in 2003, uses AAC-LC
- High Efficiency AAC Profile – defined in 2003, uses AAC-LC, SBR
- High Efficiency AAC v2 Profile – defined in 2006, uses AAC-LC, SBR, PS
- Extended High Efficiency AAC xHE-AAC – defined in 2012, uses USAC
One of many improvements in MPEG-4 Audio is an Object Type called Long Term Prediction (LTP), which is an improvement of the Main profile using a forward predictor with lower computational complexity.[29]
AAC error protection toolkit
Applying error protection enables error correction up to a certain extent. Error correcting codes are usually applied equally to the whole payload. However, since different parts of an AAC payload show different sensitivity to transmission errors, this would not be a very efficient approach.
The AAC payload can be subdivided into parts with different error sensitivities.
- Independent error correcting codes can be applied to any of these parts using the Error Protection (EP) tool defined in MPEG-4 Audio standard.
- This toolkit provides the error correcting capability to the most sensitive parts of the payload in order to keep the additional overhead low.
- The toolkit is backwardly compatible with simpler and pre-existing AAC decoders. A great deal of the toolkit's error correction functions are based around spreading information about the audio signal more evenly in the datastream.
Error Resilient (ER) AAC
Error Resilience (ER) techniques can be used to make the coding scheme itself more robust against errors.
For AAC, three custom-tailored methods were developed and defined in MPEG-4 Audio
- Huffman Codeword Reordering (HCR) to avoid error propagation within spectral data
- Virtual Codebooks (VCB11) to detect serious errors within spectral data
- Reversible Variable Length Code (RVLC) to reduce error propagation within scale factor data
AAC Low Delay
The audio coding standards MPEG-4 Low Delay (
Licensing and patents
No licenses or payments are required for a user to stream or distribute audio in AAC format.[51] 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"
It used to be common for
The AAC patent holders include
Extensions and improvements
Some extensions have been added to the first AAC standard (defined in MPEG-2 Part 7 in 1997):
- Perceptual Noise Substitution (PNS), added in pseudorandomdata.
- Long Term Predictor (LTP), added in MPEG-4 in 1999. It is a forward predictor with lower computational complexity.[29]
- Error Resilience (ER), added in MPEG-4 Audio version 2 in 2000, used for transport over error prone channels[58]
- 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.
- Parametric Stereo (PS)and HE-AAC; used for even lower bitrates. Defined in 2004 and 2006.
- MPEG-4 Scalable To Lossless (SLS), Not yet published,[59] can supplement an AAC stream to provide a lossless decoding option, such as in Fraunhofer IIS's "HD-AAC" product
Container formats
In addition to the
Products that support AAC
This section needs additional citations for verification. (September 2017) |
HDTV Standards
Japanese ISDB-T
In December 2003, Japan started broadcasting terrestrial DTV
International ISDB-Tb
In December 2007, Brazil started broadcasting terrestrial DTV standard called International
DVB
The
Hardware
iTunes and iPod
In April 2003,
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
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.[64]
As of September 2009, Apple has added support for
Other portable players
- 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
For a number of years, many mobile phones from manufacturers such as
- W890iand later support HE-AAC v2.
- Nokia XpressMusicand other new generation Nokia multimedia phones like N- and E-Series also support AAC format in LC, HE, M4A and HEv2 profiles. These also supports playing LTP-encoded AAC audio.
- BlackBerry phones running the BlackBerry 10 operating system support AAC playback natively. Select previous generation BlackBerry OS devices also support AAC.
- bada OS
- Apple's iPhone supports AAC and FairPlay protected AAC files formerly used as the default encoding format in the iTunes Store until the removal of DRM restrictions in March 2009.
- Android 2.3[65] and later supports AAC-LC, HE-AAC and HE-AAC v2 in MP4 or M4A containers along with several other audio formats. Android 3.1 and later supports raw ADTS files. Android 4.1 can encode AAC.[66]
- WebOS by HP/Palm supports AAC, AAC+, eAAC+, and .m4a containers in its native music player as well as several third-party players. However, it does not support Apple's FairPlay DRM files downloaded from iTunes.[67]
- Silverlightruntime supports AAC-LC, HE-AAC and HE-AAC v2 decoding.
Other devices
- Apple's iPad: Supports AAC and FairPlay protected AAC files used as the default encoding format in the iTunes Store
- TCPMP, a popular video player, was withdrawn after version 0.66 due to patent issues, but can still be downloaded from sites other than corecodec.org. CorePlayer, the commercial follow-on to TCPMP, includes AAC support. Other Palm OS programs supporting AAC include Kinoma Player and AeroPlayer.
- Windows Mobile: Supports AAC either by the native Windows Media Player or by third-party products (TCPMP, CorePlayer)[citation needed]
- Epson: Supports AAC playback in the P-2000 and P-4000 Multimedia/Photo Storage Viewers
- Sony Reader: plays M4A files containing AAC, and displays metadata created by iTunes. Other Sony products, including the A and E series Network Walkmans, support AAC with firmware updates (released May 2006) while the S series supports it out of the box.
- Sonos Digital Media Player: supports playback of AAC files
- Barnes & Noble Nook Color: supports playback of AAC encoded files
- Roku SoundBridge: a network audio player, supports playback of AAC encoded files
- Squeezebox: network audio player (made by Slim Devices, a Logitech company) that supports playback of AAC files
- PlayStation 3: supports encoding and decoding of AAC files
- Xbox 360: supports streaming of AAC through the Zune software, and of supported iPods connected through the USB port
- Photo Channel as of December 11, 2007. All AAC profiles and bitrates are supported as long as it is in the .m4a file extension. The 1.1 update removed MP3 compatibility, but according to Nintendo, users who have installed this may freely downgrade to the old version if they wish.[68]
- Livescribe Pulse and Echo Smartpens: record and store audio in AAC format. The audio files can be replayed using the pen's integrated speaker, attached headphones, or on a computer using the Livescribe Desktop software. The AAC files are stored in the user's "My Documents" folder of the Windows OS and can be distributed and played without specialized hardware or software from Livescribe.
- Google Chromecast: supports playback of LC-AAC and HE-AAC audio[69]
Software
Almost all current computer media players include built-in decoders for AAC, or can utilize a
Adobe Flash Player, since version 9 update 3, can also play back AAC streams.[70][71] 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
The following is a non-comprehensive list of other software player applications:
- 3ivx MPEG-4: a suite of DirectShow and QuickTime plugins which support AAC encoding or AAC/ HE-AAC decoding in any DirectShow application
- CorePlayer: also supports LC and HE AAC
- ffdshow: a free open source DirectShow filter for Microsoft Windows that uses FAAD2 to support AAC decoding
- audio player for Windowsthat 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
- 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: Rhapsodysupports 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
Nero Digital Audio
In May 2006, Nero AG released an AAC encoding tool free of charge, Nero Digital Audio (the AAC codec portion has become Nero AAC Codec),[73] which is capable of encoding LC-AAC, HE-AAC and HE-AAC v2 streams. The tool is a command-line interface tool only. A separate utility is also included to decode to PCM WAV.
Various tools including the
FAAC and FAAD2
FAAC and FAAD2 stand for Freeware Advanced Audio Coder and Decoder 2 respectively. FAAC supports audio object types LC, Main and LTP.[74] FAAD2 supports audio object types LC, Main, LTP, SBR and PS.[75] Although FAAD2 is free software, FAAC is not free software.
Fraunhofer FDK AAC
A
FFmpeg and Libav
The native AAC encoder created in
Both FFmpeg and Libav can use the Fraunhofer FDK AAC library via libfdk-aac, and while the FFmpeg native encoder has become stable and good enough for common use, FDK is still considered the highest quality encoder available for use with FFmpeg.[77] Libav also recommends using FDK AAC if it is available.[78] FFmpeg 4.4 and above can also use the Apple audiotoolbox encoder.[77]
Although the native AAC encoder only produces AAC-LC, ffmpeg's native decoder is able to deal with a wide range of input formats.
See also
- Comparison of audio coding formats
- AAC-LD
- MPEG-4 Part 14(container format)
- ALAC – a lossless codec developed by Apple
- Vorbis – a royalty-free competitor to AAC and MP3
- Opus – an open, royalty-freecodec for both pre-encoded and interactive use, standardized in 2012
Notes
- ^ only used on web player, Google Home, Amazon Alexa, and Microsoft Windows app.
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Which encoder provides the best quality? ... the likely answer is: libfdk_aac
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External links
- Fraunhofer audio codecs
- AudioCoding.com Archived 2006-08-25 at the Wayback Machine – home of FAAC and FAAD2
- Official MPEG web site
- AAC improvements and extensions (2004)
- RFC 3016- RTP Payload Format for MPEG-4 Audio/Visual Streams
- RFC 3640- RTP Payload Format for Transport of MPEG-4 Elementary Streams
- RFC 4281- The Codecs Parameter for "Bucket" Media Types
- RFC 4337- MIME Type Registration for MPEG-4