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Advanced Audio Coding
ITunes-aacp.png

iTunes AAC file, the most popular usage of the format
Filename extension .m4a, .m4b, .m4p, .m4v, .m4r, .3gp, .mp4, .aac
Internet media type audio/aac, audio/aacp, audio/3gpp, audio/3gpp2, audio/mp4, audio/MP4A-LATM, audio/mpeg4-generic
Type of format Lossy compression
Standard(s) ISO/IEC 13818-7, ISO/IEC 14496-3

Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates.[1]

AAC has been standardized by ISO and IEC, as part of the MPEG-2 and MPEG-4 specifications.[2][3] The MPEG-2 standard contains several audio coding methods, including the MP3 coding scheme. AAC is able to include 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). The MPEG-2 audio tests showed that AAC meets the requirements referred to as "transparent" for the ITU at 128 kbit/s for stereo, and 320 kbit/s for 5.1 audio.

AAC is also the default or standard audio format for: Apple's iPhone, iPod, iTunes, Sony's PlayStation 3 and is supported by Sony's Playstation Portable, latest generation of Sony Walkman, phones from Sony Ericsson, Nseries Phones and the latest S40 models from Nokia, Android based phones, Nintendo's Wii (with the Photo Channel 1.1 update installed for Wii consoles purchased before late 2007), the Nintendo DSi and the MPEG-4 video standard

'High-Efficiency AAC' is part of digital radio standards like DAB+ and Digital Radio Mondiale, and mobile television standards DVB-H and ATSC-M/H.

Contents

History

AAC was developed with the cooperation and contributions of companies including Fraunhofer IIS, AT&T Bell Laboratories, Dolby, Sony Corporation and Nokia, and was officially declared an international standard by the Moving Pictures Experts Group in April 1997. MPEG-2 AAC-LC profile consists of a base format very much like AT&T's Perceptual Audio Coding (PAC) coding format,[4][5][6] with the addition of temporal noise shaping (TNS),[7] the Dolby Kaiser Window described below, a nonuniform quantizer, and a reworking of the bitstream format to handle up to 16 stereo, 16 mono, 16 LFE, 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.

Standardization

It is specified both as Part 7 of the MPEG-2 standard, and Subpart 4 in Part 3 of the MPEG-4 standard.[8] As such, it can be referred to as MPEG-2 Part 7 and MPEG-4 Part 3 AAC depending on its implementation, however it is most often referred to as MPEG-2 AAC, MPEG-4 AAC, or AAC for short.

AAC was first specified in the standard MPEG-2 Part 7 (known formally as ISO/IEC 13818-7:1997) in 1997 as a new "part" (distinct from ISO/IEC 13818-3, a.k.a. MPEG-2 BC - backwards compatible[9][10]) in the MPEG-2 family of international standards. It is also known as MPEG-2 NBC (Non-Backward Compatible), because it is not compatible with the MPEG-1 Audio formats (MP3, MP2, MP1).[9][11][12][13] It defined three profiles: Low complexity profile (AAC LC), Main profile (AAC Main) and Scalable sampling rate profile (AAC SSR).

It was updated in MPEG-4 Part 3 (MPEG-4 Audio) (known formally as ISO/IEC 14496-3:1999) in 1999. The MPEG-4 Part 3 standard also defined usage of other audio compression formats (a.k.a. (Audio Object Types), such as TwinVQ, CELP, HVXC, Text-To-Speech Interface, Structured Audio and others. A notable addition in this version of the AAC standard is Perceptual Noise Substitution (PNS). MPEG-4 Audio is defined in a way that it remains backwards compatible to MPEG-2 AAC.[14] The MPEG-2 Part 7 profiles - 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 (using the same names AAC LC, AAC Main and AAC SSR).[15] MPEG-4 Audio Object Types are combined in four MPEG-4 Audio profiles: Main (which includes most of the MPEG-4 Audio Object Types), Scalable (AAC LC, AAC LTP, CELP, HVXC, TwinVQ, Wavetable Synthesis, TTSI), Speech (CELP, HVXC, TTSI) and Low rate Synthesis (Wavetable Synthesis, TTSI).[16][17] The reference software for MPEG-4 Part 3 is specified in MPEG-4 Part 4 and the conformance bit-streams are specified in MPEG-4 Part 5.

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.[18][19][20] It also defined four new audio profiles: High Quality Audio Profile, Low Delay Audio Profile, Natural Audio Profile and Mobile Audio Internetworking Profile.[21]

The HE-AAC Profile (AAC LC with SBR) and AAC Profile (AAC LC) were first standardized in ISO/IEC 14496-3:2001/Amd 1:2003.[22] The HE-AAC v2 Profile (AAC LC with SBR and Parametric Stereo) was first specified in ISO/IEC 14496-3:2005/Amd 2:2006.[23][24][25] The Parametric Stereo audio object type used in HE-AAC v2 was first defined in ISO/IEC 14496-3:2001/Amd 2:2004.[26][27][28]

The current version of the AAC standard is defined in ISO/IEC 14496-3:2009.[29]

AAC+ v2 is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005.[26]

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 show that AAC demonstrates greater sound quality and transparency than MP3 for files coded at the same bit rate.[1]

Improvements include:

  • More sample frequencies (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)
  • Arbitrary bit-rates and variable frame length. Standardized constant bit rate with bit reservoir.
  • Higher efficiency and simpler filterbank (rather than MP3's hybrid coding, AAC uses a pure MDCT)
  • 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)
  • Can 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)
  • Adds additional modules (tools) to increase compression efficiency: TNS, Backwards Prediction, 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. However, in terms of whether AAC is better than MP3, the advantages of AAC are not entirely decisive, and the MP3 specification, although antiquated, has proven surprisingly robust in spite of considerable flaws. AAC and HE-AAC are better than MP3 at low bit rates (typically less than 128 kilobits per second).[citation needed] This is especially true at very low bit rates where the superior stereo coding, pure MDCT, and more optimal transform window sizes leave MP3 unable to compete. However, as bit rate increases, the efficiency of an audio format becomes less important relative to the efficiency of the encoder's implementation, and the intrinsic advantage AAC holds over MP3 no longer dominates audio quality.

How AAC works

AAC is a wideband audio coding algorithm that exploits two primary coding strategies to reduce dramatically the amount of data needed to represent high-quality digital audio.

  1. Signal components that are perceptually irrelevant are discarded;
  2. 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

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 bitrate speech coding to high-quality audio coding and music synthesis.

  • The MPEG-4 audio coding algorithm family spans the range from low bitrate 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[30][31]:

  • 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) (MPEG-4 AAC-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.[15] Here is a list of some audio profiles defined in the MPEG-4 standard:[23][32]

  • 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
  • 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
  • HE-AAC v2 Profile - defined in 2006, uses AAC LC, SBR, PS

(One of many improvements in MPEG-4 Audio is the Object Type - Long Term Prediction (LTP), which is an improvement of the Main profile using a forward predictor with lower computational complexity.[14])

Depending on the AAC profile and the MP3 encoder, 96 kbit/s AAC can give nearly the same or better perceptual quality as 128 kbit/s MP3.[33]

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 tool kit'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 MPEG-4 Low Delay Audio Coder (AAC-LD) is designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. It is closely derived from the MPEG-2 Advanced Audio Coding (AAC) format.

Licensing and patents

No licenses or payments are required to be able to stream or distribute content in AAC format.[34] This reason alone makes AAC a much more attractive format to distribute content than MP3, particularly for streaming content (such as Internet radio).

However, a patent license is required for all manufacturers or developers of AAC codecs.[35] It is for this reason FOSS implementations such as FAAC and FAAD are distributed in source form only, in order to avoid patent infringement. (See below under Products that support AAC, Software.)

Products that support AAC

HDTV Standards

Japanese ISDB-T

In 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

In 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

The 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.[36] DVB broadcasts which use the h.264 codec for video normally use the HE-AAC codec for audio.[citation needed]

Hardware

iTunes and iPod

In April 2003, Apple Computer 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 proprietary Digital Rights Management (DRM)-restricted form of 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 free of DRM from participating record labels. 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.

iTunes supports a "Variable bit rate" (VBR) encoding option which encodes AAC tracks in an "Average bit rate" (ABR) scheme. As of September 2009, Apple has added support for HE-AAC (which is fully part of the MP4 standard) but iTunes still lacks support for true VBR encoding. The underlying QuickTime API does offer a true VBR encoding profile however.

Other Portable Players

Mobile phones

For 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 and such phones with integrated music players did not gain mainstream popularity until 2005 when the trend of having AAC as well as MP3 support continued. Most new smartphones and music-themed phones support playback of these formats.

  • Sony Ericsson phones support various AAC formats in MP4 container. AAC-LC is supported in all phones beginning with K700, phones beginning with W550 have support of HE-AAC. The latest devices such as the P990, K610, W890i and later support HE-AAC v2.
  • Nokia XpressMusic and other new generation Nokia multimedia phones like N- and E-Series: also support AAC format in LC, HE, M4A and HEv2 profiles
  • BlackBerry: RIM's latest series of Smartphones such as the 8100 ("Pearl"), 9500 ("Storm") and 8800 support AAC.
  • Apple's iPhone supports AAC and FairPlay protected AAC files used as the default encoding format in the iTunes store.
  • The HTC Dream (Also known as the T-Mobile G1) is described as supporting certain subset of the full AAC format. As of 2009-04-13 at least several forms of AAC files played while others did not play.

Other devices

  • Palm OS PDAs: Many Palm OS based PDAs and smartphones can play AAC and HE-AAC with the 3rd party software Pocket Tunes. Version 4.0, released in December 2006, added support for native AAC and HE-AAC files. The AAC codec for 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 PalmOS programs supporting AAC include Kinoma Player and AeroPlayer.
  • Microsoft Windows Mobile platforms support 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. This support is not available with their older models, however.
  • The Sony Reader portable eBook 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.
  • Nearly every major car stereo manufacturer offers models that will play back.m4a files recorded onto CD in a data format. This includes Pioneer, Sony, Alpine, Kenwood, Clarion, Panasonic, and JVC.
  • The Sonos Digital Media Player supports playback of AAC files.
  • The Roku SoundBridge network audio player supports playback of AAC encoded files.
  • The Squeezebox network audio player (made by Slim Devices, a Logitech company) supports playback of AAC files.
  • The PlayStation 3 supports encoding and decoding of AAC files.
  • The Xbox 360 supports streaming of AAC through the Zune software, and off supported iPods connected through the USB port
  • The Wii video game console supports AAC files through version 1.1 of the Photo Channel as of December 11, 2007. All AAC profiles and bitrates are supported as long as it is in the.m4a file extension. This update removed MP3 compatibility, but users who have installed this may freely downgrade to the old version if they wish.[37]
  • The Livescribe Pulse Smartpen records and stores 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.

Software

Flash Player

A very common program supporting AAC playback is Flash player, version 9, update 3 and above.[38][39] 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.[40]

Other software media players

Almost 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 utilized this way with the corresponding filters to enable AAC playback in any DirectShow based player. Software player applications of particular note include:

Some of these players (e.g., foobar2000, Winamp, and VLC) also support the decoding of ADTS (Audio Data Transport Stream) or MP4-contained AAC streamed over HTTP using the SHOUTcast protocol. Plug-ins for Winamp and foobar2000 enable the creation of such streams.

Nero Digital Audio

In May 2006, Nero AG released an AAC encoding tool free of charge, Nero Digital Audio (Nero AAC Codec),[41] which is capable of encoding LC-AAC, HE-AAC and HE-AAC v2 streams. The tool is a Command Line Interface tool only, and a separate utility is included to decode to PCM WAV.

Various tools including the foobar2000 audio player, MeGUI encoding front end and dBpoweramp can provide a GUI for the encoder.

FAAC and FAAD2

FAAC and FAAD2 stand for Freeware Advanced Audio Coder and Decoder 2 respectively, collectively make up an open source implementation of AAC. FAAC supports audio object types LC, Main and LTP.[42] FAAD2 supports audio object types LC, Main, LTP, SBR and PS.[43]

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 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.[14]
  • Error Resilience (ER), added in MPEG-4 Audio version 2 in 2000, used for transport over error prone channels;[44]
  • 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 or eAAC+, the combination of Parametric Stereo (PS) and HE-AAC; used for even lower bitrates; defined in 2004 and 2006;
  • MPEG-4 Scalable To Lossless (SLS), defined in 2006, 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 MP4, 3GP and other ISO base media file format-based container formats for storage, AAC audio data may be packaged in a more basic format called Audio Data Interchange Format (ADIF),[45] consisting of a single header followed by the raw AAC audio data blocks.[46] Alternatively, it may be packaged in a streaming format called Audio Data Transport Stream (ADTS), consisting of a series of frames, each frame having a header followed by the AAC audio data.[45] Both formats are defined in MPEG-2 Part 7, but are only considered informative by MPEG-4, so an MPEG-4 decoder does not need to support either format.[45] Two more formats are defined in MPEG-4 Part 3: Low-overhead MPEG-4 Audio Transport Multiplex (LATM), which provides a way to combine separate audio payloads, and Low Overhead Audio Stream (LOAS), a self-synchronizing streaming format.[45]

See also

Notes

  1. ^ a b Brandenburg, Karlheinz (1999), MP3 and AAC Explained, http://www.telos-systems.com/techtalk/hosted/Brandenburg_mp3_aac.pdf 
  2. ^ ISO (2006) ISO/IEC 13818-7:2006 - Information technology -- Generic coding of moving pictures and associated audio information -- Part 7: Advanced Audio Coding (AAC), Retrieved on 2009-08-06
  3. ^ ISO (2006) ISO/IEC 14496-3:2005 - Information technology -- Coding of audio-visual objects -- Part 3: Audio, Retrieved on 2009-08-06
  4. ^ Johnston, J. D. and Ferreira, A. J., "Sum-difference stereo transform coding," ICASSP '92, March, 1992, pp. II-569-572.
  5. ^ Sinha, D. and Johnston, J. D., "Audio compression at low bit rates using a signal adaptive switched filterbank," IEEE ASSP, 1996, pp. 1053-1057.
  6. ^ Johnston, J. D., Sinha, D., Dorward, S. and Quackenbush, S., "AT&T perceptual audio coder (PAC)" in Collected Papers on Digital Audio Bit-Rate Reduction, Gilchrist, N. and Grewin, C. (Ed.), Audio Engineering Society, 1996.
  7. ^ Herre, J. and Johnston, J. D., "Enhancing the performance of perceptual audio coders by using temporal noise shaping," AES 101st Convention, no. preprint 4384, 1996
  8. ^ ISO/IEC (1 September 2009) (PDF), ISO/IEC 14496-3:2009 - Information technology -- Coding of audio-visual objects -- Part 3: Audio, IEC, http://webstore.iec.ch/preview/info_isoiec14496-3%7Bed4.0%7Den.pdf, retrieved 2009-10-07 
  9. ^ a b MPEG.ORG. "AAC". http://www.mpeg.org/MPEG/audio/aac.html. Retrieved 2009-10-28. 
  10. ^ ISO (15 January 2006) (PDF), ISO/IEC 13818-7, Fourth edition, Part 7 - Advanced Audio Coding (AAC), http://webstore.iec.ch/preview/info_isoiec13818-7%7Bed4.0%7Den.pdf, retrieved 2009-10-28 
  11. ^ Gabriel Bouvigne (2003). "MPEG-2/MPEG-4 - AAC". MP3'Tech. http://www.mp3-tech.org/aac.html. Retrieved 2009-10-28. 
  12. ^ ISO (1998-10). "MPEG Audio FAQ Version 9 - MPEG-1 and MPEG-2 BC". ISO. http://mpeg.chiariglione.org/faq/mp1-aud/mp1-aud.htm. Retrieved 2009-10-28. 
  13. ^ ISO (1996-03). "Florence Press Release". ISO. http://mpeg.chiariglione.org/meetings/firenze/prfloren.htm. Retrieved 2009-10-28. 
  14. ^ a b c Karlheinz Brandenburg, Oliver Kunz, Akihiko Sugiyama (1999). "MPEG-4 Natural Audio Coding - General Audio Coding (AAC based)". chiariglione.org. http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/09-natural_audio_paper/gacoding.html. Retrieved 2009-10-06. 
  15. ^ a b Karlheinz Brandenburg, Oliver Kunz, Akihiko Sugiyama. "MPEG-4 Natural Audio Coding - Audio profiles and levels". chiariglione.org. http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/09-natural_audio_paper/profiles.html. Retrieved 2009-10-06. 
  16. ^ ISO/IEC JTC 1/SC 29/WG 11 (15 May 1998) (PDF), ISO/IEC FCD 14496-3 Subpart 1 - Draft - N2203, ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg4/documents/w2203/w2203.pdf, retrieved 2009-10-07 
  17. ^ Karlheinz Brandenburg, Oliver Kunz, Akihiko Sugiyama (15 May 1998), MPEG-4 Natural Audio Coding - Audio profiles and levels, chiariglione.org, http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/09-natural_audio_paper/profiles.html, retrieved 2009-10-07 
  18. ^ ISO (2000). "ISO/IEC 14496-3:1999/Amd 1:2000 - Audio extensions". ISO. http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=31568. Retrieved 2009-10-07. 
  19. ^ ISO/IEC JTC 1/SC 29/WG 11 (1999-07) (PDF), ISO/IEC 14496-3:/Amd.1 - Final Committee Draft - MPEG-4 Audio Version 2, ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg4/documents/w2803/w2803_n.pdf, retrieved 2009-10-07 
  20. ^ Heiko Purnhagen (19 February 2000). "AES 108th Convention: MPEG-4 Version 2 Audio ­ What is it about?". Heiko Purnhagen. http://140.130.175.70/html/mpeg4/sound.media.mit.edu/mpeg4/audio/general/index.html#aes108. Retrieved 2009-10-07. 
  21. ^ Fernando Pereira (2001-10). "Levels for Audio Profiles". MPEG Industry Forum. http://www.m4if.org/resources/profiles/audio.php. Retrieved 2009-10-15. 
  22. ^ ISO (2003). "ISO/IEC 14496-3:2001/Amd 1:2003 - Bandwidth extension". ISO. http://www.iso.org/iso/catalogue_detail.htm?csnumber=38148. Retrieved 2009-10-07. 
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please note that ref#8 has been changed to: "http://www.aacplus.net/products/assets/CT_aacPlus_whitepaper.pdf"

External links








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