Information AboutAdif |
| CATEGORIES ABOUT ADVANCED AUDIO CODING | |
| audio codecs | |
| computer file formats | |
| mpeg | |
| lossy compression algorithms | |
representing MPEG-4 AAC DRM encoded files, as used in the iTunes Store]] Advanced Audio Coding ('''AAC''') is a standardized, Lossy Compression and Encoding scheme for ''' Digital Audio '''. AAC is promoted as the successor to the MP3 format by MP3’s creator, Fraunhofer IIS . Depending on the encoder used, AAC generally achieves better sound quality than MP3 at the same Bitrate , particularly below 192 Kbit /s.http://www.soundexpert.info/ AAC’s most famous usage is as the default audio format of Apple 's IPhone , IPod , ITunes , and the format used for all ITunes Store audio (with extensions for proprietary Digital Rights Management (DRM) where used). AAC is also the standard audio format for Sony ’s PlayStation 3 , the MPEG-4 video standard, and HE-AAC is part of Digital Radio standards like DAB+ and Digital Radio Mondiale . HISTORY AAC was developed with the cooperation and contributions of companies including Dolby , Fraunhofer IIS , AT&T , Sony and Nokia , and was officially declared an international standard by the Moving Pictures Experts Group in April 1997. Standardization It is specified both as ''Part 7 of the MPEG-2 standard'', and ''Part 3 of the MPEG-4 standard''. As such, it can be referred to as MPEG-2 Part 7 and '''MPEG-4 Part 3''' depending on its implementation , however it is most often referred to as '''MPEG-4 AAC''', or '''AAC''' for short. AAC was first specified in the standard MPEG-2 Part 7 (known formally as / IEC 13818-3) in the MPEG-2 family of International Standard s. It was updated in MPEG-4 Part 3 (known formally as ISO/IEC 14496-3:1999) in 1999. The reference software is specified in MPEG-4 Part 4 and the conformance bitstreams are specified in MPEG-4 Part 5. A notable addition in this version of the standard is Perceptual Noise Substitution (PNS). HE-AAC (AAC with SBR) was first standardized in ISO/IEC 14496-3:2001/Amd.1. HE-AAC v2 (AAC with Parametric Stereo) was first specified in ISO/IEC 14496-3:2001/Amd.4. http://www.codingtechnologies.com/products/assets/CT_aacPlus_whitepaper.pdf The current version of the AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2ISO/IEC 14496-3:2005/Amd.2 {Link without Title} ) AacPlus v2 is also standardized by ETSI ( European Telecommunications Standards Institute ) as TS 102005. The MPEG4 standard also contains other ways of compressing sound. These are low bit rate and generally used for speech. AAC’s improvements over MP3 AAC was designed to have better performance than MP3 (which was specified in MPEG-1 and MPEG-2 ) by the ISO / IEC in 11172-3 and 13818-3. Improvements include:
Overall, the AAC format allows developers more flexibility to design codecs than MP3 does. 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 conclusive, and the MP3 specification, while outdated, has proven surprisingly robust. AAC and HE-AAC are better than MP3 at low bitrates (typically less than 192 kbit/s). HOW AAC WORKS 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. Furthermore:
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.
AAC encoders can switch dynamically between a single MDCT block of length 1024 points or 8 blocks of 128 points.
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 use for a particular application. The standard offers four default profiles:
Depending on the AAC profile and the MP3 encoder, 96 kbit/s AAC can give nearly the same or better perceptional quality as 128 kbit/s MP3. {Link without Title} 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. The most stringent requirements are a maximum algorithmic delay of only 20 ms and a good audio quality for all kind of audio signals including speech and music. The AAC-LD coding scheme bridges the gap between speech coding schemes and high quality audio coding schemes. 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.
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
PROMOTING ASPECTS Licensing and patents No licenses or payments are required to be able to stream or distribute content in AAC format. 1 This reason alone makes AAC a much more attractive format for distributing content, particularly streaming content (such as Internet radio). However, a patent license is required for all manufacturers or developers of AAC codecs, that require encoding or decoding. 2 It is for this reason FOSS implementations such as FAAC And FAAD are distributed in Source form only, in order to avoid patent infringement. AAC requires a patent license, and thus uses Proprietary technology. But contrary to popular belief, it is not the property of a single company, having been developed in a standards-making organization. Marketing aspects AAC was promoted as the successor to MPEG 1.0 audio, layer III (MP3) for audio coding at medium to high bitrates, although lower bitrates are its forte. While much less popular than MP3, its sustainability is currently maintained by it being the default Digital Rights Management ). PRODUCTS THAT SUPPORT AAC 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 and are encoded using FairPlay. Apple has often attributed the proprietary nature of Fairplay-encoded files to demands by major record labels for copy protection. On May 29, 2007, Apple began selling songs and music videos free of DRM from participating record labels, starting with EMI . While these files mostly adhere to the AAC standard and are playable on many non-Apple products, they do include custom iTunes information such as album artwork and a purchase receipt. Along with a significantly higher bit rate, these DRM-free tracks are promoted under the brand “iTunes Plus”. Apple added support for "Variable bit rate" (VBR), which encodes AAC tracks in "Average bit rate" (ABR) in iTunes v5.0 and above. It also added certain enhancements in higher-end iPods such as chapters (bookmarks that can incorporate web links and pictures set to appear at certain times during playback of audio books and podcasts) which are not features of AAC itself, but of the proprietary Apple file format that wraps the AAC bitstream. Apple has still not added support for AAC+ which is fully part of the MP4 standard or proper VBR encoding to iTunes (April 2007) Other Portable Players
Sony PlayStation 3 With the launch of the PlayStation 3 ( November 11 , 2006 ), Sony announced support for the AAC format for music (MPEG-4 AAC) and video with AAC audio. This allows for easy interoperability with the PSP system that supports AAC already. 128 kbit/s AAC is the default format for audio CDs imported to the PS3 hard disk.http://manuals.playstation.net/document/en/ps3/current/music/cdimport.html Microsoft Xbox 360 To listen to music files encoded in the AAC audio format, an Xbox 360 console will need the AAC codec (a processing file), which is available as a free download from Xbox Live Marketplace. Optional iPod Support or Optional Media Center Codec is the file required to play an iPod with the Xbox. 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.
Other Devices
Software 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 .http://www.xbox.com/en-US/support/systemuse/xbox360/digitalmedia/ipod.htm 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 raw 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, tool only, and a separate utility is included to decode to PCM WAV . Various tools including the Foobar2000 audio player and MeGUI 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. EXTENSIONS AND IMPROVEMENTS Some extensions have been added to the original AAC standard:
SEE ALSO AAC Technology General Category Related Technologies
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