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AAC
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 many bit rates.
AAC has been standardized by ISO and IEC, as part of the MPEG-2 & MPEG-4 specifications. 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 128kbit/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 320kbit/s for 5.1 audio.
AAC’s best known use is as the default audio format of Apple’s iPhone, iPod, iTunes, and the format used for all iTunes Store audio.
AAC is also the standard audio format for Sony’s PlayStation 3 and is supported by Sony’s Playstation Portable, latest generation of Sony Walkman, Walkman Phones from Sony Ericsson, Nseries Phones from Nokia, 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.
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 PAC coding format[1],[2][3] with the addition of TNS,[4] 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 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 ISO/IEC 13818-7:1997) in 1997 as a new “part” (distinct from ISO/IEC 13818-3) in the MPEG-2 family of international standards.
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 bit-streams 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.[5]
The current version of the AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2[6])
AAC+ v2 is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005.[5]
The MPEG-4 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 improve on the MP3 format (which was specified in MPEG-1 and MPEG-2) by the ISO/IEC in 11172-3 and 13818-3.
Advanced Audio Coding is designed to be the successor of the MP3 format and demonstrates greater sound quality and transparency than MP3 files coded at the same bit rate[citation needed].
Improvements include:
* More sample frequencies (from 8 kHz to 96 kHz) than MP3 (16 kHz 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 samples, allowing more efficient coding than MP3’s 576 sample blocks)
* Higher coding accuracy for transient signals (AAC uses a blocksize of 128 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.
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