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 15 low frequency enhancement (LFE, limited to 120 Hz) channels and up to 15 data streams. AAC is able to achieve good audio quality at data rates of 320 kbit/s for five channels. 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 192 kbit/s (VBR), as with MP3.
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.
AAC’s improvements over MP3
AAC was designed to fix many of the serious performance flaws in the MP3 format.
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.
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 universally[weasel words] accepted as better than MP3 at low bit rates (typically less than 128 kilobits per second). 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.
AAC hardware support
Portable players:
Apple iPod, iTouch
Creative Zen Portable
Microsoft Zune
SanDisk Sansa
Sony PlayStation Portable (PSP) with firmware 2.0 or greater
Sony Walkman
SonyEricsson Walkman Phones-W series, e.g. W890i
Nintendo DSi To be released in America mid-2009
Slacker G2 Personal Radio Player
Mobile phones:
Nokia, Motorola, Samsung, Sony Ericsson, BenQ-Siemens, Philips etc.
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: also support AAC format.
BlackBerry: RIM’s latest series of Smartphones such as the 8100 ("Pearl") and 8800 support AAC.
Apple's iPhone supports AAC and FairPlay protected AAC files used as the default encoding format in the iTunes store.
Software:
The Axara Audio Converter program (available for multiple portable players) also offers support for AAC to varying degre.
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Program
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M4A Audio Converter
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Release date: 25-11-2008
Install size: 7,6 Mb.
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