| 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.[1] 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 and the MPEG-4 video standard. HE-AAC is
part of digital radio standards like DAB+ and Digital Radio Mondiale. 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 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 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. [2] The current version
of the AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC
v2[3]) AacPlus v2 is also standardized by ETSI (European Telecommunications
Standards Institute) as TS 102005.[2] 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: * 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 bitrates and
variable frame length. Standardized constant bit rate with bit reservoir.
* Higher efficiency and simpler filterbank (hybrid ¡ú pure MDCT)
* Higher coding efficiency for stationary signals (blocksize: 576 ¡ú
1024 samples)
* Higher coding efficiency for transient signals (blocksize:
192 ¡ú 128 samples)
* 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
(separate for every scale band)
* 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. 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). | 
