AAC (Advanced Audio Coding)

AAdvanced Audio Coding (AAC) is a standardized, lossy compression and 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. AAC has been standardized by ISO and IEC, as part of the MPEG-2 and MPEG-4 specifications.Part of the AAC known as High-Efficiency Advanced Audio Coding (HE-AAC) which is part of MPEG-4 Audio is also adopted into digital radio standards like DAB+ and Digital Radio Mondiale, as well as mobile television standards DVB-H and ATSC-M/H.

AAC supports inclusion of 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 iPhone, iPod, iPad, Nintendo DSi, iTunes, DivX Plus Web Player and PlayStation 3. It is supported on PlayStation Portable, Wii (with the Photo Channel 1.1 update installed), Sony Walkman MP3 series and later, mobile phones made by Sony Ericsson; Nokia; Android; and webOS-based mobile phones. AAC has also seen moderate adoption on in-dash car audio especially on high-end units such as the Pioneer AVIC series.

Some AAC's improvements over MP3

· 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.
· Can use Kaiser-Bessel derived window function to eliminate spectral leakage at the expense of widening the main lobe.
· 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 efficiency and simpler filterbank (rather than MP3's hybrid coding, AAC uses a pure MDCT).
· 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).
· Much better handling of audio frequencies above 16 kHz.