AAC Audio File Format описание:

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 indistinguishable audio quality at data rates of 320 kbit/s (64kbit/s/channel) for five channels. The quality is close to CD also at 96 kbit/s (48kbit/s/channel) for stereo.

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 (with extensions for proprietary digital rights management).

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, Sony Ericsson Walkman Phone, Nintendo's Wii (with the Photo Channel 1.1 update installed for Wii consoles purchased before late 2007) 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 (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 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 (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.

Audio Transcoder supports the following conversions with AAC format files:

• AAC to FLAC
• AAC to M4A
• AAC to M4B
• AAC to MP4
• AAC to APE
• AAC to MPC

• AAC to MP2
• AAC to MP3
• AAC to MPC
• AAC to OGG
• AAC to WAV
• AAC to WMA
• AAC to SPX
• AC3 to AAC
• AIFF to AAC
• APE to AAC
• FLAC to AAC
• M4A to AAC
• MP+ to AAC
• MP1 to AAC
• MP2 to AAC
• MP3 to AAC
• MP4 to AAC
• MPC to AAC
• MPP to AAC
• OFR to AAC
• OFS to AAC
• OGG to AAC
• SPX to AAC
• TTA to AAC
• WAV to AAC
• WMA to AAC
• WV to AAC

Note that Audio Transcoder is not only a great Audio Converter. It is all-in-one solution to convert any audio files - WAV, WMA, OGG,MP3,MP4, AAC and many others, rip Audio CD's, edit music tags.