AAC

Advanced Audio Coding is the audio coding technology that is introduced in MPEG-2. AAC is also known as MPEG-2 Part 7. As such it is the succesor of MPEG-1/2 Layer-3, more commonly known as MP3. AAC provides a capability of up to 48 main audio channels with sampling frequencies between 8 kHz and 96 kHz, as well as 16 low frequency effect channels, 16 overdub/multilingual channels, and 16 data streams. MPEG-2 AAC has been extended with additional features and capabilities in MPEG-4 AAC.

AAC is a so-called perceptual coding technology. This means that the compression is based on the imperfections of the human ear. All frequencies that can not be heard will be left out in order to reduce the amount of data. Compared to MP3, te coding scheme is futher refined and some new coding tools are added. As a result, AAC is approximately 30% more bit rate efficient than MP3.

Different versions: Profiles and objects
AAC provides for different encoding methods. These are called Profiles in MPEG-2 or Object Types in MPEG-4. Some of the versions are:

• MPEG-2 AAC Main
• MPEG-2 AAC LC (Low Complexity)
• MPEG-2 AAC SSR (Scalable Sampling Rate)
• MPEG-4 AAC Main
• MPEG-4 AAC LC (Low Complexity)
• MPEG-4 AAC SSR (Scalable Sampling Rate)
• MPEG-4 AAC HE (High Efficiency)
• MPEG-4 AAC LD (Low Delay)
• MPEG-4 AAC LTP (Long Term Prediction)

These different versions are optimised for different situations. Especially the AAC Low Complexity Profile/Object Type is rather popular because it is the most easy method to implement an encoder and decoder.