About AAC, description of the AAC format
Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme fordigital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates.
The idea of AAC’s algorythm is to exploit two primary coding strategies to dramatically reduce the amount of data needed to convey high-quality digital audio. AAC has the sampling frequencies between 8 Hz and 96 kHz and any number of channels between 1 and 48. AAC uses the modified discrete cosine transform (MDCT) together with the increased window lengths of 2,048 points. AAC is much more capable of encoding audio with streams of complex pulses and square waves than MP3 or Musicam. AAC can be switched dynamically between MDCT block lengths of 2,048 points to 256 points.
AAC takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want use for a particular application. So the AAC LD coding scheme bridges the gap between speech coding schemes and high quality audio coding schemes.
Nowadays AAC is not only acceptable in iPhone, iPod, iPad but also supported on PlayStation Portable, Wii (with the Photo Channel 1.1 update installed for Wii consoles purchased before late 2007), Sony Walkman MP3series 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 of the other advantages of AAC are:
greater range of sample frequencies (from 8 kHz to 96 kHz)
better handling of audio frequencies above 16 kHz
up to 48 channels
variable frame length and random bitrates
simpler and more efficient filter bank
transient (block size: 192 > 128 samples) and stationary (block size: 576 > 1024 samples) signals
and elimination spectral leakage.
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