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AAC - Advanced Audio Coding

Full Form AAC - Advanced Audio Coding

What is AAC?

AAC stands for "Advanced Audio Coding", which is a file format used for storing digital audio. It was designed for compressing and encoding scheme digital audio files. This technology can be an excellent option for coding audio files at medium to high bit rates, and it achieves better sound quality than MP3 files at the same bit rate. It was built to be the successor of the MP3 (ISO/MPEG Audio Layer -3)  file formats. It provides support for DRM (Digital Radio Mondiale), which enforces copyright.

AAC+ and AAC++ can be considered as an upgrade to the AAC.

AAC technology makes use of two coding techniques so that it can minimize the data which is required to impart high-quality audio. Irrelevant signals are discarded, and redundancies are wiped out automatically.

Following are the steps for encoding the audio files:

Modified discrete cosine transform (also referred to as MDCT) is used, which converts the signals from time-domain to the frequency-domain. To convert an exact number of time samples into frequency samples, filter banks are used.
 The frequency-domain is quantized with the help of a psychoacoustic model. After that, it is coded.
 Internal error connection codes are applied appropriately.
 The signal is transmitted or stacked.
 To avoid sample corruption, the Luhn mod N algorithm is applied for each frame.

AAC has the ability to sample frequencies between 8Hz to 96kHz up to 48 channels. It can also compress audio, which contains streams of complex pulses and square waves better than MP3.

AAC is an international standard audio format for major companies such as Dolby Laboratories Inc., Nokia Corp, and Sony Corp, etc. It is also used as a default audio format for iPhone, iPod, iPad, iTunes, Nintendo DSi, and 3DS, DivX Plus Web Player, Nokia S40 phones and PlayStation 3, etc. It is also used by most of the manufacturers of in-dash car audio systems.

AAC's Advantages over MP3

It achieves more sample rates (from kHz to 96 kHz) than MP3 (from 16 kHz to 48 kHz).
It can handle up to 48 channels, whereas MP3 supports two channels in MPEG-1 and 5.1 channels in MPEG-2 mode.
Rather than MP3's hybrid coding, AAC uses a pure MDCT, which helps in getting higher efficiency and a simpler filter bank.
AAC uses a block size of 1024 or 960 samples, whereas MP3 uses 576 sample blocks so that AAC allows more efficient coding for stationary signals.
AAC provides higher coding accuracy for transient signals than MP3.
AAC is capable of better handling of audio frequencies above 16 kHz.

Hence, AAC allows developers more flexibility than MP3 to design codecs.
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