Advanced Audio Coding(AAC)

 AAC (Advanced Audio Coding) is a file format for storing digital audio. It's commonly used for storing music on the Internet, PCs and portable music players and phones.

It is similar to MP3, but it was designed to be its successor and offers better quality and smaller file sizes. It also supports DRM, which enforces copyright.

AAC+ and AAC++ are newer versions of the standard.

AAC technique involves exploiting two primary coding strategies in order to minimize the amount of data required to impart high-quality digital audio. Signal components that are irrelevant are discarded. Redundancies in the coded audio signal are wiped out.

The encoding of digital audio files involves the following steps:

1. Modified discrete cosine transform (MDCT) is used to convert the signal from time-domain to frequency-domain. Filter banks are used to convert an exact number of time samples into frequency samples.

1. The frequency domain is quantized using a psychoacoustic model and is then encoded.

1. Appropriate internal error correction codes are applied.

1. The signal is stacked or transmitted.

1. Luhn mod N algorithm is used for each frame to avoid sample corruption

AAC can sample frequencies ranging from 8Hz to 96kHz and up to 48 channels. It is also able to compress audio that contains streams of complex pulses and square waves better than MP3.

AAC is an international standard used by some major companies like Dolby Laboratories Inc., Sony Corp. and Nokia Corp. AAC is also used as a default audio codec for .m4v format by Apple in iTunes Store video files

Filename extension	MPEG/3GPP container .m4a, .mp4, .3gp Apple container .m4a, .m4b, .m4p, .m4r, .m4v ADTS stream .aac
Internet media type	audio/aac audio/aacp audio/3gpp audio/3gpp2 audio/mp4 audio/mp4a-latm audio/mpeg4-generic
Developed by	Bell, Fraunhofer, Dolby, Sony, Nokia, LG Electronics, NEC, NTT Docomo, Panasonic[1]
Initial release	1997; 23 years ago[2]
Type of format	Audio compression format, lossy compression
Contained by	MPEG-4 Part 14, 3GP and 3G2, ISO base media file format and Audio Data Interchange Format (ADIF)
Standard	ISO/IEC 13818-7, ISO/IEC 14496-3