AAC, Advanced Audio Coding, is a technique for compressing digital audio files. Officially part of the MPEG-4 standard, it is most widely used to create small digital audio files. The current variant is specified in ISO/IEC standard 14496-3.
AAC is conceptually similar to the ubiquitous MP3 format. Like MP3, it takes advantage of deficiencies in human hearing to discard digital bits corresponding to sounds unlikely to be heard. The human hearing system cannot hear quiet sounds in the presence of loud sounds of a similar frequency; for example, a voice conversation cannot be heard while a jumbo jet flies low overhead. Known as auditory masking, this phenomenon allows the discarding of data with minimal loss of fidelity.
Also like MP3, AAC is a lossy algorithm, meaning the original digital audio cannot be recreated from the compressed bits alone. In terms of audio fidelity, however, there is no loss of data if the compressed audio is properly encoded. AAC claims an advantage over MP3 in this regard: while MP3 requires a bit rate of approximately 256 kilobits per second (kbps) to achieve transparency, AAC can achieve the same quality at 128 kbps. This allows AAC files to be roughly half the size of MP3 files of the same quality, and one-tenth the size of CD digital data.
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AAC provides several other advantages over MP3. It is capable of handling much higher and lower frequencies of sound, provides up to 48 channels of audio, and allows the creation of low-latency audio necessary for two-way communication. For example, AAC can be used for compressing telephone conversations on the fly, whereas using MP3 would introduce delays in the audio. AAC also allows for Digital Rights Management, or DRM, which can be used to control how the audio file is used.
An area where AAC excels is in low bit-rate compression. Audio books, for example, don't have the dynamic range of music since they consist of little more than spoken word. Potentially, this allows for much higher levels of compression. Using MP3 at such levels of compression produces scratchy, hissy compressed audio not unlike low-quality cassette tape recordings. AAC was specifically designed to handle such applications properly.
Unlike MP3, there is no public, free version of the encoding/decoding algorithms of AAC. All AAC users must license the technology from the VIA Licensing Corporation. AAC is most widely used in Apple Computer's line of iPod portable music players and is part of the next-generation DVD specifications.