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AAC is a digital audio compression technique similar to MP3, but with better audio fidelity at lower bit rates. It can handle higher and lower frequencies, up to 48 channels of audio, and low-latency audio. AAC is used in Apple’s iPods and next-gen DVDs, but requires licensing from VIA Licensing Corporation.
AAC, Advanced Audio Coding, is a technique for compressing digital audio files. Officially part of the MPEG-4 standard, it is widely used to create small digital audio files. The current variant is specified in the ISO/IEC 14496-3 standard.
AAC is conceptually similar to the ubiquitous MP3 format. Like MP3, it exploits human hearing deficiencies to discard digital bits corresponding to sounds that can hardly be heard. The human auditory system is unable to hear soft 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 data to be discarded with minimal loss of fidelity.
Also, like MP3, AAC is a lossy algorithm, which means that the original digital audio cannot be recreated from the compressed bits alone. In terms of audio fidelity, however, there is no data loss if the compressed audio is encoded correctly. AAC claims an advantage over MP3 in this regard: while MP3 requires a bit rate of about 256 kilobits per second (kbps) to achieve transparency, AAC can achieve the same quality at 128 kbps. This allows AAC files to be about half the size of MP3 files of the same quality and one-tenth the size of CD’s digital data.
AAC offers many other advantages over MP3. It is capable of handling much higher and lower sound frequencies, provides up to 48 channels of audio, and allows for the creation of low-latency audio required for two-way communication. For example, AAC can be used to compress phone conversations on the fly, whereas using MP3s would introduce audio delays. AAC also allows for digital rights management, or DRM, which can be used to control how the audio file is used.
One area where AAC excels is low bit rate compression. Audiobooks, for example, lack the dynamic range of music as they consist of little more than spoken words. Potentially, this allows for much higher compression levels. Using MP3s at such compression levels produces scratchy, hissing compressed audio not unlike low-quality cassette recordings. The CAA was specifically designed to properly handle such applications.
Unlike MP3, there is no free, public version of AAC’s encoding/decoding algorithms. All AAC users must license the technology from 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 specification.
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