A codec — from enCOder/DECoder or COmpressor/DECompressor — is software or hardware designed to both compress and decompress data streams consisting of audio and/or video data. Compression both reduces the size of digital audio and video files, which means they take up less storage space, and also transmit more quickly. There are a number of different codecs designed with different goals in mind. One important codec is Advanced Audio Coding, often referred to by the acronym AAC. It’s formal name is ISO/IEC 14496-3:2001 (International Organization for Standardization/International Electrotechnical Commission).
The codec was developed by the companies in the MPEG group (Motion Pictures Expert Group). It forms part of the MPEG-2 and MPEG-4 standards, and AAC_MP4 compression was further developed from AAC_MP2. Advanced Audio Coding is considered superior to MP3, both in amount of compression and in sound. Compression can either be lossless — created so that the original can be exactly restored, or lossy — removing bits of data as it compresses files, with the result of greater compression, but an inability to completely restore the original file. Advanced Audio Coding is a lossy compression format.
There are several different Advanced Audio Coding profiles. The file extension MP4 refers to a standard MPEG-4 file, while M4A refers an to unprotected AAC. M4P is for protected AAC. The differences between files with an .M4A or .MP4 extension, on the one hand, and an .AAC extension, on the other hand, are that the .M4A and .MP4 extensions signal container formats that have a tagging standard, and so can carry song and album information with them, which the .AAC files cannot. Files with a .AAC extension are not accepted by many players.
The Advanced Audio Coding is an open standard. It has been adopted into Apple® QuickTime®, is used in Apples® iTunes®, and is the default format on the Apple® iPod®, where it serves to ensure high quality sound. Because it supports protected files, and therefore Digital Rights Management (DRM), the use of AAC in these venues helps to control access to the copy-protected material purchased from the iTunes® store.