Acronym for Adaptive TRansform Acoustic Encoding. An algorithm used for audio encoding that reduces the size of audio files. It basically works on the concept that low amplitude frequency components in a program signal are temporarily masked by adjacent frequency high amplitude components, and thus can be removed from the audio signal without a significant reduction in quality. The resulting decrease in data allows for smaller sound files. Opinions differ on how much of a sacrifice in quality there is for different types of music, but it is a commonly used algorithm. More recently ATRAC Pro has been developed. It attempts to address specific concerns of the pro audio community, most notably sound quality. There are other forms of ATRAC that are optimized for specific data storage mediums.