Adaptive transform Coder 3, the bitstream designation of Dolby Digital. A variable, lossy audio compression method using perceptual coding to drop sound data you cannot hear. It can be used to deliver from 1 to 5.1 tracks of audio along with metadata on how best to play back a particular recording on a particular system. The '.1' track is a low frequency effects (LFE) track limited to 120 Hz. Metadata can offer control information on down-mixing, dynamic range and level normalization as well as informational data such as artist, copyright etc. The proper ATSC designation is presently A/52A. Encodes audio sampled at 32KHz, 44.1KHz and 48KHz into 32Kbps to 640Kbps data streams. Compression varies with quality with an average of 12 1.