A digital format that uses Sony’s proprietary ATRAC (Adaptive TRansform Acoustic Coding) data compression to fit up to 74 minutes of digital audio on a 64mm, recordable, erasable magneto-optical disk. Commercial prerecorded MD releases use an optical-only playback format and are not recordable. MD audio is compressed during recording, using a lossy compression, so that the audio quality has lower fidelity than the same data written to a CD. See also Digital Compact Cassette, DAT.