MinidiscMiniDisc (MD) is a disc-based digital medium for storing data, usually audio. The technology was announced by Sony in 1991. MD Data, a version for storing computer data was announced by Sony in 1993, but it never gained significant ground, so today MDs are used primarily for audio storage.
The disc is permanently housed in a cartridge (6.8cm x 7.2cm x 5mm) with a sliding door, not unlike 3.5-inch floppy diskettes. The audio discs can be premastered or recordable (blank). Premastered MDs contain audio data and work in a way very similar to CDs. Recordable MDs can be recorded on repeatedly; Sony claims up to one million times. Today, there are 60-, 74-, and 80-minute discs available.
The audio on a MiniDisc is compressed using the ATRAC format while a CD contains 16 bit linear PCM audio. While the difference in sound quality could be easily spotted when Minidisc appeared, with the development of ATRAC, today it is virtually impossible to tell the difference. The latest version of Sony's ATRAC is "ATRAC DSP Type S" (Sharp and Panasonic have their own (but fully interoperable) ATRAC codecs).
In recent years, MiniDiscs have gained two new recording options. These are known as MDLP (MiniDisc Long Play), and use a new codec called ATRAC3. In addition to the standard, CD-quality mode, now also called SP, MDLP adds LP2 mode, which allows twice more recording time (160 minutes or 2 hours and 40 minutes on an 80-minute disc) of good-quality stereo sound, and LP4, which allows four times more recording time (320 minutes or 5 hours and 20 minutes on an 80-minute disc) of medium-quality stereo sound.
The bitrate of the standard SP mode is 292 kb/s and it uses full stereo coding with separate left and right channels. For the vast majority of people the sound quality is identical with CDs. LP2 mode uses a bitrate of 132 kb/s and also uses full stereo coding. For most people the sound quality is almost the same as SP. The last mode, LP4 has a bitrate of 66 kb/s and uses joint stereo coding. The sound quality is noticeably worse than the first two modes, but is sufficient for speech or for music listened on lower quality headphones or small speakers.