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MiniDisc
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MiniDisc

   

MiniDisc (MD) is a disc-based data storage device for storing any kind of data, usually audio. The technology was announced by Sony in 1991 and introduced January 12 1992. 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.

Table of contents
1 Design
2 Upgrades
3 External links

Design

Housing

The disc is permanently housed in a cartridge (6.8 cm × 7.2 cm × 5 mm) with a sliding door, not unlike 3.5-in. 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. Rerecording is possible through a magneto-optical system. A laser heats one side of the disc to its Curie point, making the material in the disc succeptible to a magnetic field, whilst a magnetic head on the other side of the disc gives the heated area a north or south polarity, equivalent to the 0s and 1s in a digital recording.

Compression

The audio on a MiniDisc is compressed using the ATRAC format (Adaptive TRansform Acoustic Coding) while a CD contains 16 bit linear PCM audio. ATRAC is similar to other compression formats in that data are discarded according to an algorithm that exploits certain psychoacoustic principles. This is known as lossy compression. Due to limitations in human hearing and perception, some sounds cannot be heard under a variety of conditions, and these data can simply be thrown away without noticeable difference to the recording. Whilst the difference in sound quality could be easily spotted when MiniDisc appeared, today it is virtually impossible to tell, due to vast improvements in the ATRAC algorithm. The latest version of Sony's ATRAC is "ATRAC DSP Type S" (Sharp and Panasonic have their own (but fully interoperable) ATRAC codecs).

Anti-skip

MiniDisc has an advantageous feature that prevents disc skipping. Older CD players had once been a source of annoyance to users as they were prone to mistracking due to vibration and shock. MiniDisc solved this problem by reading the data into a memory buffer at a higher speed than was required (the size of the buffer varies from model to model) before being read out to the digital-to-analog converter at the standard rate required by the format. If the MiniDisc player was bumped, playback could continue unimpeded while the laser repositioned itself to continue reading data from the disc. If the memory buffer is sufficiently full, this feature allows the player to stop the spindle motor for long periods, increasing battery life. The memory buffer concept was incorporated shortly afterwards into portable CD players.

Operation

The data structure and operation of a MiniDisc is similar to that of a computer's hard disk drive. The bulk of the disc contains data pertaining to the music itself, and a small section contains the User Table of Contents, providing the playback device with vital information about the number and location of tracks on the disc. Tracks and discs can be named. Tracks may easily be added, erased, combined and divided, and their preferred order of playback modified. Erased tracks are not actually erased at the time, but are marked so. When a disc becomes full, the recorder can simply slot track data into sections where erased tracks reside. This can lead to some fragmentation but unless many erasures and replacements are performed, the only likely problem is excessive searching, reducing battery life.

Upgrades

Recording modes

In recent years, MiniDiscs have gained new recording options. One is known as MDLP (MiniDisc Long Play), using 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 to on lower quality headphones, small speakers or in the car or other noisy environments. Source material in mono or with at least a narrow stereo image when recorded in LP4 also performs adequately.

Data transfer

Another option is a range of recorders that support NetMD, a standard that allows users to transfer music in a variety of file formats from their computer to their recorder, at high speed. Generally, up to 32× speed is common.

Hi-MD

The latest upgrade is the Hi-MD, named along the same line as Sony's Hi-8 camcorder format. Hi-MD discs have the same form factor, but store up to one gigabyte of data about that of a mini-DVD. Current price is about seven U.S. dollars, significantly more than DVD-RW discs.

External links