MiniDisc

minidisc

A MiniDisc (MD) is a magneto-optical disc-based data storage device initially intended for storage of up to 74 minutes and, later, 80 minutes, of digitized audio. In the form of Hi-MD, it has also developed into a general-purpose storage medium. MiniDisc was released in 1992, first in Japan, and then in Europe and the U.S. The music format was originally based exclusively on ATRAC audio data compression, but the option of linear PCM recording was later introduced to attain audio quality comparable to that of a compact disc.

Sony’s MiniDisc was one of two rival digital systems introduced in 1992, that were both targeted as a replacement for the Philips analog cassette audio tape system: the other was Digital Compact Cassette (DCC), created by Philips and Matsushita. Sony had originally intended for Digital Audio Tape (DAT) to be the dominant home digital audio recording format, replacing the analog cassette. Unfortunately, due to technical delays, DAT was not launched until 1989, and by then, the U.S. dollar had fallen so far in relation to the yen, the introductory DAT machine Sony had intended to market for about $400 in the late 1980s now had to retail for $800 or even $1000 to break even, putting it out of reach for most users. Relegating DAT for pro use, Sony immediately set to work to come up with a simpler, more economical digital home format.

The disc is permanently housed in a cartridge with a sliding door, similar to the casing of a 3.5″ floppy disk. This shutter is opened automatically by a mechanism upon insertion. The audio discs can either be recordable (blank) or premastered. Recordable MiniDiscs use a magneto-optical system to record data. A laser heats one side of the disc to its Curie point, making the material in the disc susceptible to a magnetic field. A magnetic head on the other side of the disc alters the polarity of the heated area, recording the digital data onto the disk. Playback is accomplished with the laser alone: taking advantage of the Faraday effect, the player senses the polarization of the reflected light and thus interprets a 1 or a 0. Recordable MDs can be recorded on repeatedly; Sony claims up to one million times.

MiniDiscs use rewritable magneto-optical storage to store the data. Unlike the Digital Compact Cassette, or the analog compact audio cassette, the disc is a random-access medium, making seek time very fast. MiniDiscs can be edited very quickly even on portable machines. Tracks can be split, combined, moved or deleted with ease either on the player or uploaded to PC and edited there. At the beginning of the disc there is a table of contents (TOC, also known as ‘System File’ area of the disc), which stores the start positions of the various tracks, as well as meta information (Title, Artist) about them and free blocks. Unlike the conventional cassette, a recorded song does not need to be stored as one piece on the disk, it can be stored in several fragments, similar to a hard drive.

All consumer-grade MiniDisc devices feature a copy-protection scheme known as Serial Copy Management System. An unprotected disc or song can be copied without limit, but the copies can no longer be digitally copied. MiniDisc has a feature that prevents disc skipping under all but the most extreme conditions. Older CD players had once been a source of annoyance to users as they were prone to mistracking from vibration and shock. MiniDisc solved this problem by reading the data into a memory buffer at a higher speed than was required before being read out to the digital-to-analog converter at the standard rate required by the format. The size of the buffer varies by model.

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