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MiniDisc

MiniDisc

Overview
The disc is permanently housed in a cartridge (68×72×5 mm) with a sliding door, similar to the casing of a 3.5" floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

. 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
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...

, making the material in the disc susceptible to a magnetic field.
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Encyclopedia
The disc is permanently housed in a cartridge (68×72×5 mm) with a sliding door, similar to the casing of a 3.5" floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

. 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
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...

, 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
Faraday effect
In physics, the Faraday effect or Faraday rotation is a Magneto-optical phenomenon, that is, an interaction between light and a magnetic field in a medium...

, the player senses the polarisation 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. As of May 2005, there were 74 minute and 80 minute discs available. 60 minute blanks, which were widely-available in the early years of the format's introduction, were phased-out long ago and are rarely seen.

MiniDiscs use a mastering process and optical playback system that is very similar to CDs
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

. The recorded signal of the premastered pits and of the recordable MD are also very similar. Eight-to-Fourteen Modulation
Eight-to-Fourteen Modulation
Eight-to-fourteen modulation is a data encoding technique – formally, a channel code – used by compact discs and pre-Hi-MD MiniDiscs. EFMPlus is a related code, used in DVDs and SACDs. EFM and EFMPlus were both invented by Kees A...

 (EFM) and a modification of CD's CIRC
Cross-Interleaved Reed-Solomon Coding
In the compact disc system, cross-interleaved Reed-Solomon code provides error detection and error correction. CIRC adds to every three data bytes one redundant parity byte.-Overview:...

 code, called Advanced Cross Interleaved Reed-Solomon Code (ACIRC) are employed.

Differences from cassette and CDs



MiniDiscs use rewritable magneto-optical storage to store the data. Unlike the Digital Compact Cassette
Digital Compact Cassette
Digital Compact Cassette was a magnetic tape sound recording format introduced by Philips and Matsushita in late 1992 and pitched as a successor to the standard analog cassette. It was also a direct competitor to Sony's MiniDisc but neither format toppled the then ubiquitous analog 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 (only with the latest version of Sony's PC based SonicStage
SonicStage
SonicStage is the name for Sony software that is used for managing portable devices when they are plugged into a computer running Microsoft Windows. It comprises a music player and library manager, similar to iTunes, Windows Media Player and RealPlayer. It is used to manage the library of ATRAC...

 V4.3 software) and edited there. Transferring data from an MD unit to a non-Windows machine can only be done in real time, preferably via optical I/O, by connecting the audio out port of the MD to an available audio in port of the computer. With the release of the Hi-MD format, Sony began to release Macintosh compatible software. However, the Mac compatible software is still not compatible with legacy MD formats (SP, LP2, LP4). This means that using an MD recorded on a legacy unit or in a legacy format still requires a Windows machine for non-real time transfers.

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. Early MiniDisc equipment had a fragment granularity of 4 seconds audio. Fragments smaller than the granularity are not kept track of, which may lead to the usable capacity of a disc actually shrinking. Also, no means of defragmenting the disc are provided in consumer grade equipment.

All consumer-grade MiniDisc devices feature a copy-protection scheme known as Serial Copy Management System
Serial Copy Management System
The Serial Copy Management System or SCMS is a copy protection scheme that was created in response to the digital audio tape invention, in order to prevent DAT recorders from making second-generation or serial copies. SCMS sets a "copy" bit in all copies, which prevents anyone from making further...

. An unprotected disc or song can be copied without limit, but the copies can no longer be digitally copied. However as a concession to this the most recent Hi-MD players can upload to PC a Digitally Recorded file which can subsequently be resaved as a WAV
WAV
Waveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...

 (PCM) file and thus replicated.

Audio data compression


The digitally encoded audio signal on a MiniDisc has traditionally been data-compressed using the ATRAC
ATRAC
Adaptive Transform Acoustic Coding is a family of proprietary audio compression algorithms developed by Sony. MiniDisc was the first commercial product to incorporate ATRAC in 1992. ATRAC allowed a relatively small disc like MiniDisc to have the same running time as CD while storing audio...

 format (Adaptive TRansform Acoustic Coding).

ATRAC was devised for MiniDisc so that the same amount of audio a CD can carry can fit on a disc far smaller than the CD (which contains uncompressed 16-bit stereo linear PCM
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

 audio).

ATRAC was also used on nearly all Walkman
Walkman
Walkman is a Sony brand tradename originally used for portable audio cassette, and now used to market Sony's portable audio and video players as well as a line of Sony Ericsson mobile phones...

 devices until the 8 series but is now only used in Sony's MiniDisc devices (as of November 2008) as ATRAC is fundamental to the MiniDisc specification.

In MiniDisc's latest progression, Hi-MD, uncompressed CD-quality linear PCM audio recording and playback is offered in addition to ATRAC compression of varying bitrates; placing Hi-MD on par with uncompressed, CD-quality audio for the first time.

Sony's ATRAC codec differs from uncompressed PCM in that it is a psychoacoustic
Psychoacoustics
Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...

 lossy
Lossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...

 audio data compression scheme, so decompression of the compressed signal will not yield the original signal, although the compressed signal may sound identical to the original to the listener. The latest version of Sony's ATRAC is ATRAC3plus. Original ATRAC3 at 132 kbit/s (also known as ATRAC-LP mode) is the format used by Sony's Connect audio download store. ATRAC3plus is not used in order to retain backwards compatibility with earlier NetMD players.

Anti-skip


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.

If the MiniDisc player were bumped, playback could continue unimpeded while the laser repositioned itself to continue reading data from the disc. This feature allows the player to stop the spindle motor for long periods, increasing battery life. The memory buffer concept introduced by MiniDisc was soon incorporated into portable CD players as well, and in hard drive based digital audio players.

A buffer of at least six seconds is required on all MiniDisc players, be they portable or stationary full-sized units. This is needed to ensure uninterrupted playback in the presence of fragmentation
Fragmentation (computer)
In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....

.

Operation



The data structure and operation of a MiniDisc is similar to that of a computer's hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

 drive. The bulk of the disc contains data pertaining to the music itself, and a small section contains the Table of Contents (TOC), 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.

The data structure of the MiniDisc, where music is recorded in a single stream of bytes while the TOC contains pointers to track positions, allows for gapless playback
Gapless playback
Gapless playback is the uninterrupted playback of consecutive audio tracks without intervening silence or clicks at the point of the track change. Gapless playback is common with compact discs, gramophone records, or tapes, but is not always available with other formats that employ compressed...

 of music, something which the majority of competing portable players, including most MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...

 players, fail to implement properly. (Notable exceptions are CD
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

 players, as well as all recent iPod
IPod
iPod is a line of portable media players created and marketed by Apple Inc. The product line-up currently consists of the hard drive-based iPod Classic, the touchscreen iPod Touch, the compact iPod Nano, and the ultra-compact iPod Shuffle...

s.)

At the end of recording, after the "Stop" button has been pressed, the MiniDisc may continue to write music data for a few seconds from its memory buffers. During this time, it may display a message ("Data Save", on at least some models) and the case will not open. After the audio data is written out, the final step is to write the TOC track denoting the start and endpoints of the recorded data. Sony notes in the manual that one should not interrupt the power or expose the unit to undue physical shock during this period.

MDLP


In 2000, Sony announced MDLP (MiniDisc Long Play), which added new recording modes based on a new codec called ATRAC3. In addition to the standard, high-quality mode, now called SP, MDLP adds LP2 mode, which allows twice as much recording time (160 minutes on an 80 minute disc) of good-quality stereo sound, and LP4, which allows four times more recording time (320 minutes on an 80 minute disc) of medium-quality stereo sound.

The bitrate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....

 of the standard SP mode is 292 kbit/s, and it uses separate stereo coding with discrete left and right channels. LP2 mode uses a bitrate of 132 kbit/s and also uses separate stereo coding. The last mode, LP4 has a bitrate of 66 kbit/s and uses joint stereo coding. The sound quality is noticeably poorer than the first two modes, but is sufficient for many users.

Tracks recorded in LP2 or LP4 mode play back as silence on non-MDLP players.

NetMD


NetMD recorders allow music files to be transferred from a computer to a recorder (but not in the other direction) over a USB connection. In LP4 mode, speeds of up to 32× real-time are possible and three Sony NetMD recorders (MZ-N10, MZ-N910, and MZ-N920) are capable of speeds up to 64× real-time. NetMD recorders all support MDLP.

NetMD is a proprietary protocol, and it is currently impossible to use it without proprietary software, such as SonicStage
SonicStage
SonicStage is the name for Sony software that is used for managing portable devices when they are plugged into a computer running Microsoft Windows. It comprises a music player and library manager, similar to iTunes, Windows Media Player and RealPlayer. It is used to manage the library of ATRAC...

. Thus, it cannot be used under non-Windows machines. A free *nix
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....

 based implementation, libnetmd, is being developed, but it cannot be used to upload music .

Hi-MD


Hi-MD is the further development of the MiniDisc-format. It was introduced in 2004. Hi-MD media will not play on non Hi-MD equipment, including NetMD players.

Recording modes


Modes marked in green are available for recordings made on the player, while those marked in red are only available for music downloaded from a PC. Capacities are official Sony figures; real world figures are usually slightly higher. Second generation Hi-MD players also support MP3 compression natively, in a multitude of bitrates. Recently, 352 kbit/s and 192 kbit/s ATRAC3plus have also been made available for 1st and 2nd generation Hi-MDs.
Name Bitrate (kbit/s) Codec Availability and capacity (min)
Standard player MDLP player Hi-MD player
80 minute disc 80 minute disc (HiMD formatted) 1 GB Hi-MD disc
Stereo SP 292 ATRAC 80 80 80 n/a n/a
Mono SP 146 ATRAC 160 160 160 n/a n/a
LP2 132 ATRAC3 n/a 160 160 290 990
- 105 ATRAC3 n/a 127 127 370 1250
LP4 66 ATRAC3 n/a 320 320 590 1970
- 48 ATRAC3plus n/a n/a n/a 810 2700
Hi-LP 64 ATRAC3plus n/a n/a n/a 610 2040
Hi-SP 256 ATRAC3plus n/a n/a n/a 140 475
PCM 1411.2 Linear PCM n/a n/a n/a 28 94


External links