MiniDisc

MiniDisc

The Sony MZ1 MiniDisc player, the first to hit the market in 1992.
Media type	Magneto-optical disc
Encoding	ATRAC, linear PCM (with Hi-MD)
Capacity	80 min (standard MiniDisc), up to 45 hours of audio (1 GB capacity) (with Hi-MD)
Read mechanism	780 nm laser
Write mechanism	Magnetic field modulation
Developed by	Sony
Usage	Audio storage, Data storage (with Hi-MD)

A MiniDisc (MD) is a magneto-optical disc-based data storage device initially intended for storage of up to 80 minutes of digitized audio. Today, in the form of Hi-MD, it has developed into a general-purpose storage medium in addition to greatly expanding its audio roots.

MiniDisc was announced by Sony in September 1992 and released that November for sale in Japan and December for the USA and Europe.^[1] The music format was originally based exclusively on ATRAC audio data compression, but the option of linear PCM recording was ultimately introduced to attain CD-quality recordings. MiniDiscs were popular in Japan and Asia as a digital upgrade from cassette tapes, but were not as popular elsewhere.^[2]

Market history

Sony Mini-Disc 74

Along with Philips and Matsushita Digital Compact Cassette (DCC) system also introduced in 1992, the MiniDisc was targeted as a replacement for the Philips analog cassette audio tape system.

Despite having a loyal customer base (primarily musicians and audio enthusiasts), MiniDisc met with only limited success. It was relatively popular in Japan during the 1990s but did not enjoy comparable sales in other world markets. Since then, Recordable CDs, flash memory and HDD-based digital audio players introduced in 1998 have become increasingly popular as playback devices.

Sony avoided the mistake that it had made in the 1970s with the Betamax video recording system, and this time licensed the MD technology to other manufacturers, with JVC, Sharp, Pioneer, Panasonic and others all producing their own MD systems.

MiniDisc technology was faced with new competition from the recordable compact disc (CD-R) when it became more affordable to consumers in 1995, while the popularity of traditional cassette tape refuses to wane in certain quarters. MiniDisc is widely respected as being a very reliable format when it comes to portable audio storage, such as field recording.

The initial low uptake of MiniDisc was attributed to the small number of pre-recorded albums available on MD as a relatively small number of record labels embraced the format. The initial high cost of equipment and blank media was also a factor. Stationary MiniDisc-player/recorders never got into the lower price ranges, and most consumers had to hook the portable player to the hi-fi in order to record. This inconvenience contrasted the earlier common use of cassette player/recorders as a more or less standard part of an ordinary hi-fi set-up, even before the break-through of portable cassette tape players. Pre-recorded MDs disappeared from the market rather suddenly in the late 1990s.

Because of the waning popularity of the format and the increasing popularity of solid-state MP3 players, Sony now produces only one model, the MZ-RH1 (also available as the MZ-M200 in North America packaged with a Sony microphone and limited Macintosh software support).^[3]

The introduction of the MZ-RH1 may herald a new lease of life for the Minidisc as for the first time users are able to freely move uncompressed digital recordings back and forth from the MZ-RH1 to a computer without the copyright protection limitations previously imposed upon the NetMD series. This allows the MiniDisc to finally compete evenly with HD recorders and MP3 players.

MD Data

MD Data, a version for storing computer data, was announced by Sony in 1993 but never gained significant ground. Its media were incompatible with standard audio MiniDiscs, which has been cited as one of the main reasons behind the format's failure.

MD Data could not write to audio-MDs, only the considerably more expensive data blanks. In 1997, MD-Data2 blanks were introduced, which held 650 MB of data. They were only implemented in Sony's short-lived MD-based camcorder as well as a small number of MultiTrack Recorders; Sony's MDM-X4, Tascam's 564 (which could also record using standard MD-Audio discs, albeit only 2 tracks), and Yamaha's MD-8, MD-4, & MD4S.

The Hi-MD format, introduced in 2004, marked a return to the data storage arena with its 1 GB discs and ability to act as a USB drive. Hi-MD units allow the recording and playback of audio and data on the same disc, and are compatible (both audio and data) with standard MiniDisc media. (An 80 minute Minidisc blank can be formatted to store 305MB of data)

Design

Physical characteristics

Minidisc in cartridge (left), exposed minidisc (right) and protective cartridge holder (rear). Detail view of the MZ-R30 MiniDisc recorder of Sony (1996)

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. 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 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.

Premastered MiniDiscs use a mastering process and optical playback system that is very similar to CDs, making them physically dissimilar to recordable discs. The recorded signal of the premastered pits and of the recordable MD are very similar to that of the CD. Eight-to-Fourteen Modulation (EFM) and a modification of CD's CIRC 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, 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 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. 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 (PCM) file and thus replicated.

Audio data compression

MD Walkman

The digitally encoded audio signal on a MiniDisc has traditionally been data-compressed using the ATRAC 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 audio). ATRAC was also used on nearly all Walkman 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 lossy 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.

Operation

MiniDisc Deck MDS-JE780

MiniDisc Recorder MDS-81, normally used in recording or broadcast radio studios.

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 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 of music, something which the majority of competing portable players, including most MP3 players, fail to implement properly. (Notable exceptions are CD players, as well as all recent iPods.)

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.

Format extensions

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 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. Thus, it cannot be used under non-Windows machines. A free *nix based implementation, libnetmd, is being developed, but it cannot be used to upload music (as of December 2005^[update]).

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

MiniDisc recorder.

Memorex Mini-Disc

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.

See also

• DAT
• UMD

References

External links

• MiniDisc Community Portal
• MiniDisc.org FAQ
• MiniDisc Forum
• Audio T-Board
• Linux and Sony MiniDisc players
• The Difference between Type-R and Type-S